data communications - jnujprdistance.comjnujprdistance.com/assets/lms/lms jnu/mba/mba...smds -...
TRANSCRIPT
Data Communications
This book is a part of the course by Jaipur National University, Jaipur.This book contains the course content for Data Communications.
JNU, JaipurFirst Edition 2013
The content in the book is copyright of JNU. All rights reserved.No part of the content may in any form or by any electronic, mechanical, photocopying, recording, or any other means be reproduced, stored in a retrieval system or be broadcast or transmitted without the prior permission of the publisher.
JNU makes reasonable endeavours to ensure content is current and accurate. JNU reserves the right to alter the content whenever the need arises, and to vary it at any time without prior notice.
I/JNU OLE
Index
ContentI. ...................................................................... II
List of FiguresII. ........................................................... V
List of TablesIII. ......................................................... VII
AbbreviationsIV. ......................................................VIII
Case StudyV. .............................................................. 129
BibliographyVI. ......................................................... 135
Self Assessment AnswersVII. ................................... 138
Book at a Glance
II/JNU OLE
Contents
Chapter I ....................................................................................................................................................... 1Introduction .................................................................................................................................................. 1Aim ................................................................................................................................................................ 1Objectives ...................................................................................................................................................... 1Learning outcome .......................................................................................................................................... 11.1 Introduction to Data Communication ...................................................................................................... 21.2 Protocols and Computer Networking Technologies ................................................................................ 21.3 OSI and TCP/IP Layers ............................................................................................................................ 4 1.3.1 Open System Interconnection Model (OSI Model) ................................................................. 4 1.3.2 TCP/IP Layers .......................................................................................................................... 91.4 Addressing ............................................................................................................................................. 10 1.4.1 Class A-E Networks ............................................................................................................... 12 1.4.2 Network/Netmask Specification ............................................................................................ 13Summary ..................................................................................................................................................... 16References ................................................................................................................................................... 16Recommended Reading ............................................................................................................................. 16Self Assessment ........................................................................................................................................... 17
Chapter II ................................................................................................................................................... 19Physical Layer ............................................................................................................................................ 19Aim .............................................................................................................................................................. 19Objectives .................................................................................................................................................... 19Learning outcome ........................................................................................................................................ 192.1 Data and Signals Fundamental ............................................................................................................... 20 2.1.1 Analog and Digital Signals .................................................................................................... 20 2.1.2 Transmission Impairments ..................................................................................................... 24 2.1.3 Data Rate Limits .................................................................................................................... 27 2.1.4 Performance ........................................................................................................................... 28Summary ..................................................................................................................................................... 31References ................................................................................................................................................... 32Recommended Reading ............................................................................................................................. 32Self Assessment ........................................................................................................................................... 33
Chapter III .................................................................................................................................................. 35Data Transmission ...................................................................................................................................... 35Aim .............................................................................................................................................................. 35Objectives .................................................................................................................................................... 35Learning outcome ........................................................................................................................................ 353.1 Digital Encoding Techniques ................................................................................................................. 36 3.1.1 Digital - to – Digital ............................................................................................................... 37 3.1.2 Analog – to – Digital .............................................................................................................. 40 3.1.3 Digital – to – Analog .............................................................................................................. 40 3.1.4 Analog – to –Analog .............................................................................................................. 403.2 Scrambling Techniques .......................................................................................................................... 413.3 Pulse Code Modulation (PCM) .............................................................................................................. 433.4 Transmission Modes .............................................................................................................................. 46 3.4.1 Parallel Transmission Mode ................................................................................................... 47 3.4.2 Serial Transmission Mode ..................................................................................................... 48Summary ..................................................................................................................................................... 49Reference..................................................................................................................................................... 49Recommended Reading ............................................................................................................................. 49Self Assessment ........................................................................................................................................... 50
III/JNU OLE
Chapter IV .................................................................................................................................................. 52Multiplexing................................................................................................................................................ 52Aim .............................................................................................................................................................. 52Objectives .................................................................................................................................................... 52Learning outcome ........................................................................................................................................ 524.1 Introduction ............................................................................................................................................ 534.2 Frequency Division Multiplexing .......................................................................................................... 544.3 Time Division Multiplexing ................................................................................................................... 554.4 Wavelength Division Multiplexing ........................................................................................................ 60Summary ..................................................................................................................................................... 62References ................................................................................................................................................... 62Recommended Reading ............................................................................................................................. 62Self assessment ........................................................................................................................................... 63
Chapter V .................................................................................................................................................... 65Transmission Media and Switching ......................................................................................................... 65Aim .............................................................................................................................................................. 65Objectives .................................................................................................................................................... 65Learning outcome ........................................................................................................................................ 655.1 Guided and Unguided Media ................................................................................................................. 665.2 Circuit Switched ..................................................................................................................................... 725.3 Datagram ............................................................................................................................................. 735.4 Virtual Circuit Networks ........................................................................................................................ 745.5 Switch Structure ..................................................................................................................................... 75Summary ..................................................................................................................................................... 78References ................................................................................................................................................... 78Recommended Reading ............................................................................................................................. 79Self Assessment ........................................................................................................................................... 80
Chapter VI .................................................................................................................................................. 82Error Detection and Correction ............................................................................................................... 82Aim .............................................................................................................................................................. 82Objectives .................................................................................................................................................... 82Learning outcome ........................................................................................................................................ 826.1 Introduction ............................................................................................................................................ 83 6.1.1 Types of Errors ....................................................................................................................... 83 6.1.2 Detection versus Correction ................................................................................................... 846.2 Hamming Distance ................................................................................................................................. 84 6.2.1 Minimum Hamming Distance ............................................................................................... 846.3 Cyclic Redundancy Check ..................................................................................................................... 85 6.3.1 Encoder .................................................................................................................................. 86 6.3.2 Decoder .................................................................................................................................. 876.4 Checksum ............................................................................................................................................... 886.5 Framing .................................................................................................................................................. 90 6.5.1 Fixed-Size Framing ................................................................................................................ 90 6.5.2 Variable-Size Framing ........................................................................................................... 906.6 Flow and Error Control .......................................................................................................................... 926.7 Data Link Protocols ............................................................................................................................... 936.8 HDLC ..................................................................................................................................................... 936.9 PPP ......................................................................................................................................................... 95Summary ..................................................................................................................................................... 97References ................................................................................................................................................... 97Recommended Reading ............................................................................................................................. 98Self Assessment ........................................................................................................................................... 99
IV/JNU OLE
Chapter VII .............................................................................................................................................. 101Multiple Accesses ..................................................................................................................................... 101Aim ............................................................................................................................................................ 101Objectives .................................................................................................................................................. 101Learning outcome ...................................................................................................................................... 1017.1 CSMA/CA ........................................................................................................................................... 102 7.1.1 CSMA/CA and Wireless Networks ...................................................................................... 1037.2 CSMA/CD ........................................................................................................................................... 1037.3 Controlled Access ................................................................................................................................ 1047.4 Channelisation ...................................................................................................................................... 106Summary ....................................................................................................................................................112References ..................................................................................................................................................112Recommended Reading ............................................................................................................................112Self Assessment ..........................................................................................................................................113
Chapter VIII ..............................................................................................................................................115Data Connections and Security................................................................................................................115Aim .............................................................................................................................................................115Objectives ...................................................................................................................................................115Learning outcome .......................................................................................................................................1158.1 IPv4 .......................................................................................................................................................116 8.1.1 Address Space .......................................................................................................................116 8.1.2 Notations ...............................................................................................................................1168.2 IPV6 ......................................................................................................................................................1198.3 Cryptography ....................................................................................................................................... 1228.4 Security ................................................................................................................................................ 125Summary ................................................................................................................................................... 126References ................................................................................................................................................. 126Recommended Reading ........................................................................................................................... 126Self Assessment ......................................................................................................................................... 127
V/JNU OLE
List of Figures
Fig. 1.1 Simplified data communication model ............................................................................................. 3Fig. 1.2 Seven layers of OSI model ............................................................................................................... 5Fig. 1.3 Operations of OSI model .................................................................................................................. 6Fig. 1.4 TCP/IP model ................................................................................................................................... 9Fig. 1.5 Addressing concepts ........................................................................................................................11Fig. 1.6 Subnet mask .................................................................................................................................... 14Fig. 2.1 Comparison between analog and digital signals ............................................................................. 20Fig. 2.2 Analog signals................................................................................................................................. 21Fig. 2.3 (a) Digital signals ............................................................................................................................ 22Fig. 2.3 (b) Two digital signals: One with two signal levels and the other with four signal levels ............. 23Fig. 2.4 Attenuation...................................................................................................................................... 24Fig. 2.5 Distortion ........................................................................................................................................ 26Fig. 2.6 Noise ............................................................................................................................................. 26Fig. 2.7 Crosstalk graph ............................................................................................................................... 27Fig. 2.8 Bandwidth ....................................................................................................................................... 29Fig. 3.1 (a) Encoding a digital signals ......................................................................................................... 36Fig. 3.1 (b) Modulating an analog signal ..................................................................................................... 36Fig. 3.2 Line coding and decoding ............................................................................................................... 37Fig. 3.3 Signal element versus data element ................................................................................................ 38Fig. 3.4 Line coding schemes ...................................................................................................................... 39Fig. 3.5 Unipolar NRZ scheme .................................................................................................................... 39Fig. 3.6 Polar NRZ-L and NRZ-I schemes .................................................................................................. 40Fig. 3.7 Digital-to-analog conversion .......................................................................................................... 40Fig. 3.8 Types of analog-to-analog modulation ........................................................................................... 41Fig. 3.9 AMI used with scrambling ............................................................................................................. 41Fig. 3.10 Two cases of B8ZS scrambling technique .................................................................................... 42Fig. 3.11 Different situations in HDB3 scrambling techniques ................................................................... 42Fig. 3.12 Components of PCM encoder ....................................................................................................... 43Fig. 3.13 Three different sampling methods for PCM ................................................................................. 43Fig. 3.14 Quantisation and encoding of a sampled signal ........................................................................... 44Fig. 3.15 Components of a PCM decoder .................................................................................................... 45Fig. 3.16 The process of delta modulation ................................................................................................... 45Fig. 3.17 Delta modulation components ...................................................................................................... 45Fig. 3.18 Delta demodulation components .................................................................................................. 46Fig. 3.19 Types of data transmission ............................................................................................................ 47Fig. 3.20 Parallel transmission ..................................................................................................................... 47Fig. 3.21 Serial transmission ........................................................................................................................ 48Fig. 4.1 Multiplexing ................................................................................................................................... 53Fig. 4.2 Categories of multiplexing ............................................................................................................. 54Fig. 4.3 Frequency division multiplexing .................................................................................................... 54Fig. 4.4 Times Division Multiplexing ......................................................................................................... 58Fig. 4.5 Synchronous Time Division Multiplexing ..................................................................................... 59Fig. 4.6 Asynchronous Time Division Multiplexing ................................................................................... 59Fig. 4.7 Wavelength Division Multiplexing ................................................................................................. 60Fig. 5.1 Types of transmission media .......................................................................................................... 66Fig. 5.2 Types of guided media .................................................................................................................... 67Fig. 5.3(a) Unshielded twisted pair cable .................................................................................................... 67Fig. 5.3(b) Unshielded twisted pair cable .................................................................................................... 68Fig. 5.4 Thinnet and thicknet coaxial cable ................................................................................................. 69Fig. 5.5 Critical angle refraction in optical fibre ......................................................................................... 70Fig. 5.6 Fibre optic cable ............................................................................................................................. 70Fig. 5.7 Electromagnetic spectrum .............................................................................................................. 71Fig. 5.8 Circuit switched network ................................................................................................................ 72
VI/JNU OLE
Fig. 5.9 Datagram network .......................................................................................................................... 73Fig. 5.10 Virtual circuit network .................................................................................................................. 74Fig. 5.11 Network switch ............................................................................................................................. 75Fig. 5.12 Distributed model ......................................................................................................................... 76Fig. 5.13 Centralised model ......................................................................................................................... 77Fig. 6.1 Single-bit error ................................................................................................................................ 83Fig. 6.2 Burst error on a data unit ................................................................................................................ 83Fig. 6.3 Design for the encoder and decoder. .............................................................................................. 86Fig. 6.4 Division in CRC encoder ................................................................................................................ 87Fig. 6.5 Division in the CRC decoder for two cases .................................................................................... 88Fig. 6.6 Process at the sender and at the receiver ........................................................................................ 89Fig. 6.7 Byte stuffing and unstuffing ........................................................................................................... 91Fig. 6.8 A frame in a bit-oriented protocol ................................................................................................... 92Fig. 6.9 Bit stuffing and unstuffing .............................................................................................................. 92Fig. 6.10 Taxonomy of protocols ................................................................................................................. 93Fig. 6.11 HDLC frame format ..................................................................................................................... 94Fig. 6.12 PPP frame format ......................................................................................................................... 95Fig. 7.1 Space/time model of the collision in CSMA ................................................................................ 102Fig. 7.2 Flow diagram for CSMA/CA ....................................................................................................... 103Fig. 7.3 CSMA/CD .................................................................................................................................... 104Fig. 7.4 Reservation access method ........................................................................................................... 104Fig. 7.5 Select and poll functions in polling access method ...................................................................... 105Fig. 7.6 Frequency Division Multiple Access ............................................................................................ 106Fig. 7.7 Time Division Multiple Access .................................................................................................... 107Fig. 7.8 Simple idea of communication with code .................................................................................... 108Fig. 7.9 Data representation in CDMA ...................................................................................................... 109Fig. 7.10 Sharing channel in CDMA ......................................................................................................... 109Fig. 7.11 Digital signal created by four stations in CDMA ........................................................................110Fig. 7.12 Decoding of the composite signal for one in CDMA ..................................................................110Fig. 8.1 Dotted-decimal notation and binary notation for an IPv4 address ................................................116Fig. 8.2 Finding the classes in binary and dotted-decimal notation ............................................................117Fig. 8.3 A network configuration for the block 205.16.37.32/28 ................................................................119Fig. 8.4(a) Hex to binary conversion ......................................................................................................... 120Fig. 8.4(b) IPv6 global address sequence of bits. ..................................................................................... 120Fig. 8.5 Components of cryptography ....................................................................................................... 122Fig. 8.6 Use three characters in an information exchange scenario: Alice, Bob and Eve ......................... 123Fig. 8.7 Categories of cryptography .......................................................................................................... 123Fig. 8.8 Symmetric key cryptography ........................................................................................................ 124Fig. 8.9 Asymmetric key cryptography ...................................................................................................... 124Fig. 8.10 Security services related to the message or entity ...................................................................... 125
VII/JNU OLE
List of Tables
Table 1.1 Comparisons between TCP/IP model and OSI reference model ................................................. 10Table 1.2 Network ID and host ID for class networks ................................................................................. 12Table 1.3 Class networks ............................................................................................................................. 12Table 6.1 A CRC code with C (7, 4) ............................................................................................................ 85Table 8.1 Number of blocks and block size in classfulIPv4 addressing .....................................................117Table 8.2 Default masks for classful addressing .........................................................................................118Table 8.3 Type prefixes for IPv6 addresses ............................................................................................... 121
VIII/JNU OLE
Abbreviations
ABM - Asynchronous Balanced ModeADPU - Application Protocol Data UnitAM - Amplitude ModulationARM - Asynchronous Response ModeARQ - Automatic Repeat RequestASCII - American Standard Code for Information InterchangeATDM - Asynchronous Time Division MultiplexingATM - Asynchronous Transfer ModeBps - Bits Per SecondCDMA - Carrier Sense Multiple AccessCDMA - Code Division Multiple AccessCIDR - Classless Inter-Domain RoutingCRC - Cyclic Redundancy CheckCSU/DSU - Channel Service Unit/ Data Service UnitCWDM - Coarse Wavelength Division MultiplexingDA - Destination AddressDEMUX - DemultiplexerDP - Destination PortDSI - Digital Speech InterpolationDWDM - Dense Wavelength Division MultiplexingEC - Error CodeEOR - End Of RecordFCS - Frame Check SequenceFDDI - Fiber Distributed Data InterfaceFDMA - Frequency Division Multiple AccessFM - Frequency ModulationFPM - Fast Packet MultiplexingGPRS - General Packet Radio ServiceHDLC - High level Data Link ControlIANA - Internet Assigned Numbers AuthorityIGRP - Interior Routing Gateway ProtocolIPV4 - Internet Protocol Version 4IPX/SPX - Internetwork Packet Exchange/Sequenced Packet ExchangeISDN - Integrated Services Digital NetworkISO - International Standardisation for OrganisationLAN - Local Area NetworkLAP - Link Access ProducerLECs - Local Exchange CarriersLLC - Logical Link ControlLSB - Least Significant BitMAN - Metropolitan Area NetworkMSB - Most Significant BitMUX - MultiplexerNETBEUI - Net Bios Extended User InterfaceNIC - Network Interface CardNPDU - Network Protocol Data UnitNRM - Normal Response ModeNSAP - Network Service Access PointOSI - Open System InterconnectionPCM - Pulse Code ModulationPPDU - Presentation Protocol Data UnitPPP - Point-to-Point ProtocolRIP - Routing Information Protocol
IX/JNU OLE
SA - Source AddressSAP - Service Access PointSMDS - Switched Multi Megabit Data ServiceSP - Source PortSPDU - Session Protocol Data UnitSTP - Shielded Twisted PairTASI - Time Assignment Speech InterpolationTCP/IP - Transfer Control Protocol/Internet ProtocolTDMA - Time Division Multiple AccessTPDU - Transfer Protocol Data UnitUD - User DataUTP - Unshielded Twisted PairVLSM - Variable Length Subnet MaskWAN - Wide Area NetworkWCMP - Wireless Control Message ProtocolWDM - Wavelength Division MultiplexingWLAN - Wireless Local Area Network
1/JNU OLE
Chapter I
Introduction
Aim
The aim of this chapter is to:
elucidate the concept of data communications•
explain OSI layers•
definesubnetmask•
Objectives
The objectives of this chapter are to:
elucidate the basic layers of OSI Model•
explain the creation of TCP/IP protocol suite•
differentiate between OSI model and the TCP/IP model•
Learning outcome
At the end of this chapter, you will be able to:
compare Physical models with Internet models•
understand addressing formats•
calculate the subnet mask• from IP addresses
Data Communications
2/JNU OLE
1.1 Introduction to Data CommunicationThe history of data communication extends well beyond modern computer connections and wireless devices; some mightfind thecommunicationchannel’searliest roots surprising.Thus,beforedelving into thehistoryofdatacommunication, let us understand what constitutes this form of information exchange. According to Dr. Carl Rebman, professor of Data Communications and Computer Networks at the University of San Diego, lexicographers have defineddatacommunicationas“Anyprocessthatpermitsthepassagefromasendertooneormorereceiversofinformationofanynature,deliveredinanyeasytouseformbyanyelectromagneticsystem.”Thisdefinitionhelpshistorians trace the roots of data communication considerably further than modern digital equipment.
AccordingtotheHistoryofComputingOrganisation,datacommunicationhasitsearliestrootsinSamuelMorse’s1837 exhibition of a telegraph system. An explanation of data communication history posted by telecommunications expertsatGeneralTelecom,LLCalsopointstoatelegraphpatentthatinventorCharlesWheatstonefiledthatsameyear. By 1843, telegraph service had become adopted by the Great Western Railway, an endorsement that allowed the service to expand across the nation.
Data Communications concerns the transmission of digital messages to devices external to the message source. “External” devices are generally thought of as being independently powered circuitry that exists beyond thechassis of a computer or other digital message source. As a rule, the maximum permissible transmission rate of a message is directly proportional to signal power and inversely proportional to channel noise. It is the aim of any communications system to provide the highest possible transmission rate at the lowest possible power and with the least possible noise.
1.2 Protocols and Computer Networking TechnologiesAll computers, in the logical and physical networks have to follow the same rules known as Protocols such as TCP/IP, IPX/SPX and NETBEUI etc.
Today, there are many computer networking technologies such as LAN, MAN, WAN, WLAN, ISDN, ATM, Frame Relay, X.25, Bluetooth, GPRS, CDMA and many others. There are different types of the physical medium that allows the communication between the network devices such as UTP, STP, Fibre optic, coaxial cable, Microwave, radio frequencies, satellite and electromagnetic waves. The devices that are involved in the data communication process are LAN cards, hub, router, switch, CSU/DSU and modems. Following elements are involved to form a network communication system.
Computers•Protocols•LAN Card•Hub/Switch/Router•Mediums (UTP/STP, Fibre optic cables, Air)•
Protocols represent a set of rules, or agreed upon ways and standards, that are used to make data transmission process. Every computer in a network should support the same protocols. TCP/IP is the most common communication protocol in the LAN, WAN and on internet. With the advancements in the communication technologies; data, voice and video signals can be transmitted over the same channel such as broadband internet.
3/JNU OLE
Text
Input information
m
SourceTrans-mitter
Trans-missionSystem
Re-ceiver
Destina-tion
Input data g(t)
Transmitted signal
s(t)
Received signal
r(t)
Output data g’(t)
Output information
m’
Digital bit stream
Digital bit stream
Analog signal
Analog signal
Text
Fig. 1.1 Simplified data communication model(Source:http://t1.gstatic.com/images?q=tbn:ANd9GcTrtDG5mRSQmN1_r5rAaJsCnKpO0lMdwmBmT4UZA7_
aASg9CVxH)
Considertheabovefiguretounderstandthefollowingexamples.
Example 1Consider that the input device and transmitter are components of a personal computer. The user of the PC wishes to sendamessagetoanotheruser.Suchas,“ThemeetingscheduledforMarch25iscancelled(m)”.Theuseractivatesthe electronic mail package on the PC and enters the message via the keyboard (input device). The character string isbrieflybufferedinmainmemory.Wecanviewitasasequenceofbits(g)inmemory.Thepersonalcomputerisconnected to some transmission medium, such as a local network or a telephone line, by an I10 device (transmitter), such as a local network transceiver or a modem.
The input data are transferred to the transmitter as a sequence of voltage shifts [g(t)] representing bits on some communications bus or cable. The transmitter is connected directly to the medium and converts the incoming stream [g(t)] into a signal [s(t)] suitable for transmission. The transmitted signal s(t) presented to the medium is subject to a number of impairments, before it reaches the receiver. Thus, the received signal r(t) may differ to some degree from s(t). The receiver will attempt to estimate the original s(t), based on r(t) and its knowledge of the medium, producingasequenceofbitsgl(t).Thesebitsaresenttotheoutputpersonalcomputer,wheretheyarebrieflybufferedin memory as a block of bits (g).
In many cases, the destination system will attempt to determine if an error has occurred and, if so, will cooperate with the source system to eventually obtain a complete, error-free block of data. These data are then presented to theuserviaanoutputdevice,suchasaprinterorascreen.Themessage(m’),asviewedbytheuser,willusuallybe an exact copy of the original message (m).
Example 2Now consider a telephone conversation. In this case, the input to the telephone is a message (m) in the form of sound waves. The sound waves are converted by the telephone into electrical signals of the same frequency. These signalsaretransmittedwithoutmodificationoverthetelephoneline.
Hence, the input signal g(t) and the transmitted signal s(t) are identical. The signal s(t) will suffer some distortion over the medium, so that r(t) will not be identical to s(t). Nevertheless, the signal r(t) is converted back into a sound wave with no attempt at correction or improvement of signal quality.
Data Communications
4/JNU OLE
Thus,m’isnotanexactreplicaofm.However,thereceivedsoundmessageisgenerallycomprehensibletothelistener. The discussion so far does not touch on other key aspects of data communications, including data-link controltechniquesforcontrollingtheflowofdataanddetectingandcorrectingerrors,andmultiplexingtechniquesfortransmissionefficiency.
1.3 OSI and TCP/IP LayersThe general problem of communication between cooperating dissimilar hosts situated on interconnected, but diverse, networks was studied by committees under the sponsorship of the International Organisation for Standardisation (ISO). Their work resulted in the Open Systems Interconnection Reference Model (OSI model, or OSIRM, for short).
Amodelisatheoreticaldescriptionofsomeaspectofthephysicaluniversethatidentifiesessentialcomponentsandis amenable to analysis. Depending on the assumptions and approximations made, the subsequent results are more or less applicable to the real environment and may be extrapolated to similar situations.
1.3.1 Open System Interconnection Model (OSI Model)As the name implies, the OSI model is designed to guide the development of open systems so that they can communicatewitheachother.Opensystemsaredefinedbytheparametersoftheinterfacesbetweentheirfunctionalblocks. Ideally, equipment from one vendor that implements a function will work with equipment from another vendorthatimplementsthenextfunction.Todothis,themodeldoesnotdefinetheequipment,onlystatesthattheymustexistattheirinterfaces.Itisthedesigners’problemtocreateequipmentthatsatisfiestheserequirements.Themodel divides the actions of each host into seven independent activities that are performed in sequence.
Figure shown below highlights the activities arrayed in two stacks that represent the cooperating hosts. The seven layerscontainprotocolsthatimplementthefunctionsneededtoensurethesatisfactorytransferofblocksofuser’sdata between them.
When sending, each layer accepts formatted data from the layer above, performs appropriate functions on it, adds information to the format and passes it to the layer below.
When receiving, each layer accepts formatted data from the layer below, performs some function on it, subtracts information from the format and passes it to the layer above. Each layer shields the layer above from the details of the services performed by the layers below. Of the seven layers in the model, the top three (5, 6, and 7) focus on conditioningorrestoringtheuser’sdata,andlayers1,2,3,and4implementdatacommunication.
5/JNU OLE
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer
The Seven Layers of OSI
User
Physical Link
ReceiveDtat
TransmitDtat
Fig. 1.2 Seven layers of OSI model(Source: http://t1.gstatic.com/images?q=tbn:ANd9GcRUIwlmRh6mvw1tLbapdsUMk9N2cC2QH95aSBqTy4jw9
9qYTZx-)
Application layerTheapplicationlayerinvokesgenericapplications(e.g.,mail,filetransfer,terminalemulation)insupportofdatageneratedbyspecificuserapplications.Whensending,theapplicationlayer:
Combinesdatareceivedfromtheuser’sapplicationwiththeappropriategenericfunctiontocreateauser’s•data block.Encapsulatestheuser’sdatablockwithaheader(applicationheader,AH)thatidentifiesthiscommunication•betweenspecificuserapplications.Passes the application protocol data unit (APDU) to the presentation layer.•
While receiving, the application layer:DecapsulatestheADPU(i.e.,removestheapplicationheaderfromtheADPUtoleavetheuser’sdatablock).•Passestheuser’sdatatotheapplicationidentifiedbytheheader.•
Data Communications
6/JNU OLE
Application
Presentation
Session
Transport
Network
Data
Physical
Application
Presentation
Session
Transport
Network
Data
Physical
Sending Process Receiving ProcessData
AH
PH
PH
PH
PH
PH
SH
SH
SH
SH
SH
TH
TH
TH
TH
NH
NH
NH
DH
DH
DT
DT
Data
Data
Data
Data
Data
Data
Data
AH
AHPH
AH
AH
AH
AH
Application protocol
Presentation protocol
Session protocol
Transport protocol
Network protocolData-link protocol
Bits
Actual Data transmission pathClient A Client B
Physical medium
Fig. 1.3 Operations of OSI model(Source: http://t0.gstatic.com/
images?q=tbn:ANd9GcSCyK94wNEs5aX6LLijuesdAB4yd9DKEE39ZhcIkwajdKR3TOQvAQ)
Peer–to–peercommunicationisrequiredtoagreeupontheuniqueidentifierforthecommunication.Usuallyitincludes a port number and may include a sequenced number. They are included in the application layer.
Presentation layerThe presentation layer conditions the APDU to compensate for differences in local data formats in the sender and receiver. When sending, the presentation layer:
Performs translation services (e.g., code changing) and may perform data compression and encryption on the •APDU.Encapsulates theAPDUby adding a header (presentation header, PH) that identifies the specific coding,•compression and encryption employed.Passes the presentation PDU (PPDU) to the session layer.•
When receiving, the presentation layer:Decapsulates the PPDU by removing the presentation header to leave the ADPU.•Performs any decoding, decompressing and decrypting required.•Passes the APDU to the application layer.•
Peer – to – peer communication is required to agree upon coding, compression and encryption algorithms. They are included in the presentation header.
7/JNU OLE
Session layerThe session layer directs the establishment, maintenance and termination of the connection. It manages data transfer includingregistrationandpasswordformalities,andmayinsertsynchronisationpointsintotheinformationflowtofacilitate restarting should a catastrophic failure occur. When sending, the session layer:
Supervises the use of passwords and other checks.•Tracks requests for retransmission and responses.•Identifiesthebeginningandcertifiestheendingoftheexchange.•Encapsulates the PPDU by adding a header (session header, SH) that identifies any specific marker •employed.Passes the session PDU (SPDU) to the transport layer.•
When receiving, the session layer:Decapsulates the SPDU by removing the session header to leave the PPDU. •Notesanyspecificmarkers.•Passes the PPDU to the presentation layer.•
Peer-to-peer communication is required to check authorisations and agree upon line discipline and the use of markers. They are functions included in the session header.
Transport layerThe transport layer is the highest layer in the stack to be concerned with communication protocols. It ensures the integrity of end-to-end communication independent of the number of networks involved and their performance. It isresponsibleforthesequenceddeliveryoftheentiremessageincludingerrorcontrol,flowcontrolandqualityofservice requirements (if they are invoked). When sending, the transport layer:
Establishes a connection-oriented duplex or connectionless simplex connection.•Calculates a frame check sequence (FCS), or uses another technique, to facilitate checking the integrity of the •SPDU at the receiver.Encapsulates the SPDU with a header (transport header, TH) to form the transport PDU (TPDU).•Copies the TPDU for retransmission (if necessary).•Passes the TPDU to the network layer.•
When receiving, the transport layer:Decapsulates the TPDU by removing the transport header to form the SPDU.•VerifiestheFCStoconfirmerror-freereception.•Acknowledges an error-free SPDU or discards it and may request a resent.•Mayinstructthesendertomodifytheflowrate,ifnecessary.•Passes the SPDU to the session layer.•
Peer-to-peer communications is required to agree on the network(s) used for this communication to frame corrupt frames and to adjust data rates. This information is included in the transport header.
Network layerThe network layer provides communications services to the transport layer. If necessary, it fragments the TPDU into packets to match the maximum frame limits of the network(s), and reassembles the packets to create the transport PDU. When sending, the network layer:
Encapsulates the TPDU with a header (network header, NH) to form the network PDU (NPDU). The network •header provides a destination address.May break the TPDU into packets to match the capabilities of the network(s).•
Data Communications
8/JNU OLE
If the TPDU is segmented it encapsulates each segment with a network header to form a NPDU. The network •header provides a destination address and a sequence number.Passes the network PDU(s) to the data link layer.•
When receiving the network layer:Removes the network header from the NPDU to form the TPDU.•Verifiesdestinationaddressandsequencenumber.•Reassembles the TPDU, if necessary.•Passes it to the transport layer. •
Peer-to-peer communication is required to initiate, maintain and terminate the network level connection. These functions are performed by the network header.
Data ink layerThe data link layer transfers data frames over a single communication link without intermediate nodes. When sending, the data link layer:
Addsaheader(DH)andatrailer(DT)toformthedatalinkPDU(DPDU).Theheaderincludesaflag,class•offrameidentifier,sequencenumberandhardwareaddressofdestinationonlink.ThetrailerincludesanFCSandaflag.Copies the frame in case retransmission is requested.•Passes the frame to the physical layer.•
When receiving the data link layer:Reconstructs the DPDU from the bit stream received from the physical layer.•Removes both header and trailer from the DPDU.•VerifiesFCSandotherlayerinformation.•Discards the frame if the checks are not conclusive.•Passes a correct NPDU on to the network layer.•Requests resend, if necessary.•
Peer-to-peer communication is required to agree on data link protocol parameters, error detection information, and error correction procedures. These are the functions of the data link header and trailer.
Physical layerThe physical layer converts the logical symbol stream into the actual signal stream and completes the connection overwhichsignalsflowbetweentheusers.Whensending,thephysicallayer:
Converts the logical data stream to a suitable electrical signals, including signal conditioning (i.e., pulse shaping, •zerostuffing,scrambling).Transmits a sequence of electrical symbols that represents the frame received from the data link layer.•
When receiving the physical layer:Receives a sequence of electrical signals.•Interprets the signals as 1s and 0s.•Deconditions the bit stream (i.e., unstuffs zeros, unscrambles).•Passes a clean logical symbol stream to the data link layer.•
Peer-to-peer communication consists of the signals that represent the total frame passed between Systems 1 and 2.
9/JNU OLE
1.3.2 TCP/IP LayersTCP/IP (Transmission Control Protocol/Internet Protocol) is the basic communication language or protocol of the Internet. It can also be used as a communications protocol in a private network. When you set up with direct access to the Internet, your computer is provided with a copy of the TCP/IP program just as every other computer that you may send messages to or get information from also has a copy of TCP/IP.
The TCP/IP model consists of four layers, each of which can have several sub layers. These layers correlate roughly tolayersintheOSIreferencemodelanddefinesimilarfunctions.SomeoftheTCP/IPlayerscorresponddirectlywith layers in the OSI reference model while other spans several OSI layers. The four TCP/IP layers are:
OSIseven-layer model
Application
Presentation
Session
Transport
Network
Data-link
Physical
TCP/IPfour-layer model
App
Transport
Internet
Network
Fig. 1.4 TCP/IP model(Source: http://t1.gstatic.com/images?q=tbn:ANd9GcQYxJzdT6a_80zuDT1-
2MzXiRWlVL22WBCHlrs6WFLozqS6K1_CXw)
TCP/IP application layer: • It refers to communications services to applications and is the interface between the network and the application. It is also responsible for presentation and controlling communication sessions. It spans the Application Layer, Presentation Layer and Session Layer of the OSI reference model. Examples include: HTTP, POP3, and SNMP.TCP/IP transport layer: • Itdefinesseveralfunctions,includingthechoiceofprotocols,errorrecoveryandflowcontrol.Thetransportlayermayprovideforretransmission,i.e.,errorrecoveryandmayuseflowcontrolto prevent unnecessary congestion by attempting to send data at a rate that the network can accommodate, or it mightnot,dependingonthechoiceofprotocols.Multiplexingofincomingdatafordifferentflowstoapplicationson the same host is also performed. Reordering of the incoming data stream when packets arrive out of order is included. It correlates with the Transport Layer of the OSI reference model. Examples include: TCP and UDP, which are called Transport Layer, or Layer 4, protocols.TCP/IP internetwork layer: • It defines end-to-end delivery of packets and defines logical addressing toaccomplishthis.Italsodefineshowroutingworksandhowroutesarelearned;andhowtofragmentapacketinto smaller packets to accommodate media with smaller maximum transmission unit sizes. It correlates with the Network Layer of the OSI reference model. Examples include: IP and ICMP.TCP/IP network interface layer: • It is concerned with the physical characteristics of the transmission medium aswellasgettingdataacrossoneparticularlinkormedium.Thislayerdefinesdeliveryacrossanindividuallinkaswellasthephysicallayerspecifications.ItspanstheDataLinkLayerandPhysicalLayeroftheOSIreference model. Examples include: Ethernet and Frame Relay.
Data Communications
10/JNU OLE
TCP/IP model vs. OSI model
No. TCP/IP Reference Model OSI Reference Model
1 DefinedaftertheadventofInternet Definedbeforeadventofinternet
2 Earlier service interface and protocols were not clearly distinguished.
Service interface and protocols are clearly distinguished
3 TCP/IP supports Internet working Internet working not supported
4 Loosely layered Strict layering
5 Protocol Dependant standard Protocol independent standard
6 More Credible Less Credible
7 TCP reliably delivers packets, IP does not reliably deliver packets All packets are reliably delivered
Table 1.1 Comparisons between TCP/IP model and OSI reference model(Source: http://www.buzzle.com/articles/tcpip-model-vs-osi-model.html)
1.4 AddressingThe concept of addressing in communications architecture is a complex one and covers a number of issues. At least four separate issues need to be discussed here:
Addressing level•Addressing scope•Connectionidentifiers•Addressing mode•
Addressing concept can be illustratedusing thefigure below,which shows a configuration using theTCP/IPprotocol architecture. The concepts are essentially the same for the OSI architecture or any other communications architecture.
Addressing level refers to the level in the communications architecture at which an entity is named. Typically, a unique address is associated with each end system (e.g., host or terminal) and each intermediate system (e.g., router) inaconfiguration.
Such an address is, in general, a network-level address. In the case of the TCP/IP architecture, this is referred to as an IP address or simply an internet address. In the case of the OSI architecture, this is referred to as a network service access point (NSAP). The network-level address is used to route a PDU through a network or networks to a system indicated by a network-level address in the PDU.
11/JNU OLE
Host A Host B
App x App x
TCP TCP
IP IP
Network Address Protocol #1
Global network address
Network attachment point address
Logical connection(TCP connection)
Port or service access point (SAP)
Logical connection (TCP connection)
Network 1 Network 2
Network Address Protocol #2
Physical Physical
App Y App Y
IPRouter J
NAP1 NAP2
Fig. 1.5 Addressing concepts(Source: http://t3.gstatic.com/images?q=tbn:ANd9GcQBHggnVikvuJDG-b2acfMY2cuMpe0y1f7low1kJMNeOG
Rixvym)
Once data arrives at a destination system, it must be routed to some process or application in the system. Typically, a system will support multiple applications and an application may support multiple users. Each application and, perhaps,eachconcurrentuserofanapplication,isassignedauniqueidentifier,referredtoasaportintheTCP/IParchitecture and as a service access point (SAP) in the OSI architecture.
Forexample,ahostsystemmightsupportbothanelectronicmailapplicationandafiletransferapplication.Atminimum,eachapplicationwouldhaveaportnumberorSAPthatisuniquewithinthatsystem.Further,thefiletransfer application might support multiple simultaneous transfers, in which case, each transfer is dynamically assigned a unique port number or SAP.
Theabovefigure illustrates two levelsofaddressingwithina system; this is typically thecase for theTCP/IParchitecture. However, the addressing can be done at each level of architecture. For example, a unique SAP can be assigned to each level of the OSI architecture.
Another issue that relates to the address of an end system or intermediate system is addressing scope. The internet address or NSAP address referred to above is a global address. The key characteristics of a global address are:
Global non-ambiguity: • Aglobaladdressidentifiesauniquesystem.Subsequently,synonymsarepermitted.That is, a system may have more than one global address.Global applicability: • It is possible at any global address to identify any other global address, in any system, by means of the global address of the other system.
IP addresses are broken into 4 octets (IPv4) separated by dots called dotted decimal notation. An octet is a byte consisting of 8 bits. The IPv4 addresses are in the following form: 192.168.10.1
There are two parts of an IP address:Network ID•Host ID•
Data Communications
12/JNU OLE
The various classes of networks specify additional or fewer octets to designate the network ID versus the host ID.
Class 1st Octet 2nd Octet 3rd Octet 4th Octet
Net ID Host ID
A
Net ID Host ID
B
Net ID Host ID
C
Table 1.2 Network ID and host ID for class networks
When a network is set up, a netmaskisalsospecified.Thenetmaskdeterminestheclassofthenetworkasshownbelow,exceptforCIDR.Whenthenetmaskissetup,itspecifiessomenumberofmostsignificantbitswitha1’svalueandtheresthavevaluesof0.Themostsignificantpartofthenetmaskwithbitssetto1’sspecifiesthenetworkaddress,and the lower part of the address will specify the host address. When setting addresses on a network, remember there can be no host address of 0 (no host address bits set), and there can be no host address with all bits set.
1.4.1 Class A-E NetworksTheaddressingschemeforclassAthroughEnetworksisshownbelow.Itistobenotedthatweusethe‘x’characterheretodenote‘doesn’tcaresituations’whichincludeallpossiblenumbersatthelocation.Itismanytimesusedtodenote networks.
Network Type Address Range Normal Netmask Comments
Class A 001.x.x.x to 126.x.x.x 255.0.0.0 For very large networks
Class B 128.1.x.x to 191.254.x.x 255.255.0.0 For medium size networks
Class C 192.0.1.x to 223.255.254.x 255.255.255.0 For small networks
Class D 224.x.x.x to 239.255.255.255 Used to support multicasting
Class E 240.x.x.x to 247.255.255.255
Table 1.3 Class networks
RFCs1518and1519defineasystemcalledClasslessInter-DomainRouting(CIDR),whichisusedtoallocateIPaddressesmoreefficiently.Thismaybeusedwithsubnetmaskstoestablishnetworksratherthantheclasssystemshown above. A class C subnet may be 8 bits but using CIDR, it may be 12 bits.
There are some network addresses reserved for private use by the Internet Assigned Numbers Authority (IANA) which can be hidden behind a computer which uses IP masquerading to connect the private network to the internet. There are three sets of addresses reserved. These addresses are shown below:
10.x.x.x•172.16.x.x - 172.31.x.x•192.168.x.x•
13/JNU OLE
Other reserved or commonly used addresses are:127.0.0.1:• The loopback interface address. All 127.x.x.x addresses are used by the loopback interface which copies data from the transmit buffer to the receive buffer of the NIC when used. This is reserved for hosts that don’tknowtheiraddressanduseBOOTPorDHCPprotocolstodeterminetheiraddresses.255:• The value of 255 is never used as an address for any part of the IP address. It is reserved for broadcast addressing. Please remember, this is exclusive of CIDR. When using CIDR, all bits of the address can never be all ones.
Below are the few examples of valid and invalid addresses:Valid addresses•
10.1.0.1 through 10.1.0.254 �10.0.0.1 through 10.0.0.254 �10.0.1.1 through 10.0.1.254 �
Invalid addresses• 10.1.0.0-HostIPcan’tbe0. �10.1.0.255-HostIPcan’tbe255. �10.123.255.4 - No network or subnet can have a value of 255. �0.12.16.89 - No Class A network can have an address of 0. �255.9.56.45 - No network address can be 255. �10.34.255.1 - No network address can be 255. �
1.4.2 Network/Netmask specificationSometimes you may see a network interface card (NIC) IP address specified in the following manner: 192.168.1.1/24
ThefirstpartindicatestheIPaddressoftheNICwhichis“192.168.1.1”inthiscase.Thesecondpart“/24”indicatesthenetmaskvaluemeaninginthiscasethatthefirst24bitsofthenetmaskareset.Thismakesthenetmaskvalue255.255.255.0.Ifthelastpartofthelineabovewere“/16”,thenetmaskwouldbe255.255.0.0.
Subnet masksSubnetting is the process of breaking down a main class A, B or C network into subnets for routing purposes. A subnet mask is the same basic thing as a netmask with the only real difference being that you are breaking a larger organisational network into smaller parts, and each smaller section will use a different set of address numbers. This will allow network packets to be routed between subnetworks.
When doing subnetting, the number of bits in the subnet mask determines the number of available subnets. Two to the power of the number of bits minus two is the number of available subnets. When setting up subnets the following must be determined:
Number of segments•Hosts per segment•
Subnetting provides the following advantages:Network traffic isolation• :Thereislessnetworktrafficoneachsubnet.Simplified administration:• Networks may be managed independently.Improved security:• Subnets can isolate internal networks so they are not visible from external networks.
Data Communications
14/JNU OLE
A 14 bit subnet mask on a class B network only allows 2 node addresses for WAN links. A routing algorithm like OSPF or EIGRP must be used for this approach. These protocols allow the variable length subnet masks (VLSM). RIPandIGRPdon’tsupportthis.
Subnet mask information must be transmitted on the update packets for dynamic routing protocols for this to work. The router subnet mask is different than the WAN interface subnet mask. One network ID is required by each of:
Subnet•WAN connection•
One host ID is required by each of:Each NIC on each host•Each router interface•
Types of subnet masks:Default• :ItfitsintoaClassA,B,orCnetworkcategory.Custom• : It is used to break a default network such as a Class A, B, or C network into subnets.
Fig. 1.6 Subnet mask(Source: http://t0.gstatic.com/images?q=tbn:ANd9GcQO467MynYXn7BVeMarpzUoqyzCaQAMLuirZPaw
z_mggSDFRQtDDw)
A
192.133.219.30/27
Host Address: 192.133.219.33Subnet mask: 255.255.255.224Default gateway: 192.133.219.30
15/JNU OLE
Subnetting an IP network is to separate a big network into smaller multiple networks for reorganisation and security purposes. All nodes (hosts) in a subnetwork see all packets transmitted by any node in a network. Performance of anetworkisadverselyaffectedunderheavytrafficloadduetocollisionsandretransmissions.ApplyingasubnetmasktoanIPaddressseparatesnetworkaddressfromhostaddress.Thenetworkbitsarerepresentedbythe1’sinthemask,andthehostbitsarerepresentedby0’s.PerformingabitwiselogicalANDoperationontheIPaddresswiththe subnet mask produces the network address. For example, applying the Class C subnet mask to our IP address 216.3.128.12 produces the following network address:
IP: 1101 1000. 0000 0011. 1000 0000. 0000 1100 (216.003.128.012)
Mask: 1111 1111. 1111 1111. 1111 1111. 0000 0000 (255.255.255.000) ----------------------------------------------------------------------------- 1101 1000. 0000 0011. 1000 0000. 0000 0000 (216.003.128.000)
Data Communications
16/JNU OLE
SummaryData communications concerns the transmission of digital messages to devices external to the message •source.The maximum permissible transmission rate of a message is directly proportional to signal power and inversely •proportional to channel noise.There are different types of physical medium that allows the communication between the network devices.•Protocols are the set of rules, agreed upon ways and standards that are used to make data transmission •process.Amodelisatheoreticaldescriptionofsomeaspectofthephysicaluniversethatidentifiesessentialcomponents•and is amenable to analysis.The OSI model is designed to guide the development of open systems so that they can communicate with each •other.Theapplicationlayerinvokesgenericapplicationsinsupportofdatageneratedbyspecificuserapplications.•The session layer directs the establishment, maintenance and termination of the connection.•The transport layer is the highest layer in the stack to be concerned with communication protocols.•TCP/IP (Transmission Control Protocol/Internet Protocol) is the basic communication language or protocol of •the Internet.Addressing level• refers to the level in the communications architecture at which an entity is named.Subnetting is the process of breaking down a main class A, B or C network into subnets for routing purposes.•Subnetting an IP network is to separate a big network into smaller multiple networks for reorganisation and •security purposes.
ReferencesWetteroth, D., 2002. • OSI reference model for telecommunications, McGraw-Hill.Kundu, S., 2005. Fundamentals of Computer Networks, 2nd ed., Prentice-Hall.•An Internet Encyclopedia, • An Internet Encyclopedia [Online] Available at: < http://www.freesoft.org/CIE/Topics/26.htm> [Accessed 24 August 2011].Toolbox.com, 1998. • TCP/IP |Protocols-l| Tcp-ip- [Online] (Updated 17 July 2008). Available at: <http://it.toolbox.com/wiki/index.php/Layers_in_TCP/IP_model> [Accessed 6 September 2011].VegasRage, 2008. • Subnetting in 6 easy steps - part 1 [Video Online] Available at: <http://www.youtube.com/watch?v=wl5_J0UtINg&feature=related> [Accessed at 6 September 2011].TrainSignalInc, 2007. • Train Signal Training: Intro to TCP/IP [Video Online] Available at: <http://www.youtube.com/watch?v=gJ5h4_0mllI&feature=related> [Accessed 6 September 2011].
Recommended ReadingWilliam, S., 2003.• Data and Computer Communications Pearson Education, 7th ed.Michael, D. and Richard, R., 2003. • Data Communications and Computer Networks: For Computer Scientists and Engineers. Prentice Hall.James, E.G. and Phillip, T. R., 2004. • Applied Data Communications: A Business-oriented Approach, John Wiley & Sons.
17/JNU OLE
Self AssessmentAll the computers, in the logical and physical networks have to follow the same rules known as __________.1.
data communicationa. protocolsb. data transferc. rate limitsd.
Which of the following statements is true?2. Every computer in a network should support the same protocols.a. Every computer in a network should support the different protocols.b. Every computer in a network should not support the protocols.c. Particular computer in a network should support the same protocols.d.
Whatisatheoreticaldescriptionofsomeaspectofthephysicaluniversethatidentifiesessentialcomponents3. and is amenable to analysis?
Dataa. Transmission methodb. Signalsc. Modeld.
______________ is required to agree upon the network(s) used to adjust data rates.4. Peer-to-peer communicationa. Client-Server communicationb. Data communicationc. Signald.
Which layer ofOSImodel invokes generic applications in support of data generated by specific user5. applications?
TCP/IPa. Data Linkb. Applicationc. Physicald.
The ______________layer conditions the APDU to compensate for differences in local data formats within the 6. sender and receiver.
presentationa. applicationb. data linkc. networkd.
Which of the following task is not performed by Session layer?7. Data transfera. Establish connectionb. Decodingc. Maintaining connectiond.
Data Communications
18/JNU OLE
The __________ provides communications services to the Transport layer.8. Data link layera. Physical layerb. Session layerc. Network layerd.
The___________________definesend-to-enddeliveryofpacketsanddefineslogicaladdressing.9. TCP/IP Internetwork Layera. TCP/IP Transport Layerb. TCP/IP Application Layerc. TCP/IP Network Interface Layerd.
Match the following10.
Network Type Description
A. Class A 1. For medium size networks
B. Class B 2. For very large networks
C. Class C 3. Used to support multicasting
D. Class D 4. For small networksA.3, B.2, C.1, D.4a. A.2, B.1, C.4, D.3b. A.1, B.3, C.2, D.4c. A.4, B.1, C.3, D.2d.
19/JNU OLE
Chapter II
Physical Layer
Aim
The aim of this chapter is to:
explicate the concept of data and signals•
explain analog and digital signals•
definetransmissionimpairments•
Objectives
The objectives of this chapter are to:
elucidate data rate limit•
explain data communication performance•
enlist transmission impairments•
Learning outcome
At the end of this chapter, you will be able to:
compare data with signals•
understand analog and digital signals•
calculate• the digital signals
Data Communications
20/JNU OLE
2.1 Data and Signals FundamentalData and signals are two of the basic building blocks of a computer network. A signal is the transmission of data while data comprises of information in raw or unorganised form. Both data and signals can be in either analog or digital form, which gives us four possible combinations: transmitting digital data using digital signals, transmitting digital data using analog signals, transmitting analog data using digital signals and transmitting analog data using analog signals.
Information that is stored within computer systems and transferred over a computer network can be divided into two categories: data and signals. Data are entities that convey meaning within a computer or computer system. If it is needed to transfer this data from one point to another, either by using a physical wire or by using radio waves, the data has to be converted into a signal. Signals are the electric or electromagnetic encoding of data and are used to transmit data.
Data Data refers to information that conveys some meaning based on some mutually agreed upon rules or conventions between a sender and a receiver and today it comes in variety of forms such as text, graphics, audio, video and animation. Data can be of two types; analog and digital. Analog data take on continuous values at some intervals.
Typical examples of analog data are voice and video. The data that are collected from the real world with the help of transducers are continuous-valued or analog in nature. On the contrary, digital data take on discrete values. Text or character strings can be considered as examples of digital data. Characters are represented by suitable codes, for example, ASCII code, where each character is represented by a 7-bit code.
SignalsIt is an electrical, electronic or optical representation of data, which can be sent over a communication medium. A signal is merely a function of the data. For example, a microphone converts voice data into voice signal, which can be sent over a pair of wire.
Analog signals are continuous-valued; digital signals are discrete-valued. The independent variable of the signal could be time (speech, for example), space (images), or the integers (denoting the sequencing of letters and numbers in the football score).
2.1.1 Analog and Digital SignalsLikethedatatheyrepresent,signalscanbeeitheranalogordigital.Ananalogsignalhasinfinitelymanylevelsof intensity over a period of time. As the wave moves from value A to value B, it passes through and includes aninfinitenumberofvaluesalongitspath.Adigitalsignal,ontheotherhand,canhaveonlyalimitednumberofdefinedvalues.Althougheachvaluecanbeofanynumber,itisoftenassimpleas1 and O. The simplest way to show signals is by plotting them on a pair of perpendicular axes. The vertical axis represents the value or strength of a signal. The horizontal axis represents time.
Value
Value
a. Analog Signal
Time Time
b. Digital Signal
Fig. 2.1 Comparison between analog and digital signals(Source: http://t2.gstatic.com/images?q=tbn:ANd9GcTok9FOVldOeBCEAFZWwargJrLV5etR9H-
Jb8IKwMM7JrYrFPYk)
21/JNU OLE
Analog signalsAnalog signals are a representation of time varying quantities in a continuous signal. Basically, a time variance is presented in a manner where some sort of information is passed using various types of methods. These methods can include electrical, mechanical, hydraulic or pneumatic systems. Unlike digital signals, which use a numeric method oftransmittinginformation,analogsignalsusesmallfluctuationsinthesignalitselftopassinformation.
Analog signals act essentially like simulations of a continuous time varying quantity. They duplicate the features of the actual quantity by presenting a different quantity. In other words, they use one method of recording information and transfer it to a different format that, in turn, presents the information in that medium.
Eachanalogsignalusesapropertyofthefinalmediumtoconveytheinformationforthesignal.Forexample,athermometer will utilise the heat of a particular object to determine its temperature. The heat is then transferred to mercury, which changes its position to display the temperature information on the gauge.
An analog signal can be any time-varying signal.•Minimum and maximum values can be either positive or negative.•They can be periodic (repeating) or non-periodic.•Sine waves and square waves are two common analog signals.•
Inte
nsity
Analog Signal
Time, Space...
Fig. 2.2 Analog signals(Source: http://t0.gstatic.com/
images?q=tbn:ANd9GcTnubMgIxBQzqBLMTCoX6qgJaSUTxMEA93EJAnC1BvSCCTJNbDy)
Digital signalsIn addition to being represented by an analog signal, information can also be represented by a digital signal. For example, it can be encoded as a positive voltage and a 0 as zero voltage. A digital signal can have more than two levels. In this case, we can send more than 1 bit for each level. Digital representation and processing of (analog) information seems extremely complex compared to analog representation.
Each analog quantity must be encoded according to a code scheme to be then described by several binary signals.
Data Communications
22/JNU OLE
Intensity
time
Abrupt amplitude variations
1 0 1 0 1
Fig. 2.3 (a) Digital signals(Source: http://t0.gstatic.com/images?q=tbn:ANd9GcR48PJkGtgulh0kAdU4d2PvoJIz4aE_Cazqgo3T_
dQUq2aFbL26uA)
Digitalsignalcanhaveonlyalimitednumberofdefinedvalues,usuallytwovalues0and1asshowninthefigure2.3 (a). Figure 2.3(b) shows two signals, one with two levels and the other with four. We send one bit per level in part‘a’ofthefigureand2bitsperlevelinpartbofthefigure.Ingeneral,ifasignalhasLlevels,eachlevelneedslog2L bits.
Where,Bit rate: The number of bits sent in 1s, expressed in bits per second (bps).Bit length: The distance one bit occupies on the transmission medium.Bit length = propagation speed x bit duration
Digital signal are commonly referred to as square waves or clock signals.•Their minimum value must be 0 volts, and their maximum value must be 5 volts.•They can be periodic (repeating) or non-periodic.•The time the signal is high (t• H) can vary anywhere from 1% of the period to 99% of the period.
23/JNU OLE
Level 1
Level 2
Level 3
Level 4
1 0 1 1 0 0 0 1
11 10 01 01 00 00 00 10
8 bits sent in 1s Bit rate = 8 bps
16 bits sent in 1s Bit rate = 16 bps
Amplitude
Amplitude
Level 2
Level 1
a. A digital signal with two levels
b. A digital signal with four levels
Time
Time
1 s
1 s
Fig. 2.3 (b) Two digital signals: One with two signal levels and the other with four signal levels(Source: http://train-srv.manipalu.com/wpress/wp-content/uploads/2010/01/clip-image022-thumb13.jpg)
Following are the parts of digital signals:Amplitude:• For digital signals, this will ALWAYS be 5 volts.Period:• The time it takes for a periodic signal to repeat. (Seconds) Frequency:• A measure of the number of occurrences of the signal per second. (Hertz, Hz) Time high (t• H): The time the signal is at 5 v.Time low (t• L): The time the signal is at 0 v.Duty cycle:• The ratio of tH to the total period (T).Rising edge:• A 0-to-1 transition of the signal.Falling edge:• A 1-to-0 transition of the signal.
Digital versus analog can refer to method of input, data storage and transfer, the internal working of an instrument, and the kind of display. The word comes from the same source as the word digit and digitus.
The digital technology breaks your voice (or television) signal into binary code a series of 1s and 0s transfers it to the other end where another device (phone, modem or TV) takes all the numbers and reassembles them into the original signal. The beauty of digital is that it knows what it should be when it reaches the end of the transmission. That way, it can correct any errors that may have occurred in the data transfer.
The nature of digital technology allows it to cram lots of those 1s and 0s together into the same space an analog signal uses. Digital offers better clarity but analog gives you richer quality. Digital like the VCR or the CD is coming down in cost and coming out in everything from cell phones to satellite dishes.
Data Communications
24/JNU OLE
For example,A digital signal has eight levels. How many bits are needed per level? We calculate the number of bits from the formula
Number of bits per level =log2 8 =3Each signal level is represented by 3 bits.
2.1.2 Transmission ImpairmentsAnalog signal consist of varying voltage with time to represent an information stream. If the transmission media were perfect, the receiver could receive exactly the same signal that the transmitter sent. But communication lines are usually not perfect so the receive signal is not the same as the transmitted signal. For digital data this difference can lead to errors. Transmission lines suffer from three major problems, namely attenuation, delay distortion and noise.
Attenuation It is the loss of energy as the signal propagates outward. The amount of energy depends on the frequency. If the attenuation is too much, the receiver may not be able to detect the signal at all or the signal may fall below the noise level. For reliable communication, the attenuation and delay over the range of frequencies of transmission should be constant.
6
4
2
0
-2
-4
6
4
2
0
-2
-4
6
4
2
0
-2
-4
6
4
2
0
-2
-4
0 1*10-6 2*10-6 3*10-6 4*10-6 5*10-6 0 1*10-6 2*10-6 3*10-6 4*10-6 5*10-6
0 1*10-6 2*10-6 3*10-6 4*10-6 5*10-6 0 1*10-6 2*10-6 3*10-6 4*10-6 5*10-6
Fig. 2.4 Attenuation
The following issues need consideration:Signalsmustbesufficientlystrongsothatthereceiverwillbeabletodetectandinterpretthem.•Theyshouldmaintainasufficienthighleveltomakethemdistinguishablefromnoise.•Too strong signals can overload the circuitry of the transmitter and result in distortion.•They should take into account that attenuation increases with the frequency.•
When sound travels through a medium, its intensity diminishes with distance. In idealised materials, sound pressure (signal amplitude) is only reduced by the spreading of the wave. Natural materials, however, all produce an effect which further weakens the sound. This further weakening results from scattering and absorption. Scattering is the reflectionofthesoundindirectionsotherthanitsoriginaldirectionofpropagation.Absorptionistheconversionofthe sound energy to other forms of energy. The combined effect of scattering and absorption is called attenuation. Ultrasonic attenuation is the decay rate of the wave as it propagates through material.
25/JNU OLE
Attenuation of sound within a material itself is often not of intrinsic interest. However, natural properties and loading conditions can be related to attenuation. Attenuation often serves as a measurement tool that leads to the formation of theories to explain physical or chemical phenomenon that decreases the ultrasonic intensity.
The amplitude change of a decaying plane wave can be expressed as:
In this expression A0 is the unattenuated amplitude of the propagating wave at some location.
The amplitude A is the reduced amplitude after the wave has travelled a distance z from that initial location. The quantityαistheattenuationcoefficientofthewavetravellinginthez-direction.Thedimensionsofαisnepers/length, where a neper is a dimensionless quantity. The term e istheexponential(orNapier’sconstant)whichisequal to approximately 2.71828.
The units of the attenuation value in Nepers per meter (Np/m) can be converted to decibels/length by dividing by 0.1151. Decibels are a more common unit when relating the amplitudes of two signals.
Attenuation is generally proportional to the square of sound frequency. Quoted values of attenuation are often given for a single frequency or an attenuation value averaged over many frequencies may be given. Also, the actual valueoftheattenuationcoefficientforagivenmaterialishighlydependentonthewayinwhichthematerialwasmanufactured. Thus, quoted values of attenuation only give a rough indication of the attenuation and should not be automatically trusted. Generally, a reliable value of attenuation can only be obtained by determining the attenuation experimentally for the particular material being used.
AttenuationcanbedeterminedbyevaluatingthemultiplebackwallreflectionsseeninatypicalA-scandisplaylikethe one shown in the image at the top of the page. The number of decibels between two adjacent signals is measured andthisvalueisdividedbythetimeintervalbetweenthem.Thiscalculationproducesanattenuationcoefficientindecibels per unit time Ut. This value can be converted to nepers/length by the following equation.
Where, v is the velocity of sound in meters per second and Ut is in decibels per second.
DistortionThe second transmission impairment is delay distortion. Communication lines have distributed inductance and capacitance which distort the amplitude of signals and also delay the signals at different frequencies by different amounts. It is caused by the fact that different Fourier components travel at different speed.
For digital data, fast components from one bit may catch up and over take slow component from bit ahead, mixing the two bits and increasing the probability of incorrect reception.
Distortion is caused by the fact that the signals of varying frequencies travel at different speeds along the medium. Any complex signal can be decomposed into different sinusoidal signals of different frequencies, resulting, in a frequency bandwidth for every signal.
One property of signal, propagation is that the speed of travel of the frequency is the highest at the centre of this bandwidth, and lowest at both ends. Therefore, at the receiving end, signals with different frequencies in a given bandwidthwillarriveatdifferenttimes.If,thesignalsreceivedaremeasuredataspecifictime,theywillnotbeexactly like the original signal resulting in its misinterpretation.
Data Communications
26/JNU OLE
For digital data, fast components from one bit may catch up and overtake low components from the bit ahead, mixing the two bits and increasing the probability of incorrect reception.
+
-
+
-
Input signal
Resultant Output Signal
Second Harmonic
ωt 0 ωt
Fig. 2.5 Distortion(Source:http://www.electronics-tutorials.ws/amplifier/amp36.gif)
NoiseNoiseisathirdimpairment.Itcanbedefinedasanunwantedenergyfromsourcesotherthanthetransmitter.Thermalnoise is caused by the random motion of the electrons in a wire and is unavoidable. As signal is transmitted through a channel, undesired signal in the form of noise gets mixed up with the signal along with the distortion introduced by the transmission media.
A
B
Fig. 2.6 Noise(Source: http://t3.gstatic.com/images?q=tbn:ANd9GcQw6-x-OnQ3osbHyALEnsDyXGfDu3ehP5peg98FRWipjB
dq5Nna7w)
Noise can be categorised into the following four types:Thermal noise:• The thermal noise is due to thermal agitation of electrons in a conductor. It is distributed across the entire spectrum and that is why it is also known as white noise. When more than one signal, signals share a single transmission medium, Intermodulation noise is generated. For example, two signals f1 and f2 will generate signals of frequencies (f1 + f2) and (f1 - f2), which may interfere with the signals of the same frequencies sent by the transmitter.Intermodulation noise: • Intermodulation noise is introduced due to nonlinearity present in any part of the communication system.Cross talk• : Similarly cross talk is a noise that is caused by the inductive coupling between two wires that are closed to each other. Sometime when talking on the telephone, you can hear another conversation in the background. This type of disturbance is called cross talk. Types of cross talk are:
27/JNU OLE
ICR
IEEE spec
60
50
40
30
20
10
00 2 4
Frequency (GHz) Time (ns)
110
100
105
95
900 0.2 0.4 0.6 0.8 1
Z (O
hms)
ICR
(dB
)
6 8 10
Inner row
Fig. 2.7 Crosstalk graph
(Source: http://t0.gstatic.com/images?q=tbn:ANd9GcSz8bgPkZr-dv72olj359BN4GQn8QSNlbNuEj2WKpHOM8SOiaRE)
Insertion Loss-to-Crosstalk-Ratio (ICR): � Theinsertionloss-to-crosstalkratio(ICR)withfive-aggressordifferentialFEXTmeetstheextrapolatedIEEE802.3apspecificationfor10+Gbpswithplentyofmargins.Differential impedance: � The differential impedance is within 100±10 ohms at 35 ps rise time ( 20% to 80%).Impulse noise: � Extra interference with the data transmission is called impulse noise.
2.1.3 Data Rate LimitsA very important consideration in data communications is how fast we can send data, in bits per second over a channel. Data rate depends on three factors:
the bandwidth available•the level of the signals we use•the quality of the channel (the level of noise)•
Two theoretical formulas were developed to calculate the data rate; one by Nyquist for a noiseless channels another by Shannon for a noisy channel.
Noiseless Channel: Nyquist bit rate
Foranoiselesschannel,theNyquistbitrateformuladefinesthetheoreticalmaximumbitrateBit Rate = 2 x bandwidth x 10g2 L
In this formula, bandwidth is the bandwidth of the channel, L is the number of signal levels used to represent data, and Bit Rate is the bit rate in bits per second.
Accordingtotheformula,wemightthinkthat,givenaspecificbandwidth,wecanhaveanybitratewewantbyincreasing the number of signa11eve s. Although the idea is theoretically correct, practically there is a limit. When we increase the number of signal1eve1s, we impose a burden on the receiver. If the number of levels in a signal is just 2, the receiver can easily distinguish between a 0 and a 1. If the level of a signal is 64, the receiver must be very sophisticated to distinguish between 64 different levels. In other words, increasing the levels of a signal reduces the reliability of the system.
For example, consider a noiseless channel with a bandwidth of 3000 Hz transmitting a signal with two signal levels. The maximum bit rate can be calculated as:
Data Communications
28/JNU OLE
For example, we need to send 265 kbps over a noiseless channel with a bandwidth of 20 kHz. How many signal levels do we need?
We can use the Nyquist formula as shown:
Since this result is not a power of 2, we need to either increase the number of levels or reduce the bit rate. If we have 128 levels, the bit rate is 280 kbps. If we have 64 levels, the bit rate is 240 kbps.
Consider another example, an extremely noisy channel in which the value of the signal-to-noise ratio is almost zero. In other words, the noise is so strong that the signal is faint. For this channel the capacity C is calculated as
This means that the capacity of this channel is zero regardless of the bandwidth. In other words, we cannot receive any data through this channel.
For example, we can calculate the theoretical highest bit rate of a regular telephone line. A telephone line normally has a bandwidth of 3000. The signal-to-noise ratio is usually 3162. For this channel the capacity is calculated as
This means that the highest bit rate for a telephone line is 34.860 kbps. If we want to send data faster than this, we can either increase the bandwidth of the line or improve the signal-to-noise ratio.
2.1.4 PerformanceThe performance here deals with the terms that are required for data communications.
BandwidthThe term bandwidth has a number of technical meanings but since the popularisation of the Internet, it has generally referred to the volume of information per unit of time that a transmission medium can handle. Bandwidth refers to how much data you can send through a network or modem connection. It is usually measured in bits per second, or“bps.”
29/JNU OLE
Suppose a bandwidth can be considered as a highway with cars travelling on it. The highway is the network connection and the cars are the data. The wider the highway, the more cars can travel on it at one time. Therefore, more cars can get to their destinations faster. The same principle applies to computer data - the more bandwidth, the more information that can be transferred within a given amount of time.
Power
Bandwidth (Hz)
Frequency (Hz)
10 100 1k 10k 100k 1M
Fig. 2.8 Bandwidth(Source: http://t2.gstatic.com/images?q=tbn:ANd9GcTAl0HE86z-8D2FItiec_
Og9pThVom2AyCTAeYvVWsKoNRufX0H)
It is important to realise that the bandwidth is expressed in terms of power. Recall that power can be expressed in termsofvoltageandcurrent,specificallyP=V2/R.Therefore,wecanalsofindadefinitionofbandwidthintermsof voltage. The power is proportional to V2 and so P/2 is proportional to ( V2 )/2 which implies that half power is achieved when the output voltage is reduced to V/(sqrt(2)) = 0.707 x V
Thebandwidthistherangeoffrequenciesoverwhichtheamplifiercircuitproducesatleast0.707ofitsratedoutputvoltage. Simultaneously, where the voltage gain will be greater than 0.707 of the expected gain. Bandwidth is usually shown on a graph of frequency against power with either or one or both axes being logarithmic.
Bandwidth in hertz• : Bandwidth in hertz is the range of frequencies contained in a composite signal or the range of frequencies a channel can pass. For example, we can say the bandwidth of a subscriber telephone line is 4 kHz.Bandwidth in bits per seconds: • The term bandwidth can also refer to the number of bits per second that a channel, a link or even a network can transmit. For example, one can say the bandwidth of a Fast Ethernet network (or the links in this network) is a maximum of 100 Mbps. This means that this network can send 100 Mbps.Relationship• : There is an explicit relationship between the bandwidth in hertz and bandwidth in bits per seconds. Basically, an increase in bandwidth in hertz means an increase in bandwidth in bits per second. The relationship depends on whether we have baseband transmission or transmission with modulation.
Latency (Delay) Thelatencyordelaydefineshowlongittakesforanentiremessagetocompletelyarriveatthedestinationfromthetimethefirstbitissentoutfromthesource.Wecansaythatlatencyismadeoffourcomponents:propagationtime, transmission time, queuing time and processing delay.
Latency =propagation time +transmission time +queuing time + processing delay
Data Communications
30/JNU OLE
Propagation timePropagation time measures the time required for a bit to travel from the source to the destination. The propagation time is calculated by dividing the distance by the propagation speed.
Propagation time =Distance /Propagation speed
The propagation speed of electromagnetic signals depends on the medium and on the frequency of the signal. For example, in a vacuum, light is propagated with a speed of 3 x 108 mfs. It is lower in air; it is much lower in cable.
Transmission timeIndatacommunications,not just1bit issendbut thewholemessage.Thefirstbitmay take timeequal to thepropagation time to reach its destination; the last bit also may take the same amount of time. However, there is a timebetweenthefirstbitleavingthesenderandthelastbitarrivingatthereceiver.Thefirstbitleavesearlierandarrives earlier; the last bit leaves later and arrives later. The time required for transmission of a message depends on the size of the message and the bandwidth of the channel.
Transmission time =Message size/Bandwidth
Queuing timeThe time needed for each intermediate or end device to hold the message before it can be processed. The queuing time isnotafixedfactor;itchangeswiththeloadimposedonthenetwork.Whenthereisheavytrafficonthenetwork,the queuing time increases. An intermediate device, such as a router, queues the arrived messages and processes them one by one. If there are many messages, each message will have to wait.
31/JNU OLE
SummaryData and signals are two of the basic building blocks of a computer network. •A signal is the transmission of data while data comprises of information in raw or unorganised form.•Data are entities that convey meaning within a computer or computer system.•Signals are the electric or electromagnetic encoding of data and are used to transmit data.•Analog data take on continuous values at some intervals. •Adigitalsignalcanonlyhavealimitednumberofdefinedvalues.•Eachanalogsignalusesapropertyofthefinalmediumtoconveytheinformationforthesignal.•A digital signal can have more than two levels.•Digital representation and processing of (analog) information seems extremely complex compared to analog •representation.Digital signal are commonly referred to as square waves or clock signals•Digital versus analog can refer to method of input, data storage and transfer, the internal working of an instrument •and the kind of display.The digital technology breaks your voice signal into binary code as a series of 1s and 0s and transfers it to the •other end where another device takes all the numbers and reassembles them into the original signal.But communication lines are usually not perfect, so the receive signal is not the same as the transmitted •signal.Transmission lines suffer from three major problems, namely attenuation, delay distortion and noise.•Attenuation is the loss of energy as the signal propagates outward.•For reliable communication, the attenuation and delay over the range of frequencies of transmission should be •constant.Too strong signals can overload the circuitry of the transmitter and result in distortion.•Ultrasonic attenuation is the decay rate of the wave as it propagates through material.•Attenuation of sound within a material itself is often not of intrinsic interest.•The units of the attenuation is Nepers per meter•Attenuation is generally proportional to the square of sound frequency.•Communication lines have distributed inductance and capacitance which distort the amplitude of signals and •also delay the signals at different frequencies by different amounts.For digital data, fast components from one bit may catch up and over take slow component from bit ahead, •mixing the two bits and increasing the probability of incorrect reception.Distortion is caused by the fact that the signals of varying frequencies travel at different speeds along the •medium.The thermal noise is due to thermal agitation of electrons in a conductor.•Intermodulation noise is introduced due to nonlinearity present in any part of the communication system.•For digital data, fast components from one bit may catch up and overtake low components from the bit ahead, •mixing the two bits and increasing the probability of incorrect reception.Cross talk is a noise that is caused by the inductive coupling between two wires that are closed to each other.•Bandwidth refers to how much data you can send through a network or modem connection.•Thelatencyordelaydefineshowlongittakesforanentiremessagetocompletelyarriveatthedestinationfrom•thetimethefirstbitissentoutfromthesource.Propagation time measures the time required for a bit to travel from the source to the destination.•
Data Communications
32/JNU OLE
ReferencesAhmad, A., 2003. • Data communication principles for fixed and wireless networks, Springer.Irvine, J. Harle, D., 2002. • Data communications and networks: an engineering approach, John Wiley and SonsFundamentals of Data and Signals• [pdf] Available at: <http://facstaff.swu.ac.th/watcharachai/course/ee462-chap2-notes.pdf> [Accessed 14 September 2011].Sudha, M., 2010. • Data and Signals [Online] Available at:< http://www.scribd.com/doc/26493680/Data-and-Signals> [Accessed 14 September 2011].Prof. Pal, A., • Transmission Impairments and Channel Capacity [Video Online] Available at: < http://freevideolectures.com/Course/2278/Data-Communication/4> [Accessed 15 September 2011].Prof. Pal, A., • Transmission Impairments and Channel Capacity [Video Online] Available at: < http://freevideolectures.com/Course/2278/Data-Communication/3>. [Accessed 15 September 2011].
Recommended ReadingSharma, R., 2008. • Data & Computer Communication, Laxmi Publications, Ltd.Smith, W. S., 2003. • Digital signal processing: a practical guide for engineers and scientists, Newnes.Schweber, L. W., 2009. • Data Communications, Tata McGraw-Hill Education.
33/JNU OLE
Self Assessment______________ can be in either analog or digital form.1.
Signalsa. Databaseb. ASCII codec. Character stringd.
Which of the following statements is true?2. Digital signals are a representation of time varying quantities in a continuous signal.a. Analog signals are a representation of time varying quantities in a continuous signal.b. Unwanted signals are a representation of time varying quantities in a continuous signal.c. Triangle signals are a representation of time varying quantities in a continuous signal.d.
Analog signal act as _________for representing continuous time varying quantity.3. dataa. simulationb. digitalc. signald.
Which of the following is the common form of analog signals?4. Triangle wavesa. Sawtooth waveb. Pulse wavec. Sine waved.
____________ representation and processing of (analog) information seems extremely complex.5. Periodic signalsa. Electric signalsb. Analog signalsc. Electromagnetic signalsd.
___________canhaveonlyalimitednumberofdefinedvalues.6. Digital signala. Analog Signalsb. Periodic Signalsc. Electromagnetic Signalsd.
Which element can be measured in terms of bits per second?7. Bit Lengtha. Latencyb. Bit Ratec. Speedd.
Data Communications
34/JNU OLE
The combined effect of scattering and absorption is called8. ______________.noisea. attenuationb. distortionc. cross talkd.
Which type of transmission impairment is caused by the inductive coupling between two wires that are closed 9. to each other?
Cross talka. Noiseb. Attenuationc. Distortiond.
Whatisknownas“thevolumeofinformationperunitoftimethatatransmissionmediumcanhandle”?10. Bit Lengtha. Bit Rateb. Propagation timec. Bandwidthd.
35/JNU OLE
Chapter III
Data Transmission
Aim
The aim of this chapter is to:
explicate the data encoding techniques•
explain scrambling techniques•
definepulsecodemodulation•
Objectives
The objectives of this chapter are to:
elucidate line coding schemes•
explain unipolar scheme for encoding•
enlist PCM encoder process•
Learning outcome
At the end of this chapter, you will be able to:
compare modulator with demodulator•
understand serial and parallel transmission•
explain characteristics of serial tra• nsmission
Data Communications
36/JNU OLE
3.1 Digital Encoding TechniquesThe basis for analog signalling is a continuous, constant-frequency signal known as the carrier signal. The frequency of the carrier signal is chosen to be compatible with the transmission medium being used. Data may be transmitted using a carrier signal by modulation. Modulation is the process of encoding source data onto a carrier signal with frequency fc. All modulation technique involves operation on one or more of the three fundamental domain parameters:
Amplitude •Frequency•Phase•
The input signal m (t) may be analog or digital and is called the modulating signals, or baseband signal. The result of modulating the carrier signal is called the modulated signal s(t).
t t
Original signal Encoded signal
Fig. 3.1 (a) Encoding a digital signals
Carrier
Modulating Wave
Modulated Result
Fig. 3.1 (b) Modulating an analog signal(Source: http://t3.gstatic.com/images?q=tbn:ANd9GcQI6oSRRLI68W0t4im8XfmTxhUWTdQPwp5JlSHSMn4J
MTCS1Ljq_A)
Following are the digital encoding techniques:
37/JNU OLE
3.1.1 Digital - to – DigitalIn this we can represent digital data by using digital signals. The conversion involves three techniques: line coding, block coding and scrambling.
Line coding Line coding is the process of converting digital data to digital signals. We assume that data, in the form of text, numbers, graphical images, audio or video are stored in computer memory as sequences of bits.
Sender
Digital data Digital dataDigital signal
•••
LinkEncoder Decoder
10 101” 101 1 10101••’101
Receiver
Fig. 3.2 Line coding and decoding
Signal element versus data element Let us distinguish between a data element and a signal element. In data communications, our goal is to send data elements. A data element is the smallest entity that can represent a piece of information: this is the bit. In digital data communications, a signal element carries data elements. A signal element is the shortest unit (time wise) of a digital signal.
In other words, data elements are what we need to send; signal elements are what we can send. Data elements are beingcarried;signalelementsarethecarriers.Wedefinearatior which is the number of data elements carried by each signal element.
Thefigurebelowshowsseveralsituationswithdifferentvaluesofr element (r =! /2).
Data Communications
38/JNU OLE
I data element I data element
1signal element
a. One data element per one signal element (r = 1)
c. Two data elements per one signal element (r=2)
d. Four data elements per three signal elements (r= 4/3)
1 signal element
01
2 data elements 4 data elements
1101
3 signal element
b. One data element per two signal elements (r=½)
2 signal element
Fig. 3.3 Signal element versus data element
Inpartcofthefigure,asignalelementcarriestwodataelements(r = 2). Finally, in part d, a group of 4 bits is being carried by a group of three signal elements (r = ). For every line coding scheme we discuss, we will give the value of r. suppose each data element is a person who needs to be carried from one place to another.
We can think of a signal element as a vehicle that can carry people. When r = 1, it means each person is driving a vehicle. When r > 1, it means more than one person is travelling in a vehicle (a carpool, for example). We can also have the case where one person is driving a car and a trailer (r = ).
Data rate versus signal rate Thedataratedefinesthenumberofdataelements(bits)sentin1s.Theunitisbitspersecond(bps).Thesignalrateis the number of signal elements sent in 1s. The unit is the baud. There are several common terminologies used in the literature. The data rate is sometimes called the bit rate; the signal rate is sometimes called the pulse rate, the modulation rate or the baud rate. One goal in data communications is to increase the data rate while decreasing the signal rate. Increasing the data rate increases the speed of transmission; decreasing the signal rate decreases the bandwidth requirement.
We now need to consider the relationship between data rate and signal rate (bit rate and baud rate). This relationship, of course, depends on the value of r. It also depends on the data pattern. If we have a data pattern of all 1s or all 0s, the signal rate may be different from a data pattern of alternating 0s and 1s. To derive a formula for the relationship, weneedtodefinethreecases:theworst,bestandaverage.Theworstcaseiswhenweneedthemaximumsignalrate; the best case is when we need the minimum.In data communications, we are usually interested in the average case. We can formulate the relationship between data rate and signal rate as
39/JNU OLE
Where N is the data rate (bps); c is the case factor, which varies for each case; S is the number of signal elements; andristhedefinedfactor.
Line coding schemesWecanroughlydividelinecodingschemesintofivebroadcategories,asshowninthefigurebelow.
Line coding
Unipolar NRZ
NRZ,RZ, and biphase (Manchester. and differential Manchester)
AMI and Pseudoternary
2B/IQ, SB/6T and 4U-PAM5
MLT-3
Polar
Bipolar
Multilevel
Multitransition
Fig. 3.4 Line coding schemes
There are several schemes in each category.
Unipolar scheme:• In a unipolar scheme, all the signal levels are on one side of the time axis, either above or below. NRZ (Non-Return-to-Zero), traditionally, a unipolar scheme was designed as a non-return-to-zero (NRZ) schemeinwhichthepositivevoltagedefinesbitIandthezerovoltagedefinesbitO.It is called NRZ because the signal does not return to zero at the middle of the bit. Figure below shows a unipolar NRZ scheme.
V1
Amplitude
Time Normalized power
½V2+½(0)2= ½V2
1 10 0
O
Fig. 3.5 Unipolar NRZ scheme
Polar schemes: • In polar schemes, the voltages are on both sides of the time axis. For example, the voltage level for 0 can be positive and the voltage level for 1 can be negative.Non-Return-to-Zero (NRZ• ): In polar NRZ encoding, we use two levels of voltage amplitude. We can have twoversionsofpolarNRZ:NRZ-LandNRZ-I,asshowninthefigurebelow.Thefigurealsoshowsthevalueof r, the average baud rate, and the bandwidth.
Inthefirstvariation,NRZ-L(NRZ-Level),thelevelofthevoltagedeterminesthevalueofthebit.Inthesecondvariation, NRZ-I (NRZ-Invert), the change or lack of change in the level of the voltage determines the value of the bit. If there is no change, the bit is 0; if there is a change, the bit is 1.
Data Communications
40/JNU OLE
O 1 1 10 0 0
Time
TimeNRZ-I
NRZ-L
ONoinversion:Nextbitis0 •InversionNextbitis1
Bandwidth
r = 1
P
1
0.5
00 I 2 f/N
Save = N/2
Fig. 3.6 Polar NRZ-L and NRZ-I schemes
3.1.2 Analog – to – DigitalIn this we can convert digital data to digital signals. Sometimes, however, we have an analog signal such as one created by a microphone or camera. A digital signal is superior to an analog signal. The tendency today is to change an analog signal to digital data.
3.1.3 Digital – to – AnalogDigital-to-analog conversion is the process of changing one of the characteristics of an analog signal based on the information in digital data. Figure 3.6 shows the relationship between the digital information, the digital-to-analog modulating process, and the resultant analog signal.
Sender
Analog signal
LinkModulator Demodulator
0101•••101 10 1 0 1 ... 1 0 1 1
Digital data Digital data
Receiver
Fig. 3.7 Digital-to-analog conversion
Asinewaveisdefinedbythreecharacteristics:amplitude,frequency,andphase.Whenwevaryanyoneofthesecharacteristics, we create a different version of that wave. So, by changing one characteristic of a simple electric signal, we can use it to represent digital data. Any of the three characteristics can be altered in this way, giving us at least three mechanisms for modulating digital data into an analog signal: amplitude shift keying (ASK), frequency shift keying (FSK), and phase shift keying (PSK).
In addition, there is a fourth (and better) mechanism that combines changing both the amplitude and phase, called quadratureamplitudemodulation(QAM).QAMisthemostefficientoftheseoptionsandisthemechanismcommonlyused today.
3.1.4 Analog – to –AnalogAnalog-to-analog conversion or analog modulation is the representation of analog information by an analog signal. Modulation is needed if the medium is band pass in nature or if only a bandpass channel is available to us. An example is radio. The government assigns a narrow bandwidth to each radio station. The analog signal produced by each station is a low-pass signal, all in the same range. To be able to listen to different stations, the low-pass signals need to be shifted, each to a different range. Analog-to-analog conversion can be accomplished in three ways: amplitude modulation (AM), frequency modulation (FM) and phase modulation (PM). FM and PM are usually categorised together.
41/JNU OLE
Analog-lo-anlog conversion
Analog-to-anlog conversion
Analog-lo-anlog conversion
Amplitude modulation Analog-lo-anlog conversion
Frequency modulation Analog-lo-anlog conversion
Phase modulation
Fig. 3.8 Types of analog-to-analog modulation
3.2 Scrambling TechniquesBi-phase schemes that are suitable for dedicated links between stations in a LAN are not suitable for long-distance communication because of their wide bandwidth requirement. The combination of block coding and NRZ line coding is not suitable for long-distance encoding either, because of the DC component. Bipolar AMI encoding, on the other hand, has a narrow bandwidth and does not create a DC component.
However,alongsequenceof0supsetsthesynchronisation.IfwecanfindawaytoavoidalongsequenceofOsinthe original stream, we can use bipolar AMI for long distances. We are looking for a technique that does not increase the number of bits and does provide synchronisation.
We are looking for a solution that substitutes long zero-level pulses with a combination of other levels to provide synchronisation. One solution is called scrambling. We modify part of the AMI rule to include scrambling, as shown inthefigurebelow.
Sender
Violated digital signals
ModifiedAMIencoding
ModifiedAMIencoding
Receiver
Fig. 3.9 AMI used with scrambling
Note that scrambling, as opposed to block coding, is done at the same time as encoding. The system needs to insert the requiredpulsesbasedonthedefinedscramblingrules.TwocommonscramblingtechniquesareB8ZSandHDB3.
R8ZS:• Bipolar with S-zero substitution (BSZS) is commonly used in North America. In this technique, eight consecutive zero-level voltages are replaced by the sequence OOOVBOVB. The V in the sequence denotes violation; this is a nonzero voltage that breaks an AMI rule of encoding (opposite polarity from the previous). The B in the sequence denotes bipolm; which means a nonzero level voltage in accordance with the AMI rule. Therearetwocases,asshowninthefigurebelow.
Data Communications
42/JNU OLE
1 0 0 0 0 0 0 0 0
V
V
0
a. Previous level is positive b. Previous level is negative
0 0 0
B
B
10
I
10 101 0
0 0 0 OOO 0 0
B
B
V
V
1
Fig. 3.10 Two cases of B8ZS scrambling technique
Note that the scrambling in this case does not change the bit rate. Also, the technique balances the positive and negative voltage levels (two positives and two negatives), which means that the DC balance is maintained. Note that the substitution may change the polarity of a 1 because, after the substitution, AMI needs to follow its rules. The letter V (violation) or B (bipolar) here is relative. The V means the same polarity as the polarity of the previous nonzero pulse; B means the polarity opposite to the polarity of the previous nonzero pulse.
HDB3:• High-density bipolar 3-zero (HDB3) is commonly used outside of North America. In this technique, which is more conservative than B8ZS, four consecutive zero-level voltages are replaced with a sequence of 000V or BOOV. The reason for two different substitutions is to maintain the even number of nonzero pulses after each substitution. The two rules can be stated as follows:
If the number of nonzero pulses after the last substitution is odd, the substitution pattern will be 000V, which �makes the total number of nonzero pulses even.If � the number of nonzero pulses after the last substitution is even, the substitution pattern will be B00V, which makes the total number of nonzero pulses even.
1 1 0 0 0 0 1 0 0 0 0 0 0 0 0 0
First substitution
Second substitution
Third substitution
B0 0 0 0 0 0 0
V B V
V
tEven Even Even EvenOdd
t t
Fig. 3.11 Different situations in HDB3 scrambling techniques
Beforethefirstsubstitution,thenumberofnonzeropulsesiseven,sothefirstsubstitutionisBODY.After �this substitution, the polarity of the 1 bit is changed because the AMI scheme, after each substitution, must follow its own rule. After this bit, we need another substitution, which is 000V because we have only one nonzero pulse (odd) after the last substitution.The third substitution is B00V because there are no nonzero pulses after the second substitution (even). �
43/JNU OLE
3.3 Pulse Code Modulation (PCM)The most common technique to change an analog signal to digital data (digitisation) is called pulse code modulation (PCM).APCMencoderhasthreeprocesses,asshowninthefigurebelow.
Analog signal
PAM signal
Quantized signal
peM encoder
Sampling; Quantizing EncodingH 1 1 •• •1 1 0 0Digital data
Fig. 3.12 Components of PCM encoder
Where, The analog signal is sampled. The sampled signal is quantised.The quantised values are encoded as streams of bits.
Sampling: • ThefirststepinPCMissampling.TheanalogsignalissampledeveryTs s, where Ts is the sample interval or period. The inverse of the sampling interval is called the sampling rate or sampling frequency and denoted by is, where is =IITs’. Therearethreesamplingmethods-ideal,natural,andflattopasshowninthefigurebelow.
Amplitude
Amplitude
b. Natural sampling
c. Flat-top sampling
Analog signal
Analog signal
Time
Time
“
Amplitude
Analog signal
Time
Fig. 3.13 Three different sampling methods for PCM
Quantisation: • The result of sampling is a series of pulses with amplitude values between the maximum and minimumamplitudesofthesignal.Thesetofamplitudescanbeinfinitewithnonintegralvaluesbetweenthetwo limits. These values cannot be used in the encoding process. The following are the steps in quantisation:
We assume that the original analog signal has instantaneous amplitudes between V � min and Vmax’ .
Data Communications
44/JNU OLE
WedividetherangeintoLzones,eachofheight∆(delta): �
We assign quantised values of 0 to L - I to the midpoint of each zone. �We approximate the value of the sample amplitude to the quantised values. �
Encoding:• The last step in PCM is encoding. After each sample is quantised and the number of bits per sample is decided, each sample can be changed to an llb-bit code word.
Normalised PAM values
Quantization code
Normalized amplitude
4Δ7
5
2
3Δ
–3Δ
–4Δ-1.22 1.50
1.50
0 +0.26
3.50
3.24 3.94
3.50
-0.44 +0.30 -0.40
-1.50
1.10 -2.26 -1.88 -1.20
-1.50
-0.30
-1.50
+0.38
-2.50
-0.24
5
101 111 111 110 010 001 010 OW
222677
-1.50
2
-0.38
010
2Δ7.5
19.7
11.0
Time
–2Δ
–Δ –6.1 –5.5
–11.3–9.4
–6.0
0
Normalised quantized values
Normalised errorQuantisation code
Encoded words
Fig. 3.14 Quantisation and encoding of a sampled signal
Inthefigureshownabove,theencodedwordsareshowninthelastrow.Aquantisationcodeof2isencodedas 010; 5 is encoded as 101; and so on. Note that the number of bits for each sample is determined from the number of quantisation levels. If the number of quantisation levels is L, the number of bits is llb =log2 L. In our example L is 8 and therefore llb is 3. The bit rate can be found from the formula
Original signal recovery: • TherecoveryoftheoriginalsignalrequiresthePCMdecoder.Thedecoderfirstusescircuitry to convert the code words into a pulse that holds the amplitude until the next pulse. After the staircase signaliscompleted,itispassedthroughalow-passfiltertosmooththestaircasesignalintoananalogsignal.Thefilterhasthesamecut-offfrequencyastheoriginalsignalatthesender.Ifthesignalhasbeensampledatthe Nyquist sampling rate and if there are enough quantisation levels, the original signal will be recreated. Note thatthemaximumandminimumvaluesoftheoriginalsignalcanbeachievedbyusingamplification.Figurebelowshowsthesimplifiedprocess.
45/JNU OLE
Make and connect samples
Make and connect samples
Amplitude
Amplitude Analog signal
Time
TimeMake and connect samples
Make and connect samples
11 •••1100Digital data
PCM decoder
Fig. 3.15 Components of a PCM decoder
Delta modulation:• PCM is a very complex technique. Other techniques have been developed to reduce the complexity of PCM. The simplest is delta modulation. PCMfindsthevalueofthesignalamplitudeforeachsample;DMfindsthechangefromtheprevioussample.Figurebelowshowstheprocess.Notethatthere are no code words here; bits are sent one after another.
Tδ
0 1 0 0 0 0 0 0 11 TimeGenerated binary data
AmplitudeT
Fig. 3.16 the process of delta modulation
Modulator:• The modulator is used at the sender site to create a stream of bits from an analog signal. The process records the small positive or negative changes, called delta 0. If the delta is positive, the process records a I; if it is negative, the process records a 0. However, the process needs a base against which the analog signal is compared. The modulator builds a second signal that resembles a staircase. Finding the change is then reduced to comparing the input signal with the gradually made staircase signal. Figure below shows a diagram of the process.
OM modulator
Comprator
Delay unit
Staircase maker
Digital data
II•-- I 100
Analog signal
Fig. 3.17 Delta modulation components
Data Communications
46/JNU OLE
The modulator, at each sampling interval, compares the value of the analog signal with the last value of the staircase signal. If the amplitude of the analog signal is larger, the next bit in the digital data is 1; otherwise, it is 0. The output of the comparator, however, also makes the staircase itself. If the next bit is 1, the staircase maker moves the last point of the staircase signal 0 up; it the next bit is 0, it moves it 0 down. Note that we need a delay unit to hold the staircase function for a period between two comparisons.
Demodulator:• The demodulator takes the digital data and using the staircase maker and the delay unit, creates theanalogsignal.Thecreatedanalogsignal,however,needstopassthroughalow-passfilterforsmoothing.Figure below shows the schematic diagram.
Analog signalDigital data
OM demodulator
Staircase maker
Delay unit
Low pass filter
11’’’1100
Fig. 3.18 Delta demodulation components
3.4 Transmission ModesFTPcantransferafileacrossthedataconnectionbyusingoneofthefollowingthreetransmissionmodes:streammode, block mode and compressed mode. The stream mode is the default mode. Data are delivered from FTP to TCP as a continuous stream of bytes. TCP is responsible for chopping data into segments of appropriate size. If thedataaresimplyastreamofbytes(filestructure),noend-of-fileisneeded.End-of-fileinthiscaseistheclosingof the data connection by the sender. If the data are divided into records (record structure), each record will have a 1-byteend-of-record(EOR)characterandtheendofthefilewillhavea1-byteend-of-file(EOF)character.
In block mode, data can be delivered from FTP to TCP in blocks. In this case, each block is preceded by a 3-byte header.Thefirstbyteiscalledtheblockdescriptor;thenext2bytesdefinethesizeoftheblockinbytes.Inthecompressedmode,ifthefileisbig,thedatacanbecompressed.Thecompressionmethodnormallyusedisrun-lengthencoding. In this method, consecutive appearances of a data unit are replaced by one occurrence and the number of repetitions.Inatextfile,thisisusuallyspaces(blanks).Inabinaryfile,nullcharactersareusuallycompressed.
There is always a need to exchange commands, data and other control information between two communicating devices. There are mainly two options for transmitting data, commands and other control information from the sender to the receiver. These are:
Parallel communication •Serial communication •
47/JNU OLE
Data transmission
Parallel
Asynchronous Synchronous Isynchronous
Serial
Fig. 3.19 Types of data transmission
(Source: http://train-srv.manipalu.com/wpress/wp-content/uploads/2010/08/clip-image012-thumb4.jpg)
3.4.1 Parallel Transmission Mode
Computer A
Parallel connection
Computer B
Fig. 3.20 Parallel transmission(Source: http://static.commentcamarche.net/en.kioskea.net/pictures/transmission-images-parallele.gif)
The binary data, consisting of 1s and 0s, may be organised into groups of n bits each. The computers always produce and consume the data in groups of bits. So, by grouping, we can simply send data n bits at a time instead of 1. This is called the parallel transmission. The mechanism for parallel transmission is conceptually very simple. Use n wires to send n bits at one time.
In that way each bit has its own wire and all the n bits of one group can be transmitted with each clock tick from one device to another device. The big advantage of parallel transmission is the factor of speed. All else being equal, parallel transmission can increase the transfer speed by a factor of n over a serial transmission. But there is also a verysignificantdisadvantageofparalleltransmission.
And that is the factor of cost. It can be proved from the fact that the parallel transmission requires ‘n’communicationlinesforexamplewires,justtotransmitdatastream.Becausethisisveryexpensive,paralleltransmission is usually limited to short distances.
Data Communications
48/JNU OLE
3.4.2 Serial Transmission Mode
0 0 1 0 1 1 1 1
Serial Interface
Serial bit stream
Serial Interface – one bit at a timeMSB LSB
1 1 1 1 0 1 0 0
Fig. 3.21 Serial transmission(Source: http://freeonlinebooks.net/iserial/01_WQS.gif)
Serial transmission is a method of data transmission in which bits of data are transmitted over a single channel. For example,assumethatanoctethasthevalueshowninthefollowinggraphic.Theleft-most0bitisthemostsignificantbit(MSB)andtheright-most1bitistheleastsignificantbit(LSB).Theeightbitsintheoctetareconvertedintoalinearseriesofbits,oneatatime,withserialinterfaceoperation.TheinterfacetransmitstheMSBfirst.
This one-at-a-time transmission contrasts with parallel data transmission that passes several bits at a time. For the octet in the preceding graphic, each of the eight bits using a parallel interface has an output element. Instead of one bit at a time, the parallel bit stream can carry all eight bits at a time. Keeping the parallel transmission of all eight bits synchronised succeeds over short distances.
Characteristics of serial transmission systemSerial data transmission is suitable for communication between two participants as well as between several participants.Characteristicfeaturesofatransmissionsystemarethedirectionofthedataflowandthedatathroughput, or the maximum possible data rate.
Direction of data flow: • Transmissionsystemsdifferastothedirectioninwhichthedataflowandwhenmessagescan be transmitted. Basically, there are three different ways of communication.
Simplex � : Here, data exchange is in only one direction.Half-duplex � : Here, the stations take turns to transmit data.Full-duplex � : Data can be exchanged in both directions simultaneously.
49/JNU OLE
SummaryThe frequency of the carrier signal is chosen to be compatible with the transmission medium being used.•Modulation is the process of encoding source data onto a carrier signal with frequency.•The result of modulating the carrier signal is called the modulated signal.•Line coding is the process of converting digital data to digital signals.•A data element is the smallest entity that can represent a piece of information.•A signal element is the shortest unit (time wise) of a digital signal. •One goal in data communications is to increase the data rate while decreasing the signal rate.•In a unipolar scheme, all the signal levels are on one side of the time axis, either above or below.•Digital-to-analog conversion• is the process of changing one of the characteristics of an analog signal.The government assigns a narrow bandwidth to each radio station.•Scrambling, as opposed to block coding, is done at the same time as encoding.•The recovery of the original signal requires the PCM decoder.•Data are delivered from FTP to TCP as a continuous stream of bytes.•Serial transmission is a method of data transmission in which bits of data are transmitted over a single •channel.
ReferenceBanzal, S., 2007, • Data and Computer Network Communication, Firewall Media.Stallings,W., 2007. Business Data Communication, 5th ed., Pearson Education India.•Dr. Benthien, W. G., 2007. • Digital Encoding and Decoding, [pdf] Available at: <http://gbenthien.net/encoding.pdf>. [Accessed 14 September 2011].www.tpub.com, Pulse code modulation. [Online]. Available at: <http://www.tpub.com/neets/book12/49l.htm>. •[Accessed 14 September 2011].Prof. Prasad, S., • Pulse Code Modulation [Video Online] Available at: < http://freevideolectures.com/Course/2314/Communication-Engineering/41> [Accessed 15 September 2011].Prof. Pal, A., 2005. • Lecture 8: Transmission of Digital Signal – II [Video Online] Available at: < http://freevideolectures.com/Course/2278/Data-Communication/8>[Accessed 15 September 2011].
Recommended ReadingGlover, A. I. and Grant, M. P., 2004• . Digital Communications, 2nd ed., Pearson Education India.Forouzan, A. B., 2006. • Data Communication and Networking, 4th ed., Tata McGraw-Hill Education.Chaudhury, P., 2009. • Computer Organisation and Design, 3rd ed., PHI Learning Pvt. Ltd.
Data Communications
50/JNU OLE
Self AssessmentConstant-frequency signal is known as the ____________.1.
carrier signala. digital signalb. periodic signalc. electric signald.
Which method uses carrier signal to transmit data?2. Demodulationa. Frequencyb. Amplitudec. Modulationd.
Which of the technique is used for digital-to-digital conversion?3. Scramblinga. Modulationb. Demodulationc. Signalsd.
Which of the following statements is false?4. A data element is the smallest entity that can represent a piece of information.a. A signal element is the shortest unit (time wise) of a digital signal. b. Thedataratedefinesthenumberofdataelements(bits)sentin4s.c. The signal rate is the number of signal elements sent in 1s.d.
The modulation rate is also called ____________.5. bit ratea. bit lengthb. signalsc. baud rated.
Which of the following statements is true?6. Decreasing the signal rate decreases the bandwidth requirement. a. Increasing the signal rate decreases the bandwidth requirement. b. Decreasing the signal rate increases the bandwidth requirement. c. Decreasing the signal rate decreases the bandwidth requirement. d.
____________ encoding technique has a narrow bandwidth and does not create a DC component.7. HDB3a. Bipolar AMIb. NRZ-Lc. R8ZSd.
51/JNU OLE
Which of the following is the process of pulse code modulation?8. Analog-to-Digitala. Digital-to-Digitalb. Quantisationc. Modulationd.
In _____________, the voltages are on the both sides of the time axis.9. polar schemesa. Unipolar Schemesb. Line coding schemesc. Bipolar schemesd.
Thesetof___________canbeinfinitewithnonintegralvaluesbetweenthetwolimits.10. noisea. amplitudesb. distortionc. wavesd.
Data Communications
52/JNU OLE
Chapter IV
Multiplexing
Aim
The aim of this chapter is to:
explicate the concept of multiplexing•
explain prominent ways of multiplexing•
definewavelengthdivisionmultiplexing•
Objectives
The objectives of this chapter are to:
elucidate time division multiplexing•
explain asynchronous transfer mode•
describe statistical time division multiplexing•
Learning outcome
At the end of this chapter, you will be able to:
compare FDM with TDM•
identify frame relay•
understand STDM and • ATDM
53/JNU OLE
4.1 IntroductionMultiplexing is a form of data transmission in which one communication channel carries several transmissions at the same time. The telephone lines that carry our daily conversations can carry thousands or even more of conversations at a time using multiplexing concept. The exact number of simultaneous transmission depends on the type of communication channel and the data transmission rate.
Economics of scale play an important role in the telephone system. It costs essentially the same amount of money to installandmaintainahigh-bandwidthtrunkaslow-bandwidthtrunkbetweentwoswitchingofficers.Consequently,telephone companies have developed elaborate schemes for multiplexing many conversations over a single physical trunk.
Accordingly, the communication channel is shared in such a way as to maximum the utilisation of the channel capacity. Thus, the method of dividing a single channel into many channels so that a number of independent signals may be transmitted on it is known as Multiplexing.
Multiplexing is the process in which two or more signals are combined for transmission over a single communications path.Thisconceptisconveyedinthefigurebelow.
MUX : MultiplexerDEMUX : Demultiplexer
1 link, n channelsn Input lines
n Output lines
Fig. 4.1 Multiplexing(Source: http://t1.gstatic.com/images?q=tbn:ANd9GcQE9Fe43xQs6mrDCop3PlN-NKR3Jie9zpx956J-HsFYt_W-
a1tOAA)
In a multiplexed system, n linessharethebandwidthofonelink.Theabovefigureshowsthebasicformatofamultiplexed system. The lines on the left direct their transmission streams to a multiplexer (MUX), which combines them into a single stream. At the receiving end, that stream is fed into a demultiplexer (DEMUX), which separates the stream back into its component transmissions (one-to-many) and directs them to their corresponding lines. In thefigure,thewordlinkreferstothephysicalpath.Thewordchannelreferstotheportionofalinkthatcarriesatransmission between a given pair of lines. One link can have many (n) channels.
Multiplexing has made communication very economical by transmitting thousands of independent signals over a single transmission line. There are three predominant ways to multiplex:
Frequency Division Multiplexing (FDM)•Time Division Multiplexing (TDM)•Wavelength Division Multiplexing (WDM)•
Data Communications
54/JNU OLE
MultiplexingMultiplexing
MultiplexingFrequency-dvision
multiplexing MultiplexingWavelength-division
multiplexing MultiplexingTime-division multiplexing
Analog Analog Digital
Fig. 4.2 Categories of multiplexing(Source: http://t1.gstatic.com/images?q=tbn:ANd9GcR4nWBQ6y_jH2PKU5r2zPPy5bLnTCx-cm_
Mem4h2yhQOl6AQB_p)
4.2 Frequency Division MultiplexingFrequency Division Multiplexing (FDM) is a networking technique in which multiple data signals are combined for simultaneous transmission via a shared communication medium. FDM uses a carrier signal at a discrete frequency for each data stream and then combines many modulated signals. When FDM is used to allow multiple users to share a single physical communications medium (for example, not broadcast through the air), the technology is called Frequency-Division Multiple Access (FDMA).
Frequency division multiplexing is the position of signal spectra in frequency such that each signal spectrum can be separatedoutfromalltheothersbyfiltering.FDMdoesnotprecludetheuseofothermodulatingmethods. There are N signals in frequency; each is band limited to fm Hz. In order to separate N signals in frequency, each is modulated with a carrier frequency fC1, fC2, …, fCN. Using DSB-LC, the spectral density of every modulated signal has a bandwidth of 2fm and each is centred at various carrier frequencies fC1, fC2,…, fCN.
These carrier frequencies are chosen far enough apart such that each signal spectral density is separated from all theothers.ThefigurebelowgivesaconceptualviewofFDM.Inthisillustration,thetransmissionpathisdividedinto three parts, each representing a channel that carries one transmission.
MUX
Input lines
Channel 1
Channel 2
Channel 3
Output lines
D E M U X
Fig. 4.3 Frequency division multiplexing(Source: http://t3.gstatic.com/images?q=tbn:ANd9GcQDKp-PygdKR6Th_
pXm1gunLEs5e5u9T29rrXPUKuMZCFaDb0CQSQ)
We consider FDM to be an analog multiplexing technique; however, this does not mean that FDM cannot be used to combine sources sending digital signals. A digital signal can be converted to an analog signal before FDM is used to multiplex them.
55/JNU OLE
FDM is an analog multiplexing technique that combines analog signals. Frequency-division multiplexing works best with low-speed devices. The frequency division multiplexing schemes used around the world is to some degree standardised. A wide spread standard is 12 400-Hz each voice channels ( 300Hz for user, plus two guard bands of 500Hz each) multiplexed into the 60 to 108 KHz band. Many carriers offer a 48 to 56 kbps leased line service to customers based on the group. Other standards upto 230000 voice channels also exist.
Advantages of FDMHere user can be added to the system by simply adding another pair of transmitter modulator and receiver •demodulators.FDMsystemsupportfullduplexinformationflowwhichisrequiredbymostofapplication.•Noise problem for analog communication has lesser effect.•
Disadvantages of FDMIn FDM system, the initial cost is high. This may include the cable between the two ends and the associated •connectors for the cable.In FDM system, a problem for one user can sometimes affect others.•In FDM system, each user requires a precise carrier frequency.•
For example, the allocated spectrum is about 1MHz, roughly 500 to 1500 KHz in different stations, each operating in a portion of the spectrum. With the interchannel separation great enough to prevent interference, this system is an example of frequency division multiplexing.
4.3 Time Division MultiplexingTime Division Multiplexing (TDM) is another popular method of utilising the capacity of a physical channel effectively. Each user of the channel is allotted a small time interval during which it may transmit a message. Thus, the total time available in the channel is divided and each user is allocated a time slice. In TDM, user send message sequentially one after another. Each user can, however, use the full channel bandwidth during the period he has control over the channel. The channel capacity is fully utilised in TDM by interleaving a number of messages belonging to different users into one long message. This message sent through the physical channel must be separated at the receiving end.
Unfortunately, TDM can only be used for digital data multiplexing. Since local loops produce analog signals, a conversionisneededfromanalogtodigitalintheendofficewherealltheindividuallocalloopscometogethertobe combined onto outgoing trucks.
Time division multiplexing is the process of dividing up one communication time slot into smaller time slots. We will use the example of a T1 which is time-division multiplexed at the DS1 rate. A T1 consists of 24 channels which are read 8,000 times per second. Each time a channel is read, a value is obtained. Thus, a time slot for a T1 is 1/8,000th of a second. Time-division multiplexing combines the values from all 24 channels on the T1 into the same 1/8,000th of a second.
InTDM,channels“share”thecommonaggregatebasedupontime,thereareavarietyofTDMschemessuchas:
Conventional Time Division Multiplexing (TDM)Conventional TDM systems usually make use of either Bit-Interleaved or Byte-Interleaved multiplexing schemes. Clocking (Bit timing) is critical in conventional TDM. All sources of I/O and aggregate clock frequencies should bederivedfromacentral,“traceable”sourceforthegreatestefficiency.
Bit-Interleaved MultiplexingIn Bit-Interleaved TDM, a single data bit from an I/O port is an output to the aggregate channel. This is followed by a data bit from another I/O port (channel), and so on, and so on, with the process repeating itself.
Data Communications
56/JNU OLE
A“timeslice”isreservedontheaggregatechannelforeachindividualI/Oport.Sincethese“timeslices”foreachI/O port are known to both the transmitter and receiver, the only requirement is for the transmitter and receiver to be in-step; that is to say, being at the right place (I/O port) at the right time. This is accomplished through the use ofasynchronisationchannelbetweenthetwomultiplexers.Thesynchronisationchanneltransportsafixedpatternthat the receiver uses to acquire synchronisation.
Total I/O bandwidth (expressed in Bits Per Second - BPS) cannot exceed that of the aggregate (minus the •bandwidth requirements for the synchronisation channel).Bit-InterleavedTDMissimpleandefficientandrequireslittleornobufferingofI/Odata.Asingledatabitfrom•each I/O channel is sampled, then interleaved and output in a high speed data stream. Unfortunately, Bit-InterleavedTDMdoesnotfitinwellwithtoday’smicroprocessor-driven,byte-basedenvironment.
Byte-Interleaved MultiplexingIn Byte-Interleaved Multiplexing, complete words (bytes) from the I/O channels are placed sequentially, one after another, onto the high speed aggregate channel. Again, a synchronisation channel is used to synchronise the multiplexers at each end of the communications facility. For an I/O payload that consists of synchronous channels only, the total I/O bandwidth cannot exceed that of the aggregate (minus the synchronisation channel bandwidth).
But for asynchronous I/O channels, the aggregate bandwidth CAN BE EXCEEDED if the aggregate byte size is LESS than the total asynchronous I/O character size (Start + Data + Stop bits). (This has to do with the actual CHARACTER transmission rate of the asynchronous data being LESS THAN the synchronous CHARACTER rate serviced by the TDM.)
Statistical Time Division Multiplexing (STDM)Statistical TDMs are such that they only utilise aggregate bandwidth when there is actual data to be transported from I/O ports. Data STDMs can be divided into two categories:
Conventional STDM �Frame Relay/X.25 Networking �
Conventional STDMTheStatisticalMultiplexer(or“statmux”)utilisesadifferentformofTDM.Thesemultiplexers typicallyuseaHDLC-likeframeforaggregatecommunicationsbetweenunits.AsI/OtrafficarrivesatthemuxitisbufferedandtheninsertedintotheI-FieldoftheHDLCframe.ThereceivingunitremovestheI/OtrafficfromtheaggregateHDLC frame.
Statistical multiplexers are ideally suited for the transport of asynchronous I/O data; as it can take advantage of •the inherent latency in asynchronous communications. However, they can also multiplex synchronous protocols by“spoofing”andprioritisation;againtakingadvantageofthelatencybetweenblocks/frames.Statistical multiplexers are typically faster at transporting I/O data End-To-End than X.25 systems but some of •these multiplexers can also perform network switching functions between I/O ports. The total I/O bandwidth can (and usually does) exceed the aggregate port bandwidth.Later, many of these multiplexers incorporated “switching”mechanismsthatallowedI/Oportsto“intelligently”connectthemselvestootherdestinationportsupon user command. While somewhat functioning as an X.25 switch, these statistical multiplexers were usually faster and provided more transparent I/O data-carrying capacity.StatisticalTDM’s biggest disadvantage is that it is I/Oprotocol sensitive.Therefore, they have difficulty•supporting “transparent” I/Odata and unusual protocols.To support these I/Odata types,many statmuxsystemshaveprovisionstosupportconventionalTDMI/Otrafficthroughtheuseofadjunct/integratedmodules.Conventional STDM was very popular in the late 1970s to mid 1980s and is still used today, although the market for these units is dwindling.
Frame Relay and X.25 networkingFrame Relay and X.25 systems are also categorised as Statistical TDMs. Both of these systems utilise aggregate HDLC frame structures and both of these systems can interoperate with both private and public systems.
57/JNU OLE
TheadvantageofFrameRelayoverX.25is that itcansupport thesametrafficasX.25,while facilitating•“bandwidthondemand”requirementsfor“bursty”traffic(forexample,LANs).PublicFrameRelayservicesare available, offering customers additional methods to interconnect LANs, rather than having dedicated Wide Area Network (WAN) links.FrameRelay,however,cannotadequatelysupportvoiceorvideotrafficbecauseofvariableEnd-To-Enddelivery•times(forexample,variabledelay).Voiceandvideotransmissionsareofa“ConstantBitRate”(CBR)natureanddonotfarewellsittinginaqueuewaitingforabigLANpackettofinishtransmitting.Care must be taken when deploying Frame Relay technologies. In the Internet world, it is so easy to overload •trunkcapacitiesattheend-pointsofanIPconnection.Infact,thisoverloadingisa“cheap”waytoaddincreasedI/Otraffic(andusers)withoutincreasingaggregatebandwidth.
Unfortunately, when Frame Relay starts chucking out I/O data frames, the impact on Internet applications is very noticeable since IP retransmissions are so long. This same detrimental effect is also experienced in wireless LANs.
Time Assignment Speech Interpolation (TASI)TASI systems represent an example of an ANALOG Statistical Time Division Multiplexing scheme. These systems enjoyedlimiteduseinthe1980sandwereparticularlyadeptatsharingvoicecircuits;specificallyPBXtrunks.
A TASI multiplexer is interconnected between the PBX and the trunk facilities. Usually, one analog trunk circuit •is used for signalling purposes between TASI units at each end of the link. The remaining voice trunks support analog TASI TDM voice conversations.In normal telephone conversations, a majority of time is spent in a latent (idle) state. TASI trunks will allocate •“snippets”ofvoicefromanotherchannelduringthisidletime.IfanindividualweretomonitortheseTASItrunks, they would hear bits and pieces of various conversations. The signalling channel is used for the signalling conversion between End-Point PBX (Private Branch Exchange) units and also for the allocation of bandwidth once incoming speech energy has been detected.As digital speech processing became more common, TASI systems were created that had analog inputs, and •digitaloutputs.Thistypeofmultiplexingtechniqueismorecommonlyknownas“DigitalSpeechInterpolation”(DSI).
Unfortunately,TASIandDSIsystemssufferfromafewdrawbacks.First,therecanbealotofvoice“clipping”noticed by users. This occurs when a little bit of speech is lost while waiting for the TASI mux to detect valid speech andallocatebandwidth.Clippingalsooccurswhentherejustisn’tbandwidthpresentatthemoment.Also,TASIand DSI units are very susceptible to audio input levels and may have problems with the transport of voiceband data (for example VF modem) signals.
Cell-Relay transmissionInCell-Relaysystems,dataisbrokenupintobasicunits(called“cells”)andtransportedthroughthenetwork.Astandardcell-sizeisdefinedasconsistingof538-bitbytes.These53bytesconsistof48bytesofPayload(data)and 5 bytes of Header (routing) information.
Cell-Relay operation is somewhat analogous to a processor bus. Instead of a 32-bit data buss, there is a 53-byte •data buss. Instead of a 64-bit address buss, there is a 5-byte address buss. While the bus operates synchronously, under control of a buss clock, the buss function itself is asynchronous (similar in operation to an ordinary microprocessorbus).Butinsteadofthetransferbeingparallel,ahigh-speedserialATMfacilityisused.That’sCell Relay!As mentioned in the paragraph above, the buss function is asynchronous. That means that the I/O data (CPU •modules in the above example) will immediately arbitrate for the facility (processor buss) when there is data destinedforit.Ifthereisaconflict,somebodylosesanddataislost.Itisuptotheapplicationtorecover(ornot!) from the error condition.
Data Communications
58/JNU OLE
Cell-Relay operation can be summarised as being similar to Conventional TDM except that ii has properties of asynchronous transfer. When operating with very high speed facilities, Cell-Relay has the ability to integrate Local Area Networks (LANs) and Wide Area Networks (WANs). Some Cell-Relay transmission services are now under development, or available on a limited basis, namely Asynchronous Transfer Mode (ATM) and Switched Multi Megabit Data Service (SMDS).
Asynchronous Transfer Mode (ATM)ATM is a cell-based transport mechanism that evolved from the development of the Broadband ISDN (B-ISDN) standards. ATM does not stand for Automatic Teller Machine or Ascom Timeplex Multiplexers (although that mightapply); rather, itdefines theasynchronous transportofcells (Cell-Relay).Perhapsevenmore important,ATMisassociatedwithaprocessknownasATMAdaptationLayer.AALdescribeshowvariousI/Otraffictypesare converted into cells.
TheAdaptationprocessandtheserialtransportofcellsarecommonlyreferredtoas“FastPacketMultiplexing”(FPM). While similar in concept, FPMs do not necessarily conform to ATM standards or switching conventions.
Switched Multi Megabit Data Service (SMDS)Similar to ATM but designed for operation at lower rates (64 KBPS - 155.520 MBPS), SMDS services are available now from many of Local Exchange Carriers (LECs). International and domestic Carrier services are available as well. SMDS offers customers alternatives to Frame Relay transport.
SMDS addressing utilises the CCITT (now ITU) E.164 addressing scheme, making addressing much more manageable for customers. Also, SMDS is available at higher rates than Frame Relay (which typically tops out at T1 rate - 1.544 MBPS).
1
2
3
4
1
2
3
4
4 3 2 1 4 3 2 1 4 3 2 1MUX
D E M U X
Dataflow
Fig. 4.4 Times Division Multiplexing (Source: http://t3.gstatic.com/images?q=tbn:ANd9GcRj22J0BWWxQGe4klZwkOIUCJ6Gk9Wgwat0sdGmfRZx
JYxcl9_v)
Notethatintheabovefigureweareconcernedwithonlymultiplexingnotswitching.Thismeansthatallthedatainamessagefromsource1alwaysgotoonespecificdestination,beit1,2,3or4.Thedeliveryisfixedandunvarying,unlike switching.
TDM is, in principle, a digital multiplexing technique. Digital data from different sources are combined into one timeshared link. However, this does not mean that the sources cannot produce analog data; analog data can be sampled, changed to digital data and then multiplexed by using TDM.
59/JNU OLE
TDM is a digital multiplexing technique for combining several low-rate channels into one high-rate one. We can divide TDM into two different schemes: synchronous and statistical. . In synchronous TDM, each input connection has an allotment in the output even if it is not sending data. Time Division multiplexing can be divided into two arts:
Synchronous Time Division MultiplexingSynchronous TDM assigns time slots of equal length to all packets regardless whether or not anything is to be sent byeachstationwithanassignedtimeslots.ThefigurebelowshowssynchronousTDM.
Multiplexer
AAAAAA
BBB
CCCC
DDDDD
A D A D C A D C B A D C B A D C B A
Fig. 4.5 SynchronousTime Division Multiplexing(Source: http://t3.gstatic.com/images?q=tbn:ANd9GcRPhNU_FtZWDMffN7xCNbkkcnhkAfz8dF4r3pivmwtC2vtkjG1x4w)
Timeslot‘X’meansitisassignedtousermaloneandcannotbeusedbyanyotheruserorotherdevices.Eachtimeits allocated time slot comes up, a user has the opportunity to send a portion of its data. If a user is unable to transmit or does not have data to send, its time slot cannot be used by any other user. STDM systems are comparatively easy to implement once the software allocates the time slots.
Asynchronous Time Division Multiplexing (ATDM)In asynchronous time division multiplexing, the total speed of the input lines can be greater than the capacity of the path. This system is more complex but allows for a means of reassigning time slots that are not in use.
ATDM networks assign time slots only when they are to be used and delete them when they are idle. Total time used foranasynchronousTDMframevarieswiththeamountoftrafficcurrentlybeinghandled.ApplicationofATDMisinhighdensityandhightrafficapplications.
C 2 B 2 A 1D 4 A 1 D 4 B 2 A 1 D 4A 1MUX
AAAA
Frame 4 Frame 3 Frame 2 Frame 1
1
2
3
4
BB
C
DDD
Asynchronous TDM: multiplexing process
Fig. 4.6 Asynchronous Time Division Multiplexing(Source: http://t3.gstatic.com/images?q=tbn:ANd9GcQV4R3eUx5u1MMXEx1Zl8pzVPxobSi6q2Uh1U6EukLzd
w4P2Logrg)
Data Communications
60/JNU OLE
4.4 Wavelength Division MultiplexingAtechniqueofsendingsignalsofseveraldifferentwavelengthsoflightintothefibresimultaneously.Infibreopticcommunications, Wavelength- Division Multiplexing (WDM) is a technology which multiplexes multiple optical carriersignalsonasingleopticalfiberbyusingdifferentwavelengths(colours)oflaserlighttocarrydifferentsignals. This allows for a multiplication in capacity, in addition to making it possible to perform Bidirectional communicationsoveronestrandoffibre.
The WDM channels are separated in wavelength to avoid cross-talk when they are (de)multiplexed by a non-ideal opticalfibre.Thewavelengthscanbeindividuallyroutedthroughanetworkorindividuallyrecoveredbywavelength-selectivecomponents.WDMallowsustousemuchofthefibrebandwidth,althoughvariousdevice,systemandnetworkissueswilllimittheutilisationofthefullfibrebandwidth.
WDM channel may contain a set of even slower time-multiplexed channels. WDM is similar to frequency-division multiplexing (FDM).
SFP SFP
SFP
SFP
SFP
SFP
Convert Signal to Optical Channel
Attenuated Hot Channel to Equalize Power
Combine Signals onto One Fiber
Amplify Power to
Overcome Fiber Loss
Separate Optical
Channels
Convert Optical
Channel to Signal
Transponders Attenuators PostAmplifier PreAmplifier Demux TranspondersMux
Fig. 4.7 Wavelength Division Multiplexing(Source: http://t2.gstatic.com/images?q=tbn:ANd9GcQWO24MxVy2cUGJ7qa-83pJoy_IFQQk-
I1s7ZeAEAoQLPpSw_U24g)
WDMisnowacost-effective,flexibleandscalabletechnologyforincreasingcapacityofafibrenetwork.WDMarchitecture is based on a simple concept – instead of transmitting a single signal on a single wavelength, transmit multiple signals, each with a different wavelength. Each remains a separate data signal at any bit rate with any protocolunaffectedbyothersignalonthefibre.
There are two types of WDM, namely, Coarse and Dense Wavelength Division Multiplexing (CWDM and DWDM).
CWDM: • WDM uses a wide spectrum and accommodates eight channels. This wide spacing of channels allows for the use of moderately priced optics but limits capacity. CWDM is typically used for lower-cost, lower-capacity, shorter-distance applications where cost is the paramount decision criteria.DWDM: • DWDM systems pack 16 or more channels into a narrow spectrum window very near the 1550 nm local attenuation minimum. Decreasing channel spacing requires the use of more precise and costly optics, but allowsforsignificantlymorescalability.TypicalDWDMsystemsprovide1-44channelsofcapacitywithsomenew systems offering up to 80-160 channels. DWDM is typically used where high capacity is needed over a limitedfibreresourceorwhereitiscostprohibitivetodeploymorefibre.
61/JNU OLE
ThekeyfeaturesandbenefitsofWDMinclude:Protocol and Bit Rate Agnostic:• Wavelengths can accept virtually any services.Fibre Capacity Expansion:• WDMaddsupto160Xbandwidthtoasinglefibre.Hi Cap/Long Haul and Lo Cap/Short Haul Applications:• CWDM and DWDM provide price performance for virtually any network.Remotely provisionable: • ROADMsprovideflexibilitytochangewithchangingnetworkrequirements.
Data Communications
62/JNU OLE
SummaryMultiplexing is a form of data transmission in which one communication channel carries several transmissions •at the same time.The method of dividing a single channel into many channels so that a number of independent signals may be •transmitted on it is known as Multiplexing.A demultiplexer (DEMUX), which separates the stream back into its component transmissions (one-to-many) •and directs them to their corresponding lines.FDM uses a carrier signal at a discrete frequency for each data stream and then combines many modulated •signals. FDM is an analog multiplexing technique that combines analog signals.•In TDM, each user of the channel is allotted a small time interval during which it may transmit a message.•TDM can only be used for digital data multiplexing.•Clocking (Bit timing) is critical in conventional TDM.•In Bit-Interleaved TDM, a single data bit from an I/O port is an output to the aggregate channel.•In Byte-Interleaved multiplexing, complete words (bytes) from the I/O channels are placed sequentially, one •after another, onto the high speed aggregate channel.Statistical TDMs are such that they only utilise aggregate bandwidth when there is actual data to be •transported.StatisticalTDMshavedifficultysupporting“transparent”I/Odataandunusualprotocols.•Time Assignment Speech Interpolation systems represent an example of an ANALOG Statistical Time Division •Multiplexing scheme.InCell-Relaysystems,dataisbrokenupintobasicunits(called“cells”)andtransportedthroughthenetwork.•Synchronous TDM assigns time slots of• equal length to all packets regardless whether or not anything is to be sent by each station with an assigned time slots.In asynchronous time division multiplexing, the total speed of the input lines can be greater than the capacity •of the path.The Wavelength Division Multiplexing channels are separated in wavelength to avoid cross-talk.•
ReferencesTanenbaum, S. A., 2003. • Computer networks, 4th ed., Prentice Hall PTR.Dhotre, A. I. and Bagad, S. V., 2008. • Data Communication, Technical Publications.General telecom, • Multiplexing Techniques [Online] Available at: < http://telecom.tbi.net/mux1.html>. [Accessed 14 September 2011].Skullbox.net, 2011. • Multiplexing [Online] (Updated 6 June 2011) Available at: < http://www.skullbox.net/multiplexing.php>[Accessed 14 September 2011].Prof. Pal, A., 2005. • Lecture 11:Multiplexing I [Video Online] Available at: < http://freevideolectures.com/Course/2278/Data-Communication/11> [Accesses 15 September 2011].Prof. Pal, A., 2005. • Lecture 12:Multiplexing II [Video Online] Available at: < http://freevideolectures.com/Course/2278/Data-Communication/12> [Accesses 15 September 2011].
Recommended ReadingWhite, C., 2010. • Data Communications and Computer Networks: A Business User’s Approach, Cengage Learning.Phillips, 2008. • Signals, Systems and Transforms, 4th ed., Pearson Education India.Tomasi, W., 2007. • Introduction to Data Communication and Networking, 3rd ed., Pearson Education India.
63/JNU OLE
Self Assessment_______________ is the process in which two or more signals are combined for transmission over a single 1. communications path.
Demultiplexinga. Multiplexingb. Data Communicationc. Pulse code modulationd.
In which form of data communication, one communication cahnnel carries several transmissions at the same 2. time?
Demultiplexinga. Data Communicationb. Pulse code modulationc. Multiplexing d.
____________________ uses a carrier signal at a discrete frequency for each data stream.3. Time Division Multiplexinga. Statistical Time Division Multiplexingb. Frequency Division Multiplexingc. Wavelength Division Multiplexingd.
FDM is a/an _____________ multiplexing technique that combines analog signals.4. analoga. digitalb. periodicc. wavyd.
Which communication technique has lesser noise effect?5. Analoga. Digitalb. Periodicc. Wavyd.
What is the another term for demultiplexer?6. Multiplexera. MUXb. DEMUXc. Multiplexingd.
A ____________ is reserved on the aggregate channel for each individual I/O port.7. muxa. FDM b. demuxc. time sliced.
Data Communications
64/JNU OLE
Which kind of TDMs are only utilised in an aggregate bandwidth when there is actual data to be transported?8. Conventional TDMa. Statistical TDMb. Synchronous TDMc. Asynchronous TDMd.
________________ TDM assigns time slots of9. equal length to all packets.Conventional TDMa. Statistical TDMb. Synchronous TDMc. Asynchronous TDMd.
Infibre optic communications, ___________________ is a technologywhichmultiplexesmultiple optical10. carrier signals.
Time Division Multiplexing (TDM)a. Frequency Division Multiplexing (FDM)b. Wavelength Division Multiplexingc. Conventional Time Division Multiplexing (CTDM)d.
65/JNU OLE
Chapter V
Transmission Media and Switching
Aim
The aim of this chapter is to:
explicate the concept of guided media•
enlist the types of guided media•
defineunguidedmedia•
Objectives
The objectives of this chapter are to:
elucidate circuit switched network•
explain datagram network•
enlist the types of unguided media•
Learning outcome
At the end of this chapter, you will be able to:
compare the rays in electromagnetic spectrum•
understand virtual circuit network•
classify classes of datagram n• etwork
Data Communications
66/JNU OLE
5.1 Guided and Unguided MediaMany different types of media can be used for the physical layer. For example, telephone twisted pair, coax cable, shieldedcoppercableandfibreopticsarethemaintypesusedforLANs.Differenttransmissiontechniquesgenerallycategorised as baseband or broadband transmission may be applied to each of these media types.
The characteristics and quality of a data transmission are determined both by the characteristics of the medium and the characteristics of the signal.
In the case of guided media, the medium itself is more important in determining the limitations of transmission.
For unguided media, the bandwidth of the signal produced by the transmitting antenna is more important than the medium in determining transmission characteristics. One key property of signals transmitted by antenna is directionality. In general, signals at lower frequencies are omni-directional; that is, the signal propagates in all directions from the antenna. At higher frequencies, it is possible to focus the signal into a directional beam.Transmission media can be divided into two broad categories: Guided and Unguided.
Transmission media
Guided (wired)
Twisted-pair cable Coaxial cable Fiber-optic cable Free space
Unguided (wireless)
Fig. 5.1 Types of transmission media(Source: http://t0.gstatic.com/images?q=tbn:ANd9GcRYCWhUfTczG9zTdJuYl5oH_h3xnATj_
q0OZnBO4XcAbxJho0rnZw)
Guided mediaGuided media, which are those that provide a conduit from one device to another, include twisted-pair cable, coaxial cableandfibre-opticcable.
Guidedtransmissionmediausesa“cabling”systemthatguidesthedatasignalsalongaspecificpath.Thedatasignalsareboundbythe“cabling”system.Guidedmediaisalsoknownasboundmedia.Here,cablingismeantina generic sense and is not meant to be interpreted as copper wire cabling only. Cable is the medium through which information usually moves from one network device to another.
Twisted pair cable and coaxial cable use metallic (copper) conductors that accept and transport signals in the form ofelectriccurrent.Opticalfibreisaglassorplasticcablethatacceptsandtransportssignalsintheformoflight.There are four basic types of guided media:
Twisted pair •Coaxial cable •Opticalfibre•
67/JNU OLE
Guided media
Open wire Twisted-Pair Coaxial Optical
Fig. 5.2 Types of guided media(Source: http://t0.gstatic.com/images?q=tbn:ANd9GcTbfVDFkISB-qSYgFTd6Pgrs_wtcg2-IqTV_
hsRtcykxk1vFXKVCg)
Twisted pair cableTwisted pair cable is least expensive and most widely used. The wires in twisted pair cabling are twisted together in pairs. Each pair would consist of a wire used for the +ve data signal and a wire used for the -ve data signal. Any noise that appears on one wire of the pair would occur on the other wire.
Because the wires are opposite polarities, they are 180 degrees out of phase. When the noise appears on both wires, it cancels or nulls itself out at the receiving end. Twisted pair cables are most effectively used in systems that use a balanced line method of transmission: polar line as opposed to unipolar line coding.
Copper Wire
Insulation
Copper Mesh
Outside Insulation
Fig. 5.3(a) Unshielded twisted pair cable(Source: http://t0.gstatic.com/
images?q=tbn:ANd9GcQZnWKcStuIA6b4CXFLevlNpxv40FDX2ZTcik5xMJIwnNt3FFuxFg)
Data Communications
68/JNU OLE
Outer jacket
Overall Shield
Pair Shields
Twisted-Pair
Color-Coded plastic
Insulation
Fig. 5.3(b) Unshielded twisted pair cable(Source: http://t1.gstatic.com/images?q=tbn:ANd9GcSj0C0du1FT0f8MlkGYhDjAlFcqzHHPWtAyQrElLX-
vQ20s3Bse)
Physical descriptionTwo insulated copper wires arranged in regular spiral pattern. �Numbers of pairs are bundled together in a cable. �Twisting decreases the crosstalk interference between adjacent pairs in the cable, by using different twist �length for neighbouring pairs.A twisted pair consists of two conductors (normally copper), each with its own plastic insulation, twisted �together. One of the wire is used to carry signals to the receiver and the other is used only a ground reference.
UTPischeap,flexible,andeasytoinstall.UTPisusedinmanyLANtechnologies,includingEthernetandTokenRing.
Transmission characteristics are:Itrequiresamplifiersforanalogsignals. �It requires repeaters for digital signals. �Attenuation is a strong function of frequency. �Higher frequency implies higher attenuation. �It is susceptible to interference and noise. �Improvement possibilities. �Shielding with metallic braids or sheathing reduces interference. �Twisting reduces low frequency interference. �Different twist length in adjacent pairs reduces crosstalk. �
Comparison between unshielded and shielded twisted pairsUnshielded twisted pair (UTP).•
Ordinary telephone wire. �Subject to external electromagnetic interference. �
69/JNU OLE
Shielded twisted pair (STP)•Shielded with a metallic braid or sheath �Reduces interference �Better performance at higher data rates �MoreexpensiveanddifficulttoworkcomparedtoUTP �
Applications of TP cableMost common transmission media for both digital and analog signals. �TP cables are used in telephone lines to provide voice and data channels. �ThelinethatconnectssubscriberstothecentraltelephoneofficeismostcommonlyUTPcable. �The DSL lines that are used by the telephone companies to provide high data rate connections also use high �bandwidth capability UTP cable. Local Area Network (LAN) also uses twisted-pair cable. �
Coaxial cableA form of network cabling used primarily in older Ethernet networks and in electrically noisy industrial environments. Thename“coax”comesfromitstwo-conductorconstructioninwhichtheconductorsrunconcentricallywitheachother along the axis of the cable. Coaxial cabling has been largely replaced by twisted-pair cabling for local area network(LAN)installationswithinbuildings,andbyfibre-opticcablingforhigh-speednetworkbackbones.
Coaxial cable (or coax) carries signals of higher frequency ranges than twisted-pair cable. Instead of having two wires, coax has a central core conductor of solid or standard wire (usually copper) enclosed in an insulating sheath, which is, in turn, encased in an outer conductor of metal foil, braid, or a combination of the two (also usually copper).
Central copper coreCopper wire braiding
Radio-frequency electric signal
Insulation
Outer sheath
Coaxial cable
Core
Core
Core
LightSignal
LightSignal
LightSignal
Protective layer
Outer sheath
Optical-fibercable
Opticalfibers
Fig. 5.4 Thinnet and thicknet coaxial cable(Source: http://t1.gstatic.com/images?q=tbn:ANd9GcRKSfeswW1Y7PjoSTFkovwW-lq-
gFuhZljPEMoEuKEGfv8izNdzMg)
Fibre optic cableFibre-optic is a glass cabling media that sends network signals using light. Fibre-optic cabling has higher bandwidth capacity than copper cabling, and is used mainly for high-speed network Fibre Distributed Data Interface (FDDI) backbones, long cable runs, and connections to high-performance workstations.
Refraction: • The direction in which a light ray is refracted depends on the change in density encountered. A beam of light moving from a less dense into a denser medium is bent towards vertical axis. When light travels
Data Communications
70/JNU OLE
into a denser medium, the angle of incidence is greater than the angle of refraction; and when light travels into a less dense medium, the angle of incidence is less than the angle of refraction.Critical angle:• A beam of light moving from a denser into a less dense medium, as the angle of incidence increases the angle of refraction also increases.
Moredense
Moredense
Moredense
Lessdense
Lessdense
Lessdense
I< critical angle, refraction
I = critical angle, refraction
I > critical angle, refraction
Fig. 5.5 Critical angle refraction in optical fibre(Source: http://t3.gstatic.com/images?q=tbn:ANd9GcRXOK21EaE6LGG9AcMXdZq5Mme7hd1BP0dJpt99lzXl
LgH4hP_3UA)
Black polyurethane outer jacket
Strength members
Core (silica)
Buffer jacket
Silicon coating
Cladding (silica)
Fig. 5.6 Fibre optic cable(Source: http://www.brainbell.com/tutorials/Networking/images/02fig03.gif)
Fibre-optic cable itself is composedof a core glassfibre surroundedby cladding.An insulated covering thensurrounds both of these within an outer protective sheath. Twotypesoffibre-opticcableavailablearesingleandmultimodefibre.
71/JNU OLE
Inmultimodefibre,manybeamsoflighttravelthroughthecable,bouncingoffthecablewalls.Thisstrategyactuallyweakensthesignal,reducingthelengthandspeedthedatasignalcantravel.Thelightis“guided”downthecentreofthefibrecalledthe“core”.Thecoreissurroundedbyanopticalmaterialcalledthe“cladding”thattrapsthelightinthecoreusinganopticaltechniquecalled“totalinternalreflection.”Thecoreandcladdingareusuallymadeofultra-pureglass,althoughsomefibresareallplasticoraglasscoreandplasticcladding.Thefibreiscoatedwithaprotectiveplasticcoveringcalledthe“primarybuffercoating”thatprotectsitfrommoistureandotherdamage.Transparentglassorplasticfibresarethose,whichallowslighttobeguidedfromoneendtotheotherwithminimalloss.
Unguided mediaUnguided media transport electromagnetic waves without using a physical conductor. This type of communication is often referred to as wireless communication. Signals are normally broadcast through free space and thus are available to anyone who has a device capable of receiving them. Figure below shows the part of the electromagnetic spectrum, ranging from 3 kHz to 900 THz, used for wireless communication.
Radio Microwave Infrared UV X-ray Gama ray
f(Hz) 100 102 104 106 108 1010 1012 1014 1016 1018 1020 1022 1024
Fig. 5.7 Electromagnetic Spectrum
MicrowaveThe transmission of signals takes place by sending microwaves, either directly or via a satellite. The receivers for microwave signals are usually disc shaped antennae from a foot to a few feet across and are often seen installed in business locations or near private homes.
Terrestrial microwaveOne of the unguided media is known as the terrestrial microwave where the signal is transmitted using a parabolic dish. A parabolic dish is used because with the receiver placed at the centre. By the property of the parabola if a wave coming at any angle falls on the surface of the parabola then it is made to pass through the focus. Hence all the signals that fall on the parabola go to the receiver and this gives maximum concentration and a focused beam. This microwave works only in the line of sight hence the sender and the receiver have to be visible to each other. If the receiver and the transmitter are both placed very high where there is no obstruction in between then it can be used for longer distances. In this way it can be used for higher frequencies and get higher data rates.
Satellite microwaveAnother unguided media that we use is satellite microwave. The relay station in this case is the satellite. The signal has to travel a very long distance in the space, hence, it gets attenuated. Therefore, the satellite receives the input signalatonefrequency,amplifiesitandtransmitsitontheotherfrequency.Thereceiver,hence,looksonlyforthefrequencythatiscomingfromthesatelliteandwillnottaketheunamplifiedsignalcomingfromthesender,whichisalso readily available in the surroundings. The frequency at which the signal is sent to the satellite is called the uplink frequency and the frequency at which the signal is transmitted by the satellite is called the down link frequency.
Broadcast radioBroadcast radio is the radio that we receive commonly. It is omni directional which means, the signal goes with the same strength in every direction and wherever we place the receiver we are capable of receiving that signal. An example of this is an FM radio.
InfraredThe other mode is infrared. Here we modulate non-coherent infrared light. The receiver and the transmitter have to be in the line of sight. This is blocked by walls. An example of this is a T.V. remote control.
Data Communications
72/JNU OLE
5.2 Circuit SwitchedLong distance transmission is typically done over a network of switched nodes. Nodes are not concerned with content of data. Data is routed by being switched from node to node.
Circuit switching is the most familiar technique used to build a communications network. It is used for ordinary telephone calls. It allows communications equipment and circuits, to be shared among users. Each user has sole access to a circuit (functionally equivalent to a pair of copper wires) during network use.
Switching Device
Switching Device
Switching Device
Switching Device
Switching Device
Switching Device
Fig. 5.8 Circuit switched network(Source: http://t2.gstatic.com/images?q=tbn:ANd9GcQIgBLl48IiB3y5LcXBsuC2SkfCu8hf9QaW3EPdsuNC0Av
62d3b)A circuit-switched network is dependent on a completed connection. When you place a telephone call, the •connection is not complete until someone at the other end of the line answers. In this way, circuit switching is like being able to reserve a lane on the highway when you visit a friend.In circuit switching, resources remain allocated during the full length of a communication, after a circuit is •established and until the circuit is terminated and the allocated resources are freed. Resources remain allocated evenifnodataisflowingonacircuit,hereby,wastinglinkcapacitywhenacircuitdoesnotcarryasmuchtrafficas the allocation permits. This is a major issue since frequencies (in FDM) or time slots (in TDM) are available in finitequantityoneachlinkandestablishingacircuitconsumesoneofthesefrequenciesorslotsoneachlinkofthecircuit.Asaresult,establishingcircuitsforcommunicationsthatcarrylesstrafficthanallocationpermitscanlead to resource exhaustion and network saturation, preventing further connections from being established. If no circuit can be established between a sender and a receiver because of a lack of resources, the connection •is blocked. A second characteristic of circuit switching is the time cost involved when establishing a connection. In a •communication network, circuit-switched or not, nodes need to lookup in a forwarding table to determine on which link to send incoming data and to actually send data from the input link to the output link. Performing a lookup in a forwarding table and sending the data on an incoming link is called forwarding. •Building the forwarding tables is called routing. In circuit switching, routing must be performed for each •communication, at circuit establishment time. During circuit establishment, the set of switches and links on the path between the sender and the receiver is •determined and messages are exchanged on all the links between the two end hosts of the communication in order to make the resource allocation and build the routing tables.
73/JNU OLE
5.3 DatagramIn Datagram networks, also called Connectionless Networks, the communication between the two sites is on a one-off basis; the packet contains the full addressing information needed to transmit it. If a site sends two datagram in quick succession to the same address they could arrive in reverse order for the intervening nodes are allowed to changetheexactpaththroughthenetworkdependingontrafficflowsandtobypassfaultynodes.Ananalogyisthepostal service; each packet is a separate letter.
In a network environment the client and the server communicate with each-other by reliable channel like TCP socket which have dedicated point-to-point channel between client and server. All data sent over the channel is received and sent in the same order. Datagram are simply a bundle of information data passed between machines. Java implements datagram on top of theUDPprotocolbyusingthreeclasses,whichareinjava.netpackageaswellasdefineasunder:
Datagram Socket•Datagram Packet•Multicast Socket•
In which the Datagram Packet is used to contain data for sent and receive by a application and the Datagram Socket is used to send or receive the Datagram Packets over the network environment. While the Multicast socket is used to broadcast the Datagram Packets to multiple recipients.
Hosts
Hosts
Source
Datagram Packet
Receiver
Fig. 5.9 Datagram network(Source: http://t2.gstatic.com/images?q=tbn:ANd9GcT8M9bNlqTu65QOY1C-s48j-
iISL4GURJ2Alf8T2F6bKN64Ht0x)
Following are the functions of the datagram network:Each packet is treated independently.•Packets can take any practical route.•Packets may arrive out of order.•Packets may go missing.•
Datagram format offers source and destination port numbers used for multiplexing and demultiplexing of data respectively. The service primitive to send a datagram is TD Unit data req (req is for request) with the Destination Address (DA), Destination Port (DP), Source Address (SA) Source port (SP), User Data (UD) as the mandatory parameters. Both the addresses of source and destination are unique. If errors occur when WDP datagram are sent from one WPP entity to another, the Wireless Control Message Protocol (WCMP) provides error handling mechanisms for WDP.
Data Communications
74/JNU OLE
The TD unit of datagram format and service primitive indicates the reception of data (Or incoming data). Here the destination address and port are optional parameters.
Ifahigherlayerrequestsaservice,whichtheWDPcannotfulfil, theerroris indicatedwiththeT-DErrorandservice primitive and an Error Code (EC) is returned indicating the reason of error to the higher layer. This primitive datagram format must only be used for local problems like large size of user data not used to indicate problems with bearer service.
5.4 Virtual Circuit NetworksInaVirtualCircuitnetwork,alsocalledaConnectionNetwork,thesenderfirstattemptstocreatea“virtualcircuit”to the receiver. If successful, the communication is assigned a virtual circuit number in each of the participating nodesandtheneachpacket’saddressisjustthisvirtualcircuitdata.Unlikearealcircuitswitchexchangethereisnoresource, other than some space reserved in circuit tables, which is reserved for the duration of the communication. Note:theterms``sender’’and``receiver’’applyonlytotheprocessofestablishingthevirtualcircuit;oncesetup,datacanflowineitherdirection.Ananalogyisaphonenetwork.Diallingthenumbersetsupthecallestablishinga circuit through all intervening phone exchanges. After that all data passes over this circuit.
A
HI J
P
GFE
CB3
3
33
22
2
2
1
1 1
1
Fig. 5.10 Virtual circuit network(Source: http://t0.gstatic.com/
images?q=tbn:ANd9GcTkplhb5iJUNQufyfTRCHIv2nUQokrP7FTbiVRam9CElVG1S7PZ)
Inthefigureabove,avirtualcircuitbetweennodesAandDisestablishedvianodesB,F,andG.Allpacketsmustgo through this circuit.
Datagram can be transmitted via the virtual circuit in two ways: Thecircuitdoesnotguaranteethedatagram’sdeliverytoitsdestination.(Ifnetworkcongestionoccurs,the•circuit can even throw the datagram away.) An example is the Frame Relay protocol.The virtual circuit can establish a connection and guarantee the data delivery, i.e., the data packets transmitted •arenumberedandthedestinationconfirmstheirreception.Ifanydatagetslost,arequesttoresendthedataismade. For example, this mechanism is used in the X.25 protocol.
Theadvantageofvirtualcircuitsisthattheyarefirstestablished(usingsignalisation)andthenthedataisinsertedonly into the established circuit. Each packet does not have to carry the globally unique address of the destination (complete routing information) in its header. It only needs the circuit ID.
The virtual mechanism is not used on the Internet, which was primarily aimed for use by the U.S. Department of Defence, since the destruction of a node in the virtual circuit would result in the transmission being interrupted—a fact that the authors of TCP/IP did not like. For this reason, IP does not use virtual circuits. Each IP datagram carries a destination IP address (complete routing information) and is therefore transported independently. If a node is destroyed, only the IP datagram currently being transmitted through that particular node are destroyed. The remaining datagram are routed via different nodes.
75/JNU OLE
Frame Relay Frame Relay is a virtual-circuit wide-area network that was designed in response to demands for a new type of WAN in the late 1980s and early 1990s. Prior to Frame Relay, some organisations were using a virtual-circuit switching network called X.25 that performed switching at the network layer. For example, the Internet, which needs wide-area networks to carry its packets from one place to another, used X.25. And X.25 is still being used by the Internet, but it is being replaced by other WANs. However, X.25 has several drawbacks: X.25 has a low 64-kbps data rate. By the 1990s, there was a need for higher data-rate WANs. X.25hasextensiveflowanderrorcontrolatboththedatalinklayerandthenetworklayer.ThiswassobecauseX.25 was designed in the 1970s, when the available transmission media were more prone to errors. Flow and error control at both layers create a large overhead and slow down transmissions. X.25 requires acknowledgments for both data link layer frames and network layer packets that are sent between nodes and between source and destination. Originally X.25 was designed for private use, not for the Internet. X.25 has its own network layer. This means that the user’s data are encapsulated in the network layer packets ofX.25.The Internet, however, hasits own network layer, which means if the Internet wants to use X.25; the Internet must deliver its network layer packet, called a datagram, to X.25 for encapsulation in the X.25 packet. This doubles the overhead.
5.5 Switch Structure
Server
Mac
PC
The packet of data from the server is sent only to the destination workstation connected to the relevant port.
PC
PC
Fig. 5.11 Network switch(Source: http://t3.gstatic.com/images?q=tbn:ANd9GcS9qwfmtxV_5w7D72jOLIroryMiJdozNHV3v53TQmwsA1
aoEmik)
Data Communications
76/JNU OLE
The network switch is the most common network device implemented with company infrastructure and as such the selection of any new switches or upgrading is a key part of most network design projects. The decision to buy new switches or upgrade equipment will be distinct after considering the network assessment and design features specified.Wirelessdesigns,asanexample,willhavenetworkswitchesinterfacingwithaccesspoints.Thatwillhavean affect on the switch such as increased utilisation, assigned switch ports, access control lists, Trunking, Spanning Tree Protocol and increased wattage draw from Power over Ethernet (PoE).
Switch selection processThe following describes the 5 components of any network switch selection process:
Considerthenetworkassessmentanddesignfeatureswhicharespecified.•Select switches that include all the design features.•Select switches with proper scalability.•Balance cost and equipment features while meeting budget guidelines.•Select IOS and/or CatOS software version.•
Centralised switching system
Distributed model•A simplest way of structuring the telecommunication switching is the terminal-to-terminal connection. This kind of switching is called distributed switching and applied only to small telephone system. Some examples ofdistributedswitchingareshownhere.Figurebelowshowsthefullinterconnectionoffiveterminals.
Terminal 1
T2
T3
T4T5
Fig. 5.12 Distributed model
Each terminal has two kinds of switches, one to make required link and other to connect a link to receive a call. By this method, for N terminals, the numbers of links required are 1/2N (N – 1).
Centralised model•The distributed system cannot be extended to large terminal cases and the increased geographical separation of terminals. A simple centralised system, which reduces the average length of transmission link, and hence the transmissioncostisshowninthefigurebelow.Butthissystemincreasesthetotalswitchingcosts.
77/JNU OLE
Centralised switching machine
T1
T2
T3T4
T5
Fig. 5.13 Centralised model
Even though the increase in the number of switching centres lower the total transmission costs, the total switching cost tend to increase for two reasons.
The local centres become more complex because they must be able to decide on a suitable routing to another �centre.Economy of scale is lost with an increased number of local centres because of additional numbers. �
Data Communications
78/JNU OLE
SummaryThe characteristics and quality of a data transmission are determined both by the characteristics of the medium •and the characteristics of the signal.For unguided media, the bandwidth of the signal produced by the transmitting antenna is more important than •the medium in determining transmission characteristics.GuidedTransmissionMediausesa“cabling”systemthatguidesthedatasignalsalongaspecificpath.•Guided Media is also known as Bound Media.•Cable is the medium through which information usually moves from one network device to another.•Twisted pair cable and coaxial cable use metallic (copper) conductors that accept and transport signals in the •form of electric current.Opticalfibreisaglassorplasticcablethatacceptsandtransportssignalsintheformoflight.•Twisted pair cables are most effectively used in systems that use a balanced line method of transmission.•Coaxial cable (or coax) carries signals of higher frequency ranges than twisted-pair cable.•Fibre-optic cabling has higher bandwidth capacity than copper cabling, and is used mainly for high-speed •network Fibre Distributed Data Interface (FDDI) backbones.Signals are normally broadcast through free space and thus are available to anyone who has a device capable •of receiving them.The frequency at which the signal is sent to the satellite is called the uplink frequency.•The frequency at which the signal is transmitted by the satellite is called the down link frequency.•A circuit-switched network is dependent on a completed connection.•In Datagram networks, also called Connectionless Networks, the communication between the two sites is on a •one-off basis; the packet contains the full addressing information needed to transmit it.Datagram are simply a bundle of information data passed between machines.•InaVirtualCircuitnetwork,alsocalledaConnectionNetwork,thesenderfirstattemptstocreatea“virtual•circuit” to the receiver.The virtual circuit can establish a connection and guarantee the data delivery.•Frame Relay is a virtual-circuit wide-area network that was designed in response to demands for a new type •of WAN.X.25hasextensiveflowanderrorcontrolatboththedatalinklayerandthenetworklayer.•A simplest way of structuring the telecommunication switching is the terminal-to-terminal connection.•
ReferencesGodbole, S. A., 2002. • Data communication and network, Tata McGraw-Hill Education.Khurana, M., 2009. • Data Communication System, Laxmi Publications, Ltd.Diablo Team, • Guided Media [Online] Available at: <http://www.sabah.edu.my/cc044.wcdd/guided.html>. [Accessed 15 September 2011].Yvan Pointurier, 2011. • Virtual circuit packet switching [Online] Available at: < http://www.cs.virginia.edu/~mngroup/projects/mpls/documents/thesis/node8.html>. [Accessed 15 August 2011].IIT Kharagpur, Data Communication [Video Online] Available at: <http://freevideolectures.com/Course/2278/•Data-Communication/19> [Accessed 15 September 2011].IIT Kharagpur, Data Communication [Video Online] Available at: < http://freevideolectures.com/Course/2278/•Data-Communication/24> [Accessed 15 September 2011].
79/JNU OLE
Recommended ReadingKundu, S., 2005. • Fundamental of Computer Networks, 2nd, ed., PHI Learning Pvt. Ltd.Gupta, C. P., 2006. Data • Communications and Computer Networks, PHI Learning Pvt. Ltd.SBanzal, S., 2007. • Data and Compute Network Communication, Firewall Media.
Data Communications
80/JNU OLE
Self AssessmentWhich of the following is not used as a transmission medium for LAN?1.
Twisted pair cablea. Open electric wireb. Fibre opticsc. Coaxial cabled.
Which of the following statements is true?2. The characteristics and quality of data transmission are determined both by the characteristics of the medium a. and the characteristics of the signal.The characteristics and quality of data transmission are determined by the characteristics of the medium b. only.The characteristics and quality of data transmission are determined both by the characteristics of the medium c. and the length of the signal.The characteristics and quality of data transmission are determined neither by the characteristics of the d. medium nor by the characteristics of the signal.
Signals at lower frequencies are ______________.3. omni-directionala. unidirectionalb. multidirectionalc. bidirectionald.
What is the another name for Guided Media?4. Unbound Mediaa. Straight Mediab. Bound Mediac. Irregular Mediad.
____________ cables are most effectively used in systems that use a balanced line method of transmission.5. Twisted Paira. Coaxialb. Fibre opticc. Thin Ethernetd.
Which cable type is subject to external electromagnetic interference?6. Shielded Twisted Paira. Thinnet Coaxial Cableb. Thicknet Coaxial Cablec. Unshielded Twisted Paird.
________________carries signals of higher frequency ranges compared to twisted-pair cable.7. Shielded twisted pair cablea. Coaxial cableb. Fibre optic cablec. Unshielded twisted pair cabled.
81/JNU OLE
Infibreoptic,coreissurroundedbyanopticalmaterialcalledthe__________.8. corea. claddingb. wrapc. beamd.
Which media transport electromagnetic waves without using a physical conductor?9. Guided a. Bi-directionalb. Unguidedc. Multidirectionald.
Which of the statements is true?10. If a site sends one datagram in quick succession to the same address they could arrive in reverse order for a. the intervening nodes.If a site sends two datagram in quick succession to different address they could arrive in reverse order for b. the intervening nodes.If a site sends two datagram in quick succession to the same address they could arrive in same order for the c. intervening nodes.If a site sends two datagram in quick succession to the same address they could arrive in reverse order for d. the intervening nodes.
Data Communications
82/JNU OLE
Chapter VI
Error Detection and Correction
Aim
The aim of this chapter is to:
explicate the concept of hamming distance•
enlist the types of errors•
definecyclicredundancycheck•
Objectives
The objectives of this chapter are to:
elucidate framing•
explain decoding•
enlist the types of unguided media•
Learning outcome
At the end of this chapter, you will be able to:
compare HDLC with PPP•
understandflowanderrorcontrol•
describe data link protocols•
83/JNU OLE
6.1 IntroductionNetworks must be able to transfer data from one device to another with acceptable accuracy. For most applications, a system must guarantee that the data received are identical to the data transmitted. Any time data are transmitted from one node to the next, they can become corrupted in passage. Many factors can alter one or more bits of a message. Someapplicationsrequireamechanismfordetectingandcorrectingerrors.Letusfirstdiscusssomeissuesrelated,directly or indirectly, to error detection and correction.
6.1.1 Types of ErrorsWheneverbitsflowfromonepointtoanother,theyaresubjecttounpredictablechangesbecauseofinterference.This interference can change the shape of the signal. In a single-bit error, a 0 is changed to a 1 or a 1 to an 0. In a burst error, multiple bits are changed. For example, an 11100 s burst of impulse noise on a transmission with a data rate of 1200 bps might change all or some of the12 bits of information.
Single-bit errorThe term single-bit error means that only 1 bit of a given data unit (such as a byte, character, or packet) is changed from 1 to 0 or from 0 to 1.
Figure below shows the effect of a single-bit error on a data unit. To understand the impact of the change, imagine that each group of 8 bits is an ASCII character with a 0 bit added to the left. In Figure 10.1,00000010 (ASCII STX) was sent, meaning start of text, but 00001010 (ASCII LF) was received, meaning line feed.
0 0 0 0 0 0 1 0 0 0 0 0 1 0 1 0
0 changed to 1
Sent Received
Fig. 6.1 Single-bit error(Source: http://3.bp.blogspot.com/_xtnf9vSJVSk/SockjyNSV3I/AAAAAAAAAHY/faq6Gr_255c/s400/1.jpg)
Single-bit errors are the least likely type of error in serial data transmission. To understand why, imagine data sent at 1 Mbps. This means that each bit lasts only 1/1,000,000 s, or 1ls. For a single-bit error to occur the noise must have a duration of only 1ls, which is very rare; noise normally lasts much longer than this.
Burst errorThe term burst error means that 2 or more bits in the data unit have changed from 1 to 0 or from 0 to 1.
0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
0 1 0 1 1 1 0 1 0 1 0 0 0 0 1 1
Sent
Length of burst error (5 bits)
Bits corrupted by burst error
ReceivedFig. 6.2 Burst error on a data unit
(Source: http://t2.gstatic.com/images?q=tbn:ANd9GcRFq_6b6ywNlHcHxULNNRbWZZcE6E3YCFBXuAghsyA8l9KYLcom)
Data Communications
84/JNU OLE
Figure above shows the effect of a burst error on a data unit. In this case, 0100010001000011 is sent, but 0101110101100011 is received. Note that a burst error does not necessarily mean that the errors occur in consecutive bits.Thelengthoftheburstismeasuredfromthefirstcorruptedbittothelastcorruptedbit.Somebitsinbetweenmay not have been corrupted.
A burst error is more likely to occur than a single-bit error. The duration of noise is normally longer than the duration of 1 bit, which means that when noise affects data, it affects a set of bits. The number of bits affected depends on the data rate and duration of noise. For example, if we are sending data at 1 kbps, a noise of 11100 s can affect 10 bits; if we are sending data at 1 Mbps, the same noise can affect 10,000 bits.
6.1.2 Detection versus CorrectionThecorrectionoferrorsismoredifficultthanthedetection.Inerrordetection,wearelookingonlytoseeifanyerror has occurred. The answer is a simple yes or no. We are not even interested in the number of errors. A single-bit error is the same for us as a burst error.
In error correction, we need to know the exact number of bits that are corrupted and more importantly, their location in the message. The number of the errors and the size of the message are important factors. If we need to correct one single error in an 8-bit data unit, we need to consider eight possible error locations; if we need to correct two errorsinadataunitofthesamesize,weneedtoconsider28possibilities.Youcanimaginethereceiver’sdifficultyinfinding10errorsinadataunitof1000bits.
6.2 Hamming DistanceOne of the central concepts in coding for error control is the idea of the hamming distance. The hamming distance between two words (of the same size) is the number of differences between the corresponding bits. We show the hamming distance between two words x and y as d(x, y).
ThehammingdistancecaneasilybefoundifwcapplytheXORoperation(ffi)onthetwowordsandcountthenumber of 1s in the result. Note that the hamming distance is a value greater than zero.
6.2.1 Minimum Hamming DistanceAlthough the concept of the hamming distance is the central point in dealing with error detection and correction codes, the measurement that is used for designing a code is the minimum hamming distance. In a set of words, the minimumhammingdistanceisthesmallesthammingdistancebetweenallpossiblepairs.Weuseadmintodefinetheminimumhammingdistanceinacodingscheme.Tofindthisvalue,wefindthehammingdistancesbetweenallwords and select the smallest one.
Three parametersBefore we continue with our discussion, we need to mention that any coding scheme needs to have at least three parameters:
the codeword size n,•the dataword size k, and •the minimum Hamming distance dmin.•
AcodingschemeCiswrittenasC(n,k)withaseparateexpressionfordmin-Forexample,wecancallourfirstcoding scheme C(3, 2) with dmin =2 and our second coding scheme C(5, 2) with dmin ::= 3.
85/JNU OLE
Hamming distance and errorBefore we explore the criteria for error detection or correction, let us discuss the relationship between the hamming distance and errors occurring during transmission. When a codeword is corrupted during transmission, the hamming distancebetweenthesentandreceivedcodeword’sisthenumberofbitsaffectedbytheerror.Inotherwords,thehamming distance between the received codeword and the sent codeword is the number of bits that are corrupted during transmission. For example, if the codeword 00000 is sent and 01101 is received, 3 bits are in error and the hamming distance between the two is d 00000, 01101) =3.
Minimum distance for error detectionNowletusfindtheminimumhammingdistanceinacodeifwewanttobeabletodetectuptoserrors.Ifserrorsoccur during transmission, the Hamming distance between the sent codeword and received codeword is s. If our code is to detect up to s errors, the minimum distance between the valid codes must be s + 1, so that the received codeworddoesnotmatchavalidcodeword.Inotherwords,iftheminimumdistancebetweenallvalidcodeword’sis s + 1, the received codeword cannot be erroneously mistaken for another codeword. The distances are not enough (s + 1) for the receiver to accept it as valid. The error will be detected. We need to clarify a point here: Although a code with dmin =s + 1 may be able to detect more than s errors in some special cases, only s or fewer errors are guaranteed to be detected.
6.3 Cyclic Redundancy CheckWe can create cyclic codes to correct errors. However, the theoretical background required is beyond the scope of this book. In this section, we simply discuss a category of cyclic codes called the cyclic redundancy check (CRC) that is used in networks such as LANs and WANs.
Table below shows an example of a CRC code. We can see both the linear and cyclic properties of this code.
Dataword Codeword Dataword Codeword
0000 0000000 1000 1000101
0001 0001011 1001 1001110
0010 0010110 1010 1010011
0011 0011101 1011 1011000
0100 0100111 1100 1100010
0101 0101100 1101 1101001
0110 0110001 1110 1110100
0111 0111010 1111 1111111
Table 6.1 A CRC code with C (7, 4)
Data Communications
86/JNU OLE
.
Generator
a3 a2 a1 a0 r2 r1 r0 b3 b2 b1 b0 q2 q1 q0
s2 s1 s0
a3 a2 a1 a0a3 a2 a1 a0
Sender
DatawordEncoder
Codeword
Unreliable transmission
Syndrome
DecoderReceiver
Dataword
Correction Logic
Checker
Codeword
Fig. 6.3 Design for the encoder and decoder.(Source: http://www.krazytech.com/wp-content/uploads/untitled.jpg)
In the encoder, the dataword has k bits (4 here); the codeword has n bits (7 here). The size of the dataword is augmented by adding n - k (3 here) 0s to the right-hand side of the word. The n-bit result is fed into the generator. The generator usesadivisorofsizen-k+1(4here),predefinedandagreedupon.Thegeneratordividestheaugmenteddatawordby the divisor (modulo-2 division). The quotient of the division is discarded; the remainder (r2rlr0) is appended to the dataword to create the codeword.
The decoder receives the possibly corrupted codeword. A copy of all n bits is fed to the checker which is a replica of the generator. The remainder produced by the checker is a syndrome of n - k (3 here) bits, which is fed to the decision logic analyser. The analyser has a simple function. If the syndrome bits are all as, the 4 leftmost bits of the codeword are accepted as the dataword (interpreted as no error); otherwise, the 4 bits are discarded (error).
6.3.1 EncoderLet us take a closer look at the encoder. The encoder takes the dataword and augments it with n - k number of as. Itthendividestheaugmenteddatawordbythedivisor,asshowninthefigurebelow.
87/JNU OLE
1 1 1 1 0 1
1 1 0 1 1 0 0 1 0 01 1 0 1
1 0 0 01 1 0 1
1 0 1 01 1 0 1
1 1 1 01 1 0 1
0 1 1 00 0 0 0
1 1 0 01 1 0 1
0 0 1
0 0 0
When the leftmost bit of the remainder is zero,
we must use 0000 instead of the original divisior.
Divisor
Remainder
Quotient
Data plus extra zeros. The number of zeros is one less than the number of bits in the divisor.
Fig. 6.4 Division in CRC encoder(Source:http://www.ecst.csuchico.edu/~sim/547/Old547/notes/NOTE9_1_files/image010.jpg)
The process of modulo-2 binary division is the same as the familiar division process we use for decimal numbers. However, as mentioned at the beginning of the chapter, in this case addition and subtraction are the same. We use the XOR operation to do both.
As in decimal division, the process is done step by step. In each step, a copy of the divisor is XORed with the 4 bits of the dividend. The result of the XOR operation (remainder) is 3 bits (in this case), which is used for the next step after 1 extra bit is pulled down to make it 4 bits long. There is one important point we need to remember in this type of division. If the leftmost bit of the dividend (or the part used in each step) is 0, the step cannot use the regular divisor; we need to use an all-0s divisor.
Whentherearenobitslefttopulldown,wehavearesult.The3-bitremainderformsthecheckbits(r2’rl’andr0). They are appended to the dataword to create the codeword.
6.3.2 DecoderThe codeword can change during transmission. The decoder does the same division process as the encoder. The remainder of the division is the syndrome. If the syndrome is all 0s, there is no error; the dataword is separated from the received codeword and accepted. Otherwise, everything is discarded. Figure below shows two cases: The left handfigureshowsthevalueofsyndromewhennoerrorhasoccurred;thesyndromeis000.Theright-handpartofthefigureshowsthecaseinwhichthereisonesingleerror.Thesyndromeisnotall0s(itis0Il).
Data Communications
88/JNU OLE
1 0 1 0 1 0 1 0
1 0 0 0 1 1 0
1 0 1 1
0 0 0 0
1 1 1 11 0 1 1
1 0 0 0
1 0 1 1
0 1 1
1 0 1 1 1 0 1 1 1 0 0 1 1 1 0
1 0 1 1
0 1 0 10 0 0 0
1 0 1 11 0 1 1
0 0 0 0
0 0 0 0
0 0 0
1 0 0 1 1 0 0 1
Syndrome Syndrome
Codeword
CodewordCodeword
Codeword
DivisionDivision
Dataword accepted
Dataword discarded
1 0 0 1 1 1 0 1 0 0 0 1 1 0
Fig. 6.5 Division in the CRC decoder for two cases
(Source: http://www.scribd.com/doc/51220907/14/Example-10-16)
6.4 ChecksumThe last error detection method we discuss here is called the checksum. The checksum is used in the Internet by severalprotocolsalthoughnotatthedatalinklayer.However,webrieflydiscussitheretocompleteourdiscussionon error checking. Like linear and cyclic codes, the checksum is based on the concept of redundancy. Several protocols still use the checksum for error detection, although the tendency is to replace it with a CRC. This means that the CRC is also used in layers other than the data link layer.
IdeaTheconceptofthechecksumisnotdifficult.Letusillustrateitwithafewexamples.
Example 1Supposeourdataisalistoffive4-bitnumbersthatwewanttosendtoadestination.Inadditiontosendingthesenumbers, we send the sum of the numbers. For example, if the set of numbers is (7, 11, 12, 0, 6), we send (7, 11, 12,0,6,36),where36isthesumoftheoriginalnumbers.Thereceiveraddsthefivenumbersandcomparestheresultwiththesum.Ifthetwoarethesame,thereceiverassumesnoerror,acceptsthefivenumbers,anddiscardsthe sum. Otherwise, there is an error somewhere and the data are not accepted.
Example 2We can make the job of the receiver easier if we send the negative (complement) of the sum, called the checksum. In this case, we send (7, 11, 12, 0, 6, and 36). The receiver can add all the numbers received (including the checksum). If the result is 0, it assumes no error; otherwise, there is an error.
One’s complementThe previous example has one major drawback. All of our data can be written as a 4-bit word (they are less than 15) exceptforthechecksum.Onesolutionistouseone’scomplementarithmetic.Inthisarithmetic,wecanrepresentunsigned numbers between 0 and 2n - 1 using only n bits. t If the number has more than n bits, the extra leftmost bits need to be added to the n rightmostbits(wrapping).Inone’scomplementarithmetic,anegativenumbercanbe represented by inverting all bits (changing a 0 to a 1 and a 1 to a 0). This is the same as subtracting the number from 2n - 1.
89/JNU OLE
Example 3Howcanwerepresentthenumber21inone’scomplementarithmeticusingonlyfourbits?
SolutionThenumber21inbinaryis10101(itneedsfivebits).Wecanwraptheleftmostbitandaddittothefourrightmostbits. We have (0101 + 1) = 0110 or 6.
Example 4Howcanwerepresentthenumber-6inone’scomplementarithmeticusingonlyfourbits?
SolutionInone’scomplementarithmetic,thenegativeorcomplementofanumberisfoundbyinvertingallbits.Positive6are 0110; negative 6 is 1001. If we consider only unsigned numbers, this is 9. In other words, the complement of 6is9.Anotherwaytofindthecomplementofanumberinone’scomplementarithmeticistosubtractthenumberfrom 2n - I (16 - 1 in this case).
Example 5Thefigurebelowshowstheprocessatthesenderandatthereceiver.Thesenderinitialisesthechecksumto0andadds all data items and the checksum (the checksum is considered as one data item and is shown in color). The result is 36. However, 36 cannot be expressed in 4 bits. The extra two bits are wrapped and added with the sum to createthewrappedsumvalue6.Inthefigure,wehaveshownthedetailsinbinary.
The sum is then complemented, resulting in the checksum value 9 (15 - 6 = 9). The sender now sends six data items to the receiver including the checksum 9. The receiver follows the same procedure as the sender. It adds all data items (including the checksum); the result is 45. The sum is wrapped and becomes 15. The wrapped sum is complemented and becomes 0. Since the value of the checksum is 0, this means that the data is not corrupted. The receiver drops the checksum and keeps the other data items. If the checksum is not zero, the entire packet is dropped.
711120603669
7111206945150
1 0 0 1 0 0 36 1 0 1 1 0 1 45
1 0 1 0
0 1 1 0 6 0 1 1 0 151 0 0 0 01 0 0 0 9
Details of wrapping and complementing
Details of wrapping and complementing
Sum SumWrapped sum Wrapped sum
Checksum Checksum
Sender site Receiver site
Packet
1, 11, 12, 0, 6, 9
Fig. 6.6 Process at the sender and at the receiver
Data Communications
90/JNU OLE
Internet checksumTraditionally, the Internet has been using a 16-bit checksum. The sender calculates the checksum by following these steps.
Sender site:Step 1:• The message is divided into 16-bit words.Step 2:• The value of the checksum word is set to 0.Step 3:• Allwordsincludingthechecksumareaddedusingone’scomplementaddition.Step 4:• The sum is complemented and becomes the checksum.Step 5: • The checksum is sent with the data.
The receiver uses the following steps for error detection.Receiver site:
Step 1:• The message (including checksum) is divided into 16-bit words.Step 2: • Allwordsareaddedusingone’scomplementaddition.Step 3: • The sum is complemented and becomes the new checksum.Step 4: • If the value of checksum is 0, the message is accepted; otherwise, it is rejected.
The nature of the checksum (treating words as numbers and adding and complementing them) is well-suited for software implementation. Short programs can be written to calculate the checksum at the receiver site or to check the validity of the message at the receiver site.
6.5 FramingData transmission in the physical layer means moving bits in the form of a signal from the source to the destination. The physical layer provides bit synchronisation to ensure that the sender and receiver use the same bit durations and timing.
The data link layer, on the other hand, needs to pack bits into frames, so that each frame is distinguishable from another. Our postal system practices a type of framing. The simple act of inserting a letter into an envelope separates onepieceofinformationfromanother;theenvelopeservesasthedelimiter.Inaddition,eachenvelopedefinesthesender and receiver addresses since the postal system is a many-to-many carrier facility.
Framing in the data link layer separates a message from one source to a destination, or from other messages to other destinations,byaddingasenderaddressandadestinationaddress.Thedestinationaddressdefineswherethepacketis to go; the sender address helps the recipient acknowledge the receipt.
Although the whole message could be packed in one frame that is not normally done. One reason is that a frame can beverylarge,makingflowanderrorcontrolveryinefficient.Whenamessageiscarriedinoneverylargeframe,even a single-bit error would require the retransmission of the whole message. When a message is divided into smaller frames, a single-bit error affects only that small frame.
6.5.1 Fixed-Size FramingFramescanbeoffixedorvariablesize.Infixed-sizeframing,thereisnoneedfordefiningtheboundariesoftheframes; the size itself can be used as a delimiter. An example of this type of framing is the ATM wide-area network, whichusesframesoffixedsizecalledcells.
6.5.2 Variable-Size FramingOur main discussion in this chapter concerns variable-size framing, prevalent in local area networks. In variable-size framing,weneedawaytodefinetheendoftheframeandthebeginningofthenext.Historically,twoapproacheswere used for this purpose: a character-oriented approach and a bit-oriented approach.
91/JNU OLE
Character-oriented protocolsIn a character-oriented protocol, data to be carried are 8-bit characters from a coding system such as ASCII.The header, which normally carries the source and destination addresses and other control information, and the trailer, which carries error detection or error correction redundant bits, are also multiples of 8 bits. To separate one frame fromthenext,an8-bit(1-byte)flagisaddedatthebeginningandtheendofaframe.Theflag,composedofprotocol-dependent special characters, signals the start or end of a frame.
Character-orientedframingwaspopularwhenonlytextwasexchangedbythedatalinklayers.Theflagcouldbeselected to be any character not used for text communication. Now, however, we send other types of information suchasgraphs,audioandvideo.Anypatternusedfortheflagcouldalsobepartoftheinformation.Ifthishappens,the receiver, when it encounters this pattern in the middle of the data, thinks it has reached the end of the frame.
Tofixthisproblem,abyte-stuffingstrategywasaddedtocharacter-orientedframing.Inbytestuffing(orcharacterstuffing),aspecialbyteisaddedtothedatasectionoftheframewhenthereisacharacterwiththesamepatternastheflag.Thedatasectionisstuffedwithanextrabyte.Thisbyteisusuallycalledtheescapecharacter(ESC),whichhasapredefinedbitpattern.WheneverthereceiverencounterstheESCcharacter,itremovesitfromthedatasectionandtreatsthenextcharacterasdata,notadelimitingflag. Bytestuffingbytheescapecharacterallowsthepresenceoftheflaginthedatasectionoftheframe,butitcreatesanotherproblem.Whathappensifthetextcontainsoneormoreescapecharactersfollowedbyaflag?Thereceiverremovestheescapecharactebutkeepstheflag,whichisincorrectlyinterpretedastheendoftheframe.Tosolvethis problem, the escape characters that are part of the text must also be marked by another escape character. In other words, if the escape character is part of the text, an extra one is added to show that the second one is part of the text. Figure below shows this situation.
Flag Address Control 000111110110011111001000 FCS Flag
Flag Address Control 0 0 0 11111 0 11 0 0 11111 0 0 1 0 0 0 FCS Flag
0001111111001111101000
0001111111001111101000
Data sent
Data received
Frame sent
Frame receivedStuffed and unstuffed
bits
Fig. 6.7 Byte stuffing and unstuffing(Source: http://t1.gstatic.com/images?q=tbn:ANd9GcTJDPcAkK7NYF-EKKqYP-pUvyf838NcLbJSOYKComnq
pS801d11ATi8MxbT)
Character-oriented protocols present another problem in data communications. The universal coding systems in usetoday,suchasUnicode,have16-bitand32-bitcharactersthatconflictwith8-bitcharacters.Wecansaythatingeneral, the tendency is moving toward the bit-oriented protocols that we discuss next.
Bit-oriented protocolsIn a bit-oriented protocol, the data section of a frame is a sequence of bits to be interpreted by the upper layer as text, graphic, audio, video and so on. However, in addition to headers (and possible trailers), we still need a delimiter to separateoneframefromtheother.Mostprotocolsuseaspecial8-bitpatternflag01111110asthedelimitertodefinethebeginningandtheendoftheframe,asshowninfigurebelow.
Data Communications
92/JNU OLE
01111110 Header01111010110••• 11011110 Trailer 01111110
Data from upper layer Variable number of bits
Flag Flag
Fig. 6.8 A frame in a bit-oriented protocol
Thisflagcancreatethesametypeofproblemwesawinthebyte-orientedprotocols.Thatis,iftheflagpatternappearsinthedata,weneedtosomehowinformthereceiverthatthisisnottheendoftheframe.Wedothisbystuffing1singlebit(insteadofIbyte)topreventthepatternfromlookinglikeaflag.Thestrategyiscalledbitstuffing.
Inbitstuffing,ifa0andfiveconsecutiveIbitsareencountered,anextra0isadded.Thisextrastuffedbitiseventuallyremovedfromthedatabythereceiver.Notethattheextrabitisaddedafterone0followedbyfive1sregardlessofthevalueofthenextbit.Thisguaranteesthattheflagfieldsequencedoesnotinadvertentlyappearintheframe. Figurebelowshowsbitstuffingatthesenderandbitremovalatthereceiver.Notethatevenifwehavea0afterfive1s, we still stuff an O. The 0 will be removed by the receiver.
Flag Header 000111110110011111001000 Trailer Flag
Flag Header 000111110110011111001000 Trailer Flag
Data from upper layer
Data to upper layer
Unstuffed
StuffedFrame sent
Frame received
Extra 2 bits
0001111111001111101000
0001111111001111101000
Fig. 6.9 Bit stuffing and unstuffing
Thismeansthatiftheflaglikepattern01111110appearsinthedata,itwillchangeto011111010(stuffed)andisnotmistakenasaflagbythereceiver.Therealflag01111110isnotstuffedbythesenderandisrecognisedbythereceiver.
6.6 Flow and Error ControlData communication requires at least two devices working together, one to send and the other to receive. Even such a basic arrangement requires a great deal of coordination for an intelligible exchange to occur. The most important responsibilitiesofthedatalinklayerareflowcontrolanderrorcontrol.Collectively,thesefunctionsareknownasdata link control.
Flow controlFlow control coordinates the amount of data that can be sent before receiving an acknowledgment and is one of themostimportantdutiesofthedatalinklayer.Inmostprotocols,flowcontrolisasetofproceduresthattellsthesenderhowmuchdataitcantransmitbeforeitmustwaitforanacknowledgmentfromthereceiver.Theflowofdata must not be allowed to overwhelm the receiver.
93/JNU OLE
Any receiving device has a limited speed at which it can process incoming data and a limited amount of memory in which to store incoming data. The receiving device must be able to inform the sending device before those limits are reached and to request that the transmitting device send fewer frames or stop temporarily. Incoming data must be checked and processed before they can be used. The rate of such processing is often slower than the rate of transmission. For this reason, each receiving device has a block of memory, called a buffer, reserved for storing incomingdatauntiltheyareprocessed.Ifthebufferbeginstofillup,thereceivermustbeabletotellthesendertohalt transmission until it is once again able to receive.
Error controlError control is both error detection and error correction. It allows the receiver to inform the sender of any frames lost or damaged in transmission and coordinates the retransmission of those frames by the sender. In the data link layer, the term error control refers primarily to methods of error detection and retransmission. Error control in the datalinklayerisoftenimplementedsimply:Anytimeanerrorisdetectedinanexchange,specifiedframesareretransmitted. This process is called automatic repeat request (ARQ).
6.7 Data Link ProtocolsThedatalinklayercancombineframing,flowcontrolanderrorcontroltoachievethedeliveryofdatafromonenode to another. The protocols are normally implemented in software by using one of the common programming languages. The division of protocols can be done into those that can be used for noiseless (error-free) channels andthosethatcanbeusedfornoisy(error-creating)channels.Theprotocolsinthefirstcategorycannotbeusedinreallifebuttheyserveasabasisforunderstandingtheprotocolsofnoisychannels.Thefigurebelowshowstheclassifications.
Protocols
For noiseless channel
For noisy channel
SimplestStop-and-Wait
Stop-and-wait ARQGo-hack-N ARQ
Selective Repeat ARQ
Fig. 6.10 Taxonomy of protocols
There is a difference between the protocols we discuss here and those used in real networks. All the protocols we discuss are unidirectional in the sense that the data frames travel from one node, called the sender, to another node, called the receiver. Although special frames, called acknowledgment (ACK) and negative acknowledgment (NAK) canflowintheoppositedirectionforflowanderrorcontrolpurposes,dataflowinonlyonedirection.
6.8 HDLCHigh-Level Data Link Control, also know as HDLC, is a bit oriented, switched and non-switched protocol. It is a data link control protocol, and falls within layer 2, the Data Link Layer, of the Open Systems Interface (OSI) model. HDLC is a protocol developed by the International Organisation for Standardisation (ISO). It falls under the ISO standards ISO 3309 and ISO 4335.
It has found itself being used throughout the world. It has been so widely implemented because it supports both half duplex and full duplex communication lines, point to point(peer to peer) and multi-point networks, and switched or non-switched channels. The procedures outlined in HDLC are designed to permit synchronous, code-transparent data transmission.OtherbenefitsofHDLCarethatthecontrolinformationisalwaysinthesameposition,andspecificbitpatterns used for control differ dramatically from those in representing data, which reduces the chance of errors.
Data Communications
94/JNU OLE
It has also led to many subsets. Two subsets widely in use are Synchronous Data Link Control (SDLC) and Link Access Procedure-Balanced (LAP-B).
HDLC offers a master-slave arrangement in which the master station (in charge of the link) issues command frames and the slave stations reply with response frames.
It is also possible for a station to assume a hybrid identity (master and slave) so that it can both issue commands and send responses. HDLC offers three modes of operation:
Normal Response Mode (NRM):• In this mode, a slave station is unable to initiate a transmission; it can only transmit in response to a command from the master station. The master station is responsible for managing the transmission. This mode is typically used for multipoint lines, where a central station (e.g., a host computer) polls a set of other stations (e.g., PCs, terminals, printers, etc.).Asynchronous Response Mode (ARM):• In this mode, a slave station can initiate a transmission on its own accord. However, the master station is still responsible for managing the transmission. This mode is now largely obsolete.Asynchronous Balanced Mode (ABM):• In this mode, all stations are of the hybrid form with equal status. Eachstationmaytransmitonitsownaccord.Thismodeisbest-suitedtopoint-to-pointconfigurations.
TheHDLCframestructureisshowninthefigurebelow.TheAddressfieldandtheControlfieldareoneortwooctetseach.TheChecksumfieldistwooctetsandusestheCRCmethod.TheDatafieldisofarbitrarylength,andmay be zero for some messages.
0 1 1 1 1 1 1 0 Address Control Data Checksum 0 1 1 1 1 1 1 0
Bits 8 8 8 > 0 16 8
Fig. 6.11 HDLC frame format(Source: http://train-srv.manipalu.com/wpress/wp-content/uploads/2009/04/041509-0833-mi0026unit71.png)
There are a number of HDLC-related protocols, often referred to as HDLC subsets. These include:The Link Access Procedure (LAP) is based on the SARM command of HDLC. Under LAP, the transmitting •station sends a SARM to the receiving station. The latter responds with a UA. At its discretion, the receiving station may interpret the receiving of a SARM command as a request for transmission in the opposite direction, in which case the roles are reversed.The Link Access Protocol Balanced (LAP-B) is an ABM subset of HDLC designed for use with X.25. It is •used for establishing a link between a DCE and a DTE. LAP-B does not support the SREJ command, and only supports a limited number of the unnumbered commands.The Link Access Protocol, D channel (LAP-D) is an HDLC subset designed for use with ISDN.•The Logical Link Control (LLC) is an HDLC subset designed as a part of the IEEE 802 series of standards for •use with LANs.
95/JNU OLE
6.9 PPPThe Point-to-Point Protocol (PPP) is a data link layer protocol which encapsulates other network layer protocols for transmissiononsynchronousandasynchronouscommunicationlines.Twoprecisedefinitionsof“point-to-point”in the context of data communications are as follows:
Anetworkconfigurationinwhichaconnectionisestablishedbetweentwoandonlytwopoints.Theconnection•may include switching facilities.A circuit connecting two points without the use of any intermediate terminal or computer.•
This definition explains the point-to-point aspect of PPP.The protocol aspect lies in the fact that PPP is theintermediate packet structure which facilitates transmission of higher level protocols, such as TCP/IP, across diverse communication links.
PPP provides a method for transmitting datagrams over serial point-to-point links. PPP contains three main components:
A method for encapsulating datagrams over serial links. PPP uses the High-Level Data Link Control (HDLC) •protocol as a basis for encapsulating datagrams over point-to-point links. AnextensibleLCPtoestablish,configureandtestthedatalinkconnection.•AfamilyofNCPsforestablishingandconfiguringdifferentnetworklayerprotocols.PPPisdesignedtoallow•the simultaneous use of multiple network layer protocols.
Toestablishcommunicationsoverapoint-to-pointlink,theoriginatingPPPfirstsendsLCPframestoconfigureand (optionally) test the data link. After the link has been established and optional facilities have been negotiated asneededbytheLCP,theoriginatingPPPsendsNCPframestochooseandconfigureoneormorenetworklayerprotocols.Wheneachofthechosennetworklayerprotocolshasbeenconfigured,packetsfromeachnetworklayerprotocolcanbesentoverthelink.ThelinkwillremainconfiguredforcommunicationsuntilexplicitLCPorNCPframes close the link, or until some external event occurs.
Frame format
01111110 11111111 00000011 Protocol Info Check 01111110
1 1 1 1or 2 2 or 4 1Variablelength
Flag FlagAddress Control
Fig. 6.12 PPP frame format (Source: http://t3.gstatic.com/images?q=tbn:ANd9GcSQM8OAHC3_Tsj8Hd0GdxhssBxJmKBr6Z_0cJIO95sEJT
CY2fnRQZIhAp8j)
ThePPPframecontainsthefollowingfields:Flag field: • EveryPPPframebeginsandendswithaonebyteflagfieldwithavalueof01111110.Address field:• Theonlypossiblevalueforthisfieldis11111111.Control field:• Theonlypossiblevalueofthisfieldis00000011.Becauseboththeaddressandcontrolfieldscancurrentlytakeonlyafixedvalue,onewonderswhythefieldsareevendefinedinthefirstplace.ThePPPspecificationstatesthatothervalues“maybedefinedatalatertime,”althoughnonehavebeendefinedtodate.Becausethesefieldstakefixedvalues,PPPallowsthesendertosimplynotsendtheaddressandcontrolbytes,thus, saving two bytes of overhead in the PPP frame.
Data Communications
96/JNU OLE
Protocol:• TheprotocolfieldtellsthePPPreceivertheupper-layerprotocoltowhichthereceivedencapsulateddata(thatis,thecontentsofthePPPframe’sinfofield)belongs.OnreceiptofaPPPframe,thePPPreceiverwillcheck the frame for correctness and then pass the encapsulated data on to the appropriate protocol. RFC 1700 definesthe16-bitprotocolcodesusedbyPPP.OfinteresttousistheIPprotocol(thatis,thedataencapsulatedin the PPP frame is an IP datagram) which has a value of 21 hexadecimal, other network-layer protocols such as AppleTalk (29) and DECnet (27), the PPP link control protocol (C021 hexadecimal) that we discuss in detail in the following section and the IP Control Protocol (8021). This last protocol is called by PPP when a link is firstactivatedinordertoconfiguretheIP-levelconnectionbetweentheIP-capabledevicesoneachendofthelink (see below). Information: • Thisfieldcontainstheencapsulatedpacket(data)thatisbeingsentbyanupper-layerprotocol(forexample,IP)overthePPPlink.Thedefaultmaximumlengthoftheinformationfieldis1,500bytes,althoughthiscanbechangedwhenthelinkisfirstconfigured,asdiscussedbelow.Checksum: • Thechecksumfieldisusedtodetectbiterrorsinatransmittedframe.Ituseseitheratwoorfourbyte HDLC-standard cyclic redundancy code.
97/JNU OLE
SummaryWhenever bits flow from one point to another, they are subject to unpredictable changes because of •interference.The term single-bit error• means that only 1 bit of a given data unit is changed from 1 to 0 or from 0 to 1.The term burst error means that 2 or more bits in the data unit have changed from 1 to 0 or from 0 to 1.•The Hamming distance between two words is the number of differences between the corresponding bits.•When a codeword is corrupted during transmission, the Hamming distance between the sent and received •codeword’sisthenumberofbitsaffectedbytheerror.A category of cyclic codes called the cyclic redundancy check (CRC) that is used in networks such as LANs •and WANs.The checksum is used in the Internet by several protocols although not at the data link layer.• The Internet has been using a 16-bit checksumData transmission in the physical layer means moving bits in the form of a signal from the source to the •destination.Framing in the data link layer separates a message from one source to a destination or from other messages •to other destinations, by adding a sender address and a destination address. Framescanbeoffixedorvariablesize.The variable-size framing, are prevalent in local area networks.•Bytestuffingbytheescapecharacterallowsthepresenceoftheflaginthedatasectionoftheframe.•In a bit-oriented protocol, the data section of a frame is a sequence of bits to be interpreted by the upper layer •as text, graphic, audio, video, and so on.Flow control coordinates the amount of data that can be sent before receiving an acknowledgment and is one •of the most important duties of the data link layer.Each receiving device has a block of memory, called a buffer• , reserved for storing incoming data until they are processed.Error control is both error detection and error correction.•HDLC, is a bit oriented, switched and non-switched protocol.•The Point-to-Point Protocol (PPP) is a data link layer protocol which encapsulates other network layer protocols •for transmission on synchronous and asynchronous communication lines.
References Amutha Jeevakumari, S.A., 2008. • Elements of Data Communication and Networks, Laxmi Publications, Ltd.Beyda, J. W., 1996. • Data communications: from basics to broadband, Prentice Hall.Error Detection, Correction, and Related Topics• [Online] Available at: < http://www.cs.nmsu.edu/~pfeiffer/classes/573/notes/ecc.html>. [Accessed 15 September 2011].www.pulsesupply.com, 2011. • High Level Data Link Control [Online] Available at: < http://www.pulsewan.com/data101/hdlc_basics.htm>. [Accessed 15 September 2011].Prof. Pal, A., 2005. • Lecture 16: Error Detection and Correction [Video Online] Available at :< http://freevideolectures.com/Course/2278/Data-Communication/16> [Accessed 15 September 2011].Prof. Pal, A., 2005. • Lecture 16: Error Detection and Correction [Video Online] Available at :< http://freevideolectures.com/Course/2278/Data-Communication/17> [Accessed 15 September 2011].
Data Communications
98/JNU OLE
Recommended ReadingAhmad, A., 2003. • Data communication principles: for fixed and wireless networks, Springer.Spohn, L. D., 2003. • Data Network Design, 3rd ed., Tata McGraw-Hill Education.Held, G., 2002. • Understanding Data Communications, 7th ed., Pearson Education India.
99/JNU OLE
Self AssessmentIn a _______ error type, multiple bits are changed.1.
single bita. burstb. noisec. multi-bitd.
In_______________, it is required to know the exact number of bits that are corrupted.2. error correctiona. error detectionb. error avoidancec. error creationd.
The _____________ between two words (of the same size) is the number of differences between the 3. corresponding bits.
Checksuma. Cyclic Redundancy Checkb. Point to Point Protocolc. Hamming distanced.
What is created to correct errors?4. Checksuma. Cyclic codesb. Hamming distancec. HDLCd.
The ___________is used in the Internet by several protocols although not at the data link layer. 5. Cyclic Redundancy Checka. hamming distanceb. checksumc. HDLCd.
The ______________ provides bit synchronisation to ensure that the sender and receiver uses the same bit 6. durations and timing.
physical layera. data link layerb. application layerc. presentation layerd.
Which of the following statements is true?7. When a message is divided into larger frames, a single-bit error affects only that small frame.a. When a message is divided into smaller frames, a single-bit error affects only that small frame.b. When a message is divided into smaller frames, a burst error affects only that small frame.c. When a message is divided into smaller frames, a single-bit error affects the entire frame.d.
Data Communications
100/JNU OLE
What kind of framing are prevalent in Local Area Networks?8. Fixed size framinga. Variable size framingb. Multi-bit framingc. Single-byte framingd.
Anytimeanerrorisdetectedinanexchange,specifiedframesareretransmittedandiscalled____________.9. Automatic Repeat Request (ARQ)a. High-Level Data Link Control (HDLC)b. Point-to-Point Protocol (PPP)c. Normal Response Mode (NRM)d.
Which protocol in data link layer offers master slave arrangement?10. HDLC (High-Level Data Link Control)a. PPP (Point-to-Point Protocol)b. SLIP (Serial Line Internet Protocol)c. SDLC (Synchronous Data Link Control)d.
101/JNU OLE
Chapter VII
Multiple Accesses
Aim
The aim of this chapter is to:
explicate the concept of carrier sense multiple access•
enlist the types carrier sense multiple access•
definecontrolledaccess•
Objectives
The objectives of this chapter are to:
elucidate reservation•
explain token passing•
enlist the types of polling•
Learning outcome
At the end of this chapter, you will be able to:
explain channelisation•
understand frequency division multiple access•
discuss• encoding and decoding
Data Communications
102/JNU OLE
7.1 CSMA/CATo minimise the chance of collision and increase the performance, the CSMA method was developed. The chance of collision can be reduced if a station senses the medium before trying to use it. Carrier sense multiple access (CSMA)requiresthateachstationfirstlistentothemedium(orcheckthestateofthemedium)beforesending.Inotherwords,CSMAisbasedontheprinciple“sensebeforetransmit”or“listenbeforetalk.”
CSMAcanreducethepossibilityofcollisionbutitcannoteliminateit.Thereasonforthisisshowninthefigurebelow a space and time model of a CSMA network. Stations are connected to a shared channel (usually a dedicated medium).
The possibility of collision still exists because of propagation delay; when a station sends a frame, it still takes time (althoughveryshort)forthefirstbittoreacheverystationandforeverystationtosenseit..
A B C D
Area where A’ssignalexists
Area where both signal exists
Arear where B’ssignalexists
Time Time
B starts at time t1
C starts at time t2
t1
t2
Fig. 7.1 Space/time model of the collision in CSMA
Inotherwords,astationmaysensethemediumandfinditidle,onlybecausethefirstbitsentbyanotherstationhasnotyetbeenreceived.Attimet1’ stationBsensesthemediumandfindsitidle,soitsendsaframe.Attimet2(t2>tI)’ stationCsensesthemediumandfindsitidlebecause,atthistime,thefirstbitsfromstationBhavenotreachedstation C. Station C also sends a frame. The two signals collide and both frames are destroyed.
CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) is a protocol for carrier transmission in 802.11 networks. Unlike CSMA/CD (Carrier Sense Multiple Access/Collision Detect) which deals with transmissions after a collision has occurred, CSMA/CA acts to prevent collisions before they happen.
In CSMA/CA, as soon as a node receives a packet that is to be sent, it checks to be sure the channel is clear (no other node is transmitting at the time). If the channel is clear, then the packet is sent. If the channel is not clear, the node waits for a randomly chosen period of time and then checks again to see if the channel is clear. This period of time is called the back-off factor and is counted down by a back-off counter. If the channel is clear when the back-off counter reaches zero, the node transmits the packet. If the channel is not clear when the back-off counter reaches zero, the back-off factor is set again and the process is repeated.
103/JNU OLE
However, in a wireless network, much of the sent energy is lost in transmission. The received signal has very little energy. Therefore, a collision may add only 5 to 10 percent additional energy. This is not useful for effective collision detection.
We need to avoid collisions on wireless networks because they cannot be detected. Carrier sense multiple access with collision avoidance (CSMA/CA) was invented for this network.
7.1.1 CSMA/CA and Wireless NetworksCSMA/CA was mostly intended for use in wireless networks. We will see how the arising issues are solved by augmenting the above protocol with hand-shaking features.
Start
Set back-off to zero
Persistence strategy
Wait DIFS
Sent RTS
Set a timer
CTS received before time-out?
No
NoNo
Yes
YesYesSuccessAbort
Back-off limit?
Wait back-off time
Increment back-off
Wait SIFS
Send the frame
Set a timer
ACK received before time-out?
Fig. 7.2 Flow diagram for CSMA/CA(Source: http://t0.gstatic.com/images?q=tbn:ANd9GcRBWkNSA3ATmMy9_
Fq2KZJOZVepmymCbmtPKWutvUbIJBkeZRdJ)
7.2 CSMA/CDThe CSMA method does not specify the procedure following a collision. Carriers sense multiple access with collision detection (CSMA/CD) augments the algorithm to handle the collision.
In this method, a station monitors the medium after it sends a frame to see if the transmission was successful. If so, thestationisfinished.If,however,thereisacollision,theframeissentagain.
TobetterunderstandCSMA/CD,letuslookatthefirstbitstransmittedbythetwostationsinvolvedinthecollision.Although each station continues to send bits in the frame until it detects the collision, we show what happens as the firstbitscollide.Infigureshownbelow,stationsAandCareinvolvedinthecollision.
Data Communications
104/JNU OLE
Station A Station B
Collision
Fig. 7.3 CSMA/CD(Source: http://t2.gstatic.com/images?q=tbn:ANd9GcQmOpE21ovN3UScSVAalTlJwTD5EWGmgBM
6nc1AV-T5Ip-VjCal)
Station A attempts to send a frame across the network. First, station A checks to see if the network is available (carrier sense).Ifthenetworkisnotavailable,stationAwaitsuntilthecurrentsenderonthemediumhasfinished.
Let’ssupposethatstationAbelievesthenetworkisavailableandtriessendingaframe.Becausethenetworkisshared (multiple access), other stations on the same network segment might also attempt to send it at the same time (station B, for instance).
ShortlyafterstationBattemptstosendtrafficacrosstheline,bothstationAandstationBrealisethatanotherdeviceis attempting to send a frame (collision detection). Each station waits a random amount of time before sending again. The time after the collision is divided into time slots; station A and station B each pick a random slot for attempting a retransmission.
Should station A and station B attempt to retransmit at the same time, they extend the amount of time each waits before trying again, decreasing the chance of resending data in the same time slot.
7.3 Controlled AccessIncontrolledaccess,thestationsconsultoneanothertofindwhichstationhastherighttosend.Astationcannotsend unless it has been authorised by other stations. Following are the three popular controlled access methods:
ReservationIn the reservation method, a station needs to make a reservation before sending data. Time is divided into intervals. In each interval, a reservation frame precedes the data frames sent in that interval. If there are N stations in the system, there are exactly N reservation mini slots in the reservation frame. Each mini slot belongs to a station. When a station needs to send a data frame, it makes a reservation in its own mini slot. The stations that have made reservations can send their data frames after the reservation frame.
1 2 3 4 5 1 2 3 4 5 1 2 3 4 5Data
station 1Data
station 3Data
station 4Data
station 1Reservation
frame
0 0 0 0 0 1 0 0 0 0 1 0 1 1 0
Fig. 7.4 Reservation access method
105/JNU OLE
PollingPolling works with topologies in which one device is designated as a primary station and the other devices are secondary stations. All data exchanges must be made through the primary device even when the ultimate destination is a secondary device.
The primary device controls the link; the secondary devices follow its instructions. It is up to the primary device to determine which device is allowed to use the channel at a given time. The primary device, therefore, is always the initiator of a session.
Primary
SelectPoll
SEL
ACK
Data
ACK
PrimaryA B
PollNAK
PollDataACK
Fig. 7.5 Select and poll functions in polling access method
If the primary wants to receive data, it asks the secondary if they have anything to send; this is called poll function. If the primary wants to send data, it tells the secondary to get ready to receive; this is called select function.
SelectThe select function is used whenever the primary device has something to send. Remember that the primary controls the link. If the primary is neither sending nor receiving data, it knows the link is available.
If it has something to send, the primary device sends it. What it does not know, however, is whether the target device is prepared to receive. So the primary must alert the secondary to the upcoming transmission and wait for anacknowledgmentofthesecondary’sreadystatus.Beforesendingdata,theprimarycreatesandtransmitsaselect(SEL)frame,onefieldofwhichincludestheaddressoftheintendedsecondary.
PollThe poll function is used by the primary device to solicit transmissions from the secondary devices. When the primary isreadytoreceivedata,itmustask(poll)eachdeviceinturnifithasanythingtosend.Whenthefirstsecondaryisapproached, it responds either with a NAK frame if it has nothing to send or with data (in the form of a data frame) if it does. If the response is negative (a NAK frame), then the primary polls the next secondary in the same manner untilitfindsonewithdatatosend.Whentheresponseispositive(adataframe),theprimaryreadstheframeandreturns an acknowledgment (ACK frame), verifying its receipt.
Token passingIn the token-passing method, the stations in a network are organised in a logical ring. In other words, for each station, there is a predecessor and a successor. The predecessor is the station which is logically before the station in the ring; the successor is the station which is after the station in the ring. The current station is the one that is accessing the channel now. The right to this access has been passed from the predecessor to the current station. The right will be passed to the successor when the current station has no more data to send.
Data Communications
106/JNU OLE
Token management is needed for this access method. Stations must be limited in the time they can have possession of the token. The token must be monitored to ensure it has not been lost or destroyed. For example, if a station that is holding the token fails, the token will disappear from the network. Another function of token management is to assignprioritiestothestationsandtothetypesofdatabeingtransmitted.Andfinally,tokenmanagementisneededto make low-priority stations release the token to high priority stations.
7.4 ChannelisationChannelisation is a multiple-access method in which the available bandwidth of a link is shared in time, frequency, or through code, between different stations. The three channelisation protocols: FDMA, TDMA and CDMA.
7.4.1 Frequency Division Multiple AccessIn Frequency-Division Multiple Access (FDMA), the available bandwidth is divided into frequency bands. Each stationisallocatedabandtosenditsdata.Inotherwords,eachbandisreservedforaspecificstation,anditbelongsto the station all the time.
Eachstationalsousesabandpassfiltertoconfinethetransmitterfrequencies.Topreventstationinterferences,theallocated bands are separated from one another by small guard bands.
Data
Common channel
1
43
2
Data Data
DataSilent
f
f
f f
f
t
t t
Fig. 7.6 Frequency Division Multiple Access
FDMAspecifiesapredeterminedfrequencybandfortheentireperiodofcommunication.Thismeansthatstreamdata(acontinuousflowofdatathatmaynotbepacketised)caneasilybeusedwithFDMA.Weneedtoemphasisethat although FDMA and FDM conceptually seem similar, there are differences between them. FDM is a physical layer technique that combines the loads from low-bandwidth channels and transmits them by using a high-bandwidth channel. The channels that are combined are low-pass. The multiplexer modulates the signals, combines them, and creates a bandpass signal. The bandwidth of each channel is shifted by the multiplexer.
FDMA, on the other hand, is an access method in the data link layer. The data link layer in each station tells its physical layer to make a bandpass signal from the data passed to it. The signal must be created in the allocated band. There is no physical multiplexer at the physical layer. The signals created at each station are automatically bandpass-filtered.Theyaremixedwhentheyaresenttothecommonchannel.
107/JNU OLE
7.4.2 Time Division Multiple Access (TDMA)In time-division multiple access (TDMA), the stations share the bandwidth of the channel in time. Each station is allocated a time slot during which it can send data. Each station transmits its data in is assigned time slot.
21
Data
Data
Data
4
DataSilent
Common channel
3
f
f
f f
f
t
Fig. 7.7 Time Division Multiple Access
The main problem with TDMA lies in achieving synchronisation between the different stations. Each station needs to know the beginning of its slot and the location of its slot.
Thismaybedifficultbecauseofpropagationdelaysintroducedinthesystemifthestationsarespreadoveralargearea. To compensate for the delays, we can insert guard times. Synchronisation is normally accomplished by having some synchronisation bits (normally referred to as preamble bits) at the beginning of each slot.
We also need to emphasise that although TDMA and TDM conceptually seem the same, there are differences between them. TDM is a physical layer technique that combines the data from slower channels and transmits them by using a faster channel. The process uses a physical multiplexer that interleaves data units from each channel.
TDMA, on the other hand, is an access method in the data link layer. The data link layer in each station tells its physical layer to use the allocated time slot. There is no physical multiplexer at the physical layer.
7.4.3 Code Division Multiple AccessCode-division multiple access (CDMA) was conceived several decades ago. Recent advances in electronic technology havefinallymadeitsimplementationpossible.CDMAdiffersfromFDMAbecauseonlyonechanneloccupiestheentire bandwidth of the link. It differs from TDMA because all stations can send data simultaneously; there is no timesharing.
Data Communications
108/JNU OLE
1 2
43
d1
d3 d4
d2
d1 • c1
d1 • c1 + + +
d3 • c3
d3 • c3
d4 • c4
d4 • c4
d2 • c2
d2 • c2Common channel
Data
Fig. 7.8 Simple idea of communication with code
Let us assume we have four stations 1, 2, 3 and 4 connected to the same channel. The data from station 1 are d1 from station 2 are d2, andsoon.Thecodeassignedtothefirststationisc1,tothesecondisc2, and so on. We assume that the assigned codes have two properties.
If we multiply each code by another, we get 0.•If we multiply each code by itself, we get 4 (the number of stations).•
With these two properties in mind, let us see how the above four stations can send data using the same common channel,asshowninthefigureaboveStation1multipliesitsdatabyitscodetogetd l . c1 Station 2 multiplies its data by its code to get d2 . c2 and so on. The data that go on the channel are the sum of all these terms, as shown in the box. Any station that wants to receive data from one of the other three multiplies the data on the channel by the code of the sender. For example, suppose stations 1 and 2 are talking to each other. Station 2 wants to hear what station1issaying.Itmultipliesthedataonthechannelbycl’thecodeofstation1.
Because (cl. cl) is 4, but (c2 . c1), (c3 . c1), and (c4 . cl) are all 0s, station 2 divides the result by 4 to get the data from station 1.
ChipsCDMA is based on coding theory. Each station is assigned a code, which is a sequence of numbers called chips. They are called orthogonal sequences and have the following properties:
Each sequence is made of N elements, where N is the number of stations. �If we multiply a sequence by a number, every element in the sequence is multiplied by that element. This �is called multiplication of a sequence by a scalar.If we multiply two equal sequences, element by element, and add the results, we get N, where N is the �number of elements in the each sequence. This is called the inner product of two equal sequences.If we multiply two different sequences, element by element, and add the results, we get O. This is called �inner product of two different sequences.Adding two sequences means adding the corresponding elements. The result is another sequence. �
109/JNU OLE
Data representationWe must follow the following rules for encoding:
If a station needs to send a 0 bit, it encodes it as -1; if it needs to send a 1 bit, it encodes it as +1. When a station is idle, it sends no signal, which is interpreted as an O.
Data bit 0 Data bit I Silence 0- I + 1
Fig. 7.9 Data representation in CDMA
Encoding and decodingFromthefigureabovewegetthatfourstationssharethelinkduringaI-bitinterval.Theprocedurecaneasilyberepeated for additional intervals. We assume that stations 1 and 2 are sending a 0 bit and channel 4 is sending a 1 bit. Station 3 is silent. The data at the sender site are translated to -1, -1, 0, and +1. Each station multiplies the corresponding number by its chip (its orthogonal sequence), which is unique for each station. The result is a new sequence which is sent to the channel. For simplicity, we assume that all stations send the resulting sequences at thesametime.Thesequenceonthechannelisthesumofallfoursequencesasdefinedbefore.Figure12.26showsthe situation.
Now imagine station 3, which we said is silent, is listening to station 2. Station 3 multiplies the total data on the channel by the code for station 2, which is [+1 -1 +1-1], to get :
bit 1
1
1 1
1
Bit a–1 –1
C1
C3 C4
C2
d1 • c1
d3 • c3 d4 • c4
d2 • c21-1 +1 +1 +IJ
[+1 +1 -1 -IJ 1+1 -1 -1 +11
1-1 -I +1 +1J
[-1 -1 -1 -1 ]
[0 0 0 0]
0 +1
[+1 -1 -1 +1]
[-1 +1 -1 +1]
Bit a
Bit 1Silent
[-1 -1 -3+1]Data
Common channel
Fig. 7.10 Sharing channel in CDMA
Signal levelThe process can be better understood if we show the digital signal produced by each station and the data recovered atthedestination.Thefigurebelowshowsthecorrespondingsignalsforeachstation(usingNRZ-Lforsimplicity)and the signal that is on the common channel.
Data Communications
110/JNU OLE
1
2
3
4
BitO 1-1 -1 -I -I]
1-1 +1 -I +I]
[0 0 0 0]
1+1 -1 -I +1]
BitO
Silent
Bit I
Time
Time
Time
Time
TimeData on the channel
Fig. 7.11 Digital signal created by four stations in CDMA
Figure below shows how station 3 can detect the data sent by station 2 by using the code for station 2. The total data on the channel are multiplied (inner product operation) by the signal representing station 2 chip code to get a new signal. The station then integrates and adds the area under the signal, to get the value -4, which is divided by 4 and interpreted as bit 0.
3
TimeData on the channel
Station2’scode
Inner product result
Summing the values
[+1 -I +1 -1]Time
Time
Time
BitO-I- 4/4-4
Fig. 7.12 Decoding of the composite signal for one in CDMA
Sequence generationTo generate chip sequences, we use a Walsh table, which is a two-dimensional table with an equal number of rows and columns.
111/JNU OLE
In the Walsh table, each row is a sequence of chips. W1 for a one-chip sequence has one row and one column. We can choose –l or +1 for the chip for this trivial table (we chose +1). According to Walsh, if we know the table for N sequencesWN’wecancreatethetablefor2NsequencesW2N, as shown in equation above. The WN with the over bar WN stands for the complement of WN’whereeach+1ischangedto-1andviceversa.
Equation above also shows how we can create W2 and W4 from W1. After we select W1, W2 can be made from four Wj ‘s, with the last one the complement of Wl’AfterW2 is generated, W4 can be made of four W2’s,withthelastone the complement of W2. Of course, Ws is composed of four W4’s,andsoon.NotethatafterWNismade,eachstation is assigned a chip corresponding to a row.
Something we need to emphasise is that the number of sequences N needs to be a power of 2. In other words, we need to have N = 2m.
Data Communications
112/JNU OLE
SummaryTo minimise the chance of collision and increase the performance, the CSMA method was developed.•Carriersensemultipleaccess(CSMA)requiresthateachstationfirstlistentothemedium(orcheckthestate•of the medium) before sending.CSMA can reduce the possibility of collision, but it cannot eliminate it.•CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) is a protocol for carrier transmission •in 802.11 networks.It needs to avoid collisions on wireless networks because they cannot be detected.•In • controlled access, thestationsconsultoneanothertofindwhichstationhastherighttosend.In the • reservation method, a station needs to make a reservation before sending data.Polling • works with topologies in which one device is designated as a primary station and the other devices are secondary stations.The • select function is used whenever the primary device has something to send.The • poll function is used by the primary device to solicit transmissions from the secondary devices.In the token-passing method, the stations in a network are organized in a logical ring.•Channelisation is a multiple-access method in which the available bandwidth of a link is shared in time, frequency, •or through code, between different stations.In frequency-division multiple access (FDMA), the available bandwidth is divided into frequency bands.•In time-division multiple access (TDMA), the stations share the bandwidth of the channel in time.•CDMA is based on coding theory. Each station is assigned a code, which is a sequence of numbers called •chips.To generate chip sequences, we use a Walsh table• , which is a two-dimensional table with an equal number of rows and columns.
ReferencesSingal, T. L., 2010. • Wireless Communications, Tata McGraw-Hill Education.Rappaport, S. T., 2009. • Wireless Communications - Principles And Practice, 2nd ed., Pearson Education India.Tom Sheldon and Big Sur Multimedia, • CSMA/CD (Carrier Sense Multiple Access/Collision Detection) [Online] Available at: < http://www.linktionary.com/c/csma.html> [Accessed 15 September 2011].LearnNetworking, 2008. • Carrier Sense Multiple Access Collision Detect (CSMA/CD) Explained [Online] Available at: < http://learn-networking.com/network-design/carrier-sense-multiple-access-collision-detect-csmacd-explained> [Accessed 15 September 2011].Prof. Ghosh, S., 2005. • Lecture 19: Ethernet - CSMA/CD [Video Online] Available at: < http://freevideolectures.com/Course/2276/Computer-Networks/19> [Accessed 15 September 2011].Prof. Gallager, R. & Prof. Zheng, L., 2006. • Principles of Digital Communications I [Video Online] Available at: < http://freevideolectures.com/Course/2376/Principles-of-Digital-Communications-I/24> [Accessed 15 September 2011].
Recommended Reading Dodd, Z. A., 2002. • The essential guide to telecommunications, 3rd ed., Prentice Hall Professional.Buehrer, M. & Buehrer, M. R., 2006. Code Division multiple access, Morgan & Claypool Publishers.•Viterbi, J. A., 1995. • CDMA: principles of spread spectrum communication, Addison-Wesley Pub. Co.
113/JNU OLE
Self AssessmentCSMA is based on the principle of ____________________.1.
sense before transmita. transmit and senseb. sense during transmitc. transmit and no sensed.
___________ can reduce the possibility of collision but it cannot eliminate it.2. Token Passinga. Pollingb. CSMAc. Reservationd.
The time period taken for CSMA/CD processes is called the ______________.3. back-off factora. collisionb. sensec. accessd.
Which of the following statements is true?4. In a. controlled access, thestationsdonotconsultoneanothertofindwhichstationhastherighttosend.In b. controlled access, thestationsconsultoneanothertofindwhichstationdoesnothavetheright tosend.In c. controlled access, thestationsconsultallofthemtofindwhichstationhastherighttosend.In d. controlled access, thestationsconsultoneanothertofindwhichstationhastherighttosend.
How is time divided in the reservation method?5. Sessiona. Intervalb. Stationsc. Access methodd.
The _______________is used by the primary device to solicit transmissions from the secondary devices.6. poll a. functionreservationb. token passingc. selectd.
Which of the following is the multiple access method where the available bandwidth of a link is shared in time, 7. frequency, or through code?
Token passinga. Pollingb. Channelizationc. CSMAd.
Data Communications
114/JNU OLE
Each station in frequency division multiple access also uses a __________ to confine the transmitter 8. frequencies.
bandpassfiltera. stream datab. low-bandwidthc. bandwidth channeld.
In ____________ only one channel occupies the entire bandwidth of the link.9. Code division multiple accessa. Carrier sense multiple accessb. Time division multiple accessc. Frequency division multiple accessd.
Which of the following statements is true?10. In Carrier Sense Multiple Access all stations can send data simultaneously; there is no timesharing.a. In Time Division Multiple Access all stations can send data simultaneously; there is no timesharing.b. In Frequency Division Multiple Access all stations can send data simultaneously; there is no timesharing.c. In Code Division Multiple Access all stations can send data simultaneously; there is no timesharing.d.
115/JNU OLE
Chapter VIII
Data Connections and Security
Aim
The aim of this chapter is to:
explain the concept of IPV4•
enlist elements in IPV4•
defineclassfuladdressing•
Objectives
The objectives of this chapter are to:
elucidate mask•
explain elements of classful addressing•
enlist elements of IPV6•
Learning outcome
At the end of this chapter, you will be able to:
compare binary with dotted decimal notation•
understand cryptography•
describe security i• n data communication
Data Communications
116/JNU OLE
8.1 IPv4An IPv4 address is a 32-bit address that uniquely and universally definestheconnectionofadevice(forexample,a computer or a router) to the Internet. IPv4 addresses are unique.
Theyareuniqueinthesensethateachaddressdefinesone,andonlyone,connectiontotheInternet.Twodeviceson the Internet can never have the same address at the same time. On the other hand, if a device operating at the network layer has m connections to the Internet, it needs to have m addresses. The IPv4 addresses are universal in the sense that the addressing system must be accepted by any host that wants to be connected to the Internet.
8.1.1 Address spaceAprotocol such as IPv4 that defines addresses has an address space.An address space is the total numberofaddressesusedbytheprotocol.IfaprotocolusesNbitstodefineanaddress,theaddressspaceis2N because each bit can have two different values (0 or 1) and N bits can have 2N values. IPv4 uses 32-bit addresses, which means that the address space is 232 or 4,294,967,296 (more than 4 billion). This means that, theoretically, if there were no restrictions, more than 4 billion devices could be connected to the Internet.
8.1.2 NotationsThere are two prevalent notations to show an IPv4 address: binary notation and dotted decimal notation.
Binary notationIn binary notation, the IPv4 address is displayed as 32 bits. Each octet is often referred to as a byte. So it is common to hear an IPv4 address referred to as a 32-bit address or a 4-byte address. The following is an example of an IPv4 address in binary notation:
01110101 10010101 00011101 00000010
Dotted decimal notationTo make the IPv4 address more compact and easier to read, Internet addresses are usually written in decimal form with a decimal point (dot) separating the bytes. The following is the dotted-decimal notation of the above address:
117.149.29.2
Figure below shows an IPv4 address in both binary and dotted-decimal notation. Note that because each byte (octet) is 8 bits, each number in dotted-decimal notation is a value ranging from 0 to 255.
192 168 1 0
192.168.1.0
11000000.10101000.00000001.00000000
Fig. 8.1 Dotted-decimal notation and binary notation for an IPv4 address(Source: http://www.dialogic.com/support/helpweb/safepipe/SP/images/courses/ip-binary-decimal.gif)
Classful addressingIPv4 addressing, at its inception, used the concept of classes. This architecture is called classful addressing. In classfuladdressing,theaddressspaceisdividedintofiveclasses:A,B,C,DandE.Eachclassoccupiessomepartoftheaddressspace.Wecanfindtheclassofanaddresswhengiventheaddressinbinarynotationordotted-decimalnotation.Iftheaddressisgiveninbinarynotation,thefirstfewbitscanimmediatelytellustheclassoftheaddress.Iftheaddressisgivenindecimal-dottednotation,thefirstbytedefinestheclass.
117/JNU OLE
First byte
First byte
Class A Class A 0-127
1128-19111
1192-22311
1224-23911
1240-25511
Class B Class B
Class C Class C
Class D Class D
Class E Class E1111
1110
110
10
0
Second byte
Second byte
Third byte
Third byte
Fourthbyte
Fourthbyte
Fig. 8.2 Finding the classes in binary and dotted-decimal notation
Classes and blocksOneproblemwithclassfuladdressingisthateachclassisdividedintoafixednumberofblockswitheachblockhavingafixedsizeasshownintablebelow
Class Number of Blocks Block Size Application
A 128 16,777,216 Unicast
B 16,384 65,536 Unicast
C 2,097,152 256 Unicast
D 1 268,435,456 Multicast
E 1 268,435,456 Reserved
Table 8.1 Number of blocks and block size in classfulIPv4 addressing
When an organisation requested a block of addresses, it was granted one in class A, B or C. Class A addresses were designed for large organisations with a large number of attached hosts or routers. Class B addresses were designed for midsize organisations with tens of thousands of attached hosts or routers. Class C addresses were designed for small organisations with a small number of attached hosts or routers.
A block in class A address is too large for almost any organisation. This means most of the addresses in class A were wasted and were not used. A block in class B is also very large, probably too large for many of the organisations that received a class B block. A block in class C is probably too small for many organisations. Class D addresses weredesignedformulticasting.EachaddressinthisclassisusedtodefineonegroupofhostsontheInternet.TheInternet authorities wrongly predicted a need for 268,435,456 groups.
Netid and host-idIn classful addressing, an IP address in class A, B, or C is divided into netid and hostid. These parts are of varying lengthsdependingontheclassoftheaddress.InclassA,onebytedefinesthenetidandthreebytesdefinethehostid.InclassB,twobytesdefinethenetidandtwobytesdefinethehostid.InclassC,threebytesdefinethenetidandonebytedefinesthehostid.
MaskAlthough the length of the netid and hostid (in bits) is predetermined in classful addressing, we can also use a mask (also called the default mask), a 32-bit number made of contiguous 1s followed by contiguous 0s. The masks for classes A, B and C are shown in table below. The concept does not apply to classes D and E.
Data Communications
118/JNU OLE
Class Binary Dotted-Decimal CIDR
A 11111111 00000000 00000000 00000000 255.0.0.0 18
B 11111111 11111111 00000000 00000000 255.255.0.0 116
C 11111111 11111111 11111111 00000000 255.255.255.0 124
Table 8.2 Default masks for classful addressing
Themaskcanhelpustofindthenetidandthehostid.Forexample,themaskforaclassAaddresshaseight1s,whichmeansthefirst8bitsofanyaddressinclassAdefinethenetid;thenext24bitsdefinethehostid.
The last column of Table 8.2 shows the mask in the form 1n where n can be 8, 16, or 24 in classful addressing. This notation is also called slash notation or Classless Interdomain Routing (CIDR) notation.
SubnettingDuring the era of classful addressing, subnetting was introduced. If an organisation was granted a large block in class A or B, it could divide the addresses into several contiguous groups and assign each group to smaller networks (called subnets) or, in rare cases, share part of the addresses with neighbours. Subnetting increases the number of 1s in the mask.
SupernettingThere was a time when most of the class A and class B addresses were depleted. The size of a class C block with a maximum number of 256 addresses did not satisfy the needs of most organisations.
Even a midsize organisation needed more addresses. One solution was supernetting. In supernetting, an organisation can combine several class C blocks to create a larger range of addresses. In other words, several networks are combined to create a supernetwork or a supernet. An organisation can apply for a set of class C blocks instead ofjust one.
For example, an organisation that needs 1000 addresses can be granted four contiguous class C blocks. The organisation can then use these addresses to create one supernetwork. Supernetting decreases the number of 1s in the mask. For example, if an organisation is given four class C addresses, the mask changes from /24 to /22.
Network addressA very important concept in IP addressing is the network address. When an organisation is given a block of addresses, theorganisationisfreetoallocatetheaddressestothedevicesthatneedtobeconnectedtotheInternet.Thefirstaddressintheclass,however,isnormally(notalways)treatedasaspecialaddress.Thefirstaddressiscalledthenetworkaddressanddefinestheorganisationnetwork.Itdefinestheorganisationitselftotherestoftheworld.Inalaterchapterwewillseethatthefirstaddressistheonethatisusedbyrouterstodirectthemessagesenttotheorganisation from the outside.
119/JNU OLE
Figure below shows an organisation that is granted a 16-address block.
All messages with receiver addresses 205.16.37.32 to 205.16.37.47
are routed to x.y.z.t/n x.y.
z.t/n
Organization network
Rest of the internet
205.16.37.33/28 205.16.37.34/28 205.16.37.39/28 205.16.37.46/28 205.16.37.47/28
1
205.
16.3
7.40
/28
205.16.37.32/28Network address :
Fig. 8.3 A network configuration for the block 205.16.37.32/28
The organisation network is connected to the Internet via a router. The router has two addresses. One belongs to the granted block; the other belongs to the network that is at the other side of the router. We call the second address x.y.z.t/n because we do not know anything about the network, it is connected to the other side. All messages destined for addresses in the organisation block (205.16.37.32 to 205.16.37.47) are sent, directly or indirectly, to x.y.z.t/n. We say directly or indirectly because we do not know the structure of the network to which the other side of the router is connected.
8.2 IPV6Despiteallshort-termsolutions,suchasclasslessaddressing,DynamicHostConfigurationProtocol(DHCP),andNAT, address depletion is still a long-term problem for the Internet. This and other problems in the IP protocol itself, such as lack of accommodation for real-time audio and video transmission and encryption and authentication of data for some applications, have been the motivation for IPv6.
StructureAn IPv6 address consists of 16 bytes (octets); it is 128 bits long.
Hexadecimal colon notationTomakeaddressesmorereadable,IPv6specifieshexadecimalcolonnotation.Inthisnotation128bitsisdividedinto eight sections, each 2 bytes in length. Two bytes in hexadecimal notation requires four hexadecimal digits. Therefore, the address consists of 32 hexadecimal digits, with every four digits separated by a colon.
The IPv6 global space begins with 2000::/3. This means that all public IPv6 addresses now use three bits which amount to value 2 in hex.Look at this notation of global address one more time:
2000::/3This is a shortcut for:
2001:0000:0000:0000:0000:0000:0000:0000/3One character stands for four bits (called a nibble). They need to be converted as two separate entities.
Data Communications
120/JNU OLE
Hex Value : 2000::
Hex
Bin 0
23 2322 2221 2120 20
0 0 0 0 0 01
2 0
Fig. 8.4(a) Hex to binary conversion
(Source: http://ciscoiseasy.blogspot.com/2011/05/lesson-56-introduction-to-ipv6-address.html)
Thisway,thehexadecimal20becomes00100000.IntheglobalIPv6addressscopethefirstthreebits(‘/3’)willalways be set like shown below:
Hex
Bin 0 0 01
23 22 21 20
2
Hex Value: 2000 ::
Fig. 8.4(b) IPv6 global address sequence of bits.(Source: http://ciscoiseasy.blogspot.com/2011/05/lesson-56-introduction-to-ipv6-address.html)
Address spaceIPv6 has a much larger address space; 2128 addresses are available. The designers of IPv6 divided the address into severalcategories.Afewleftmostbits,calledthetypeprefix,ineachaddressdefineitscategory.Thetypeprefixisvariableinlength,butitisdesignedsuchthatnocodeisidenticaltothefirstpartofanyothercode.Inthisway,thereisnoambiguity;whenanaddressisgiven,thetypeprefixcaneasilybedetermined.Table8.3showstheprefixfor each type of address. The third column shows the fraction of each type of address relative to the whole address space.
121/JNU OLE
Type Prefix Type Fraction
00000000 Reserved 1/256
00000001 Unassigned 1/256
0000001 ISO network addresses 1/128
0000010 IPX (Novell) network addresses 1/128
0000011 Unassigned 1/128
00001 Unassigned 1/32
0001 Reserved 1/16
001 Reserved 1/8
010 Provider-based unicast address 1/8
011 Unassigned 1/8
100 Geographic-based unicast ad-dress 1/8
101 Unassigned 1/8
110 Unassigned 1/8
1110 Unassigned 1116
11110 Unassigned 1132
1111 10 Unassigned 1/64
1111 110 Unassigned 1/128
1111 1110a Unassigned 1/512
1111 111010 Link Local addresses 1/1024
1111 1110 11 Site Local addresses 1/1024
11111111 Multicast addresses 1/256
Table 8.3 Type prefixes for IPv6 addresses
Unicast addressesA unicast address definesasinglecomputer.Thepacketsenttoaunicastaddressmustbedeliveredtothatspecificcomputer.IPv6definestwotypesofunicastaddresses:
Geographically based �Provider-based. �
Multicast addressesMulticastaddressesareusedtodefineagroupofhostsinsteadofjustone.Apacketsenttoamulticastaddressmustbe delivered to each member of the group.
Data Communications
122/JNU OLE
Anycast addressesIPv6alsodefinesanycastaddresses.Ananycastaddress,likeamulticastaddress,alsodefinesagroupofnodes.However, a packet destined for an anycast address is delivered to only one of the members of the anycast group, the nearest one (the one with the shortest route). one possible use of anycast address is to assign an anycast address to all routers of an ISP that covers a large logical area in the Internet. The routers outside the ISP deliver a packet destined for the ISP to the nearest ISP router. No block is assigned for anycast addresses.
Reserved addressAnothercategoryintheaddressspaceisthereservedaddress.TheseaddressesstartwitheightOs(typeprefixis00000000).
Local addressThese addresses are used when an organisation wants to use IPv6 protocol without being connected to the global Internet. In other words, they provide addressing for private networks. Nobody outside the organisation can send a message to the nodes using these addresses.
8.3 CryptographyCryptography, awordwithGreekorigins,means“secretwriting.”However,weusethetermtorefertothescienceand art of transforming messages to make them secure and immune to attacks.
Figure below shows the components involved in cryptography.
Sender
Plaintext
EncryptionCiphertext
DecryptionPlaintext
Receiver
Fig. 8.5 Components of cryptography
Plain text and cipher textThe original message, before being transformed, is called plaintext. After the message is transformed, it is called ciphertext. An encryption algorithm transforms the plaintext into ciphertext; a decryption algorithm transforms the ciphertext back into plaintext. The sender uses an encryption algorithm, and the receiver uses a decryption algorithm.
CipherEncryption and decryption algorithms are referred to as ciphers. The term cipher is also used to refer to different categories of algorithms in cryptography. This is not to say that every sender-receiver pair needs their very own unique cipher for a secure communication. On the contrary, one cipher can serve millions of communicating pairs.
KeyA key is a number (or a set of numbers) that the cipher, as an algorithm, operates on. To encrypt a message, we need an encryption algorithm, an encryption key, and the plaintext.
These create the ciphertext. To decrypt a message, we need a decryption algorithm, a decryption key, and the ciphertext. These reveal the original plaintext.
123/JNU OLE
Alice, Bob and EveIn cryptography, it is customary to use three characters in an information exchange scenario; we use Alice, Bob, and Eve. Alice is the person who needs to send secure data. Bob is the recipient of the data. Eve is the person who somehow disturbs the communication between Alice and Bob by intercepting messages to uncover the data or by sending her own disguised messages. These three names represent computers or processes that actually send or receive data, or intercept or change data.
plain text
attacker Eve
sender Alice
receiver Bob
plain textciphertext
encryption decryption
Fig. 8.6 Use three characters in an information exchange scenario: Alice, Bob and Eve
(Source:http://flylib.com/books/3/320/1/html/2/FILES/02fig02.gif)
Types of cryptographyCryptography algorithms (ciphers) can be divided into two groups: symmetric key (also called secret-key) cryptography algorithms and asymmetric (also called public-key) cryptography algorithms.
Cryptography
Symmetric Key (Secret-Key)
Asymmetric Key (Public-Key)
Fig. 8.7 Categories of cryptography
Symmetric key cryptographyIn symmetric-key cryptography, the same key is used by both parties. The sender uses this key and an encryption algorithm to encrypt data; the receiver uses the same key and the corresponding decryption algorithm to decrypt the data.
Data Communications
124/JNU OLE
Combine keys
Alice
Bob’spublic key
Alice’sprivate key
Alice’sandBob’sshared secret
Alice’sandBob’sshared secret
Bob’sprivate key
Alice’spublic key
Bob
751A696C24D97009
751A696C24D97009
Combine keys
Fig. 8.8 Symmetric key cryptography(Source: http://img.search.com/thumb/4/4c/Public_key_shared_secret.svg/300px-Public_key_
shared_secret.svg.png)
Asymmetric key cryptographyIn asymmetric or public-key cryptography, there are two keys: a private key and a public key. The private key is kept bythereceiver.Thepublickeyisannouncedtothepublic.Infigurebelow,imagineAlicewantstosendamessageto Bob. Alice uses the public key to encrypt the message. When the message is received by Bob, the private key is used to decrypt the message.
Alice
Big random number
Alices’publickey
Alices private key
52ED879E7OF71D92
Key generation function
Fig. 8.9 Asymmetric key cryptography(Source: http://img.search.com/thumb/3/3f/Public_key_making.svg/250px-Public_key_making.svg.png)
In public-key encryption/decryption, the public key that is used for encryption is different from the private key that isusedfordecryption.Thepublickeyisavailabletothepublic;’theprivatekeyisavailableonlytoanindividual.
125/JNU OLE
8.4 SecurityNetworksecuritycanprovideoneofthefiveservicesasshowninfigurebelow.Fouroftheseservicesarerelatedtothemessageexchangedusingthenetwork:messageconfidentiality,integrity,authentication,andnonrepudiation.Thefifthserviceprovidesentityauthenticationoridentification.
Confidentiality
Integrity
Authentication
Nonrepudiatton
Message
Entity Authentication
Security services
Fig. 8.10 Security services related to the message or entity
Message confidentialityMessageconfidentialityorprivacymeansthatthesenderandthereceiverexpectconfidentiality.Thetransmittedmessage must make sense to only the intended receiver. To all others, the message must be garbage. When a customer communicateswithherbank,sheexpectsthatthecommunicationistotallyconfidential.
Message integrityMessage integrity means that the data must arrive at the receiver exactly as they were sent. There must be no changes during the transmission, neither accidentally nor maliciously. As more and more monetary exchanges occur over the Internet, integrity is crucial. For example, it would be disastrous if a request for transferring $100 changed to a request for $10,000 or $100,000. The integrity of the message must be preserved in a secure communication.
Message authenticationMessage authentication is a service beyond message integrity. In message authentication the receiver needs to be sureofthesender’sidentityandthatanimposterhasnotsentthemessage.
Message non-repudiationMessage non-repudiation means that a sender must not be able to deny sending a message that he or she, in fact, did send. The burden of proof falls on the receiver. For example, when a customer sends a message to transfer money from one account to another, the bank must have proof that the customer actually requested this transaction.
Entity authenticationInentityauthentication(oruseridentification)theentityoruserisverifiedpriortoaccesstothesystemresources(files,forexample).Forexample,astudentwhoneedstoaccessheruniversityresourcesneedstobeauthenticatedduring the logging process. This is to protect the interests of the university and the student.
Data Communications
126/JNU OLE
SummaryAnIPv4addressisa32-bitaddressthatuniquelyanduniversallydefinestheconnectionofadevice(forexample,•a computer or a router) to the Internet.An address space is the total number of addresses used by the protocol.•In binary notation, the IPv4 address is displayed as 32 bits.•To make the IPv4 address more compact and easier to read, Internet addresses are usually written in decimal •form with a decimal point (dot) separating the bytes.Inclassfuladdressing,theaddressspaceisdividedintofiveclasses:A,B,C,DandE.•In classful addressing, an IP address in class A, B, or C is divided into netid and hostid.•A mask being of 32-bit number made of contiguous 1s followed by contiguous 0s.•Subnetting increases the number of 1s in the mask.•Supernetting decreases the number of 1s in the mask. •A very important concept in IP addressing is the network address.•Tomakeaddressesmorereadable,IPv6specifieshexadecimalcolonnotation.•IPv6 has a much larger address space of 2128 addresses.•Encryption and Decryption algorithms are referred to as ciphers.•A key is a number (or a set of numbers) that the cipher, as an algorithm, operates on.•In cryptography, it is customary to use three characters in an information exchange scenario; we use Alice, Bob •and Eve.In symmetric-key cryptography, the same key is used by both parties.•In asymmetric or public-key cryptography, there are two keys: a private key and a public key.•
ReferencesBlanchet, M., 2006. • Migrating to IPv6: a practical guide to implementing IPv6 in mobile and fixed networks, John Wiley and Sons.Clerk, E. G., 2009. • Comptia Network+Cert.St.Gd, Tata McGraw-Hill Education.Parr, B., 2011. • IPV4 & IPV6: A Short guide [Online] Available at: < http://mashable.com/2011/02/03/ipv4-ipv6-guide/>[Accessed 16 September 2011].Kessler, C. G., 2011. • An Overview of Cryptography [Online] Available at :< http://www.garykessler.net/library/crypto.html>[Accessed 16 September 2011].Prof. Lewis, R. H., 2005. • Lecture 27: Public key cryptography [Video Online] Available at: < http://freevideolectures.com/Course/2656/CSCI-E-2-Bits/27> [Accessed 16 September 2011].Prof. Pal, A., • Lecture 40: Secured Communication – I [Video Online] Available at: < http://freevideolectures.com/Course/2278/Data-Communication/40> [Accessed 16 September 2011].
Recommended ReadingBeyda, J. W., 1996. • Data communications: from basics to broadband, 2nd ed., Prentice Hall.Stallings, W., 2010. • Cryptography and network security: principles and practice, 5th ed., Prentice Hall.Blanchet, M., 2006. • Migrating to IPv6: a practical guide to implementing IPv6 in mobile and fixed networks, John Wiley and Sons.
127/JNU OLE
Self AssessmentWhich of the following statements is true?1.
Two devices on the Internet can never have the same address at the same time.a. Two devices on the Internet can have the same address at the same time.b. Two persons on the Internet can never have the same address at the same time.c. One device on the Internet can never have the same address.d.
IPv4 addressing, uses the concept of _________________.2. securitya. cipherb. classful addressingc. public keyd.
Inwhichclassaddressingnotation,thefirstfewbitscanimmediatelytellustheclassoftheaddress?3. Dotted decimal notationa. Binary notationb. Mask c. Hostedd.
Inwhichclassaddressingnotation,thefirstbytedefinestheclass?4. Dotted decimal notationa. Binary notationb. Mask c. Hostedd.
__________numbersofbytesaredefinedbythehostidinclassfuladdressing.5. Onea. Twob. Threec. Fourd.
Which term has a 32-bit number made of contiguous 1s followed by contiguous 0s?6. Netida. Hostidb. Classc. Maskd.
Which of the following statements is true?7. Subnetting increases the number of 1s in the mask.a. Subnetting decreases the number of 1s in the mask.b. Subnetting increases the number of 0s in the mask.c. Netid increases the number of 1s in the mask.d.
Data Communications
128/JNU OLE
________________ decreases the number of 1s in the mask.8. Subnettinga. Maskingb. Supernettingc. IPV4d.
What is the another name for secret writing?9. Cryptographya. Classful addressingb. Netidc. Hostidd.
In cryptography, ____________ is the person who needs to send secure data.10. Boba. Eveb. Alicec. Martind.
129/JNU OLE
Case Study I
The Company: Progressive Gaming International Corporation
ProgressiveGamingisaleading,diversifiedsupplierofgamingequipmentandsystemstotheinternationalgamingindustry.Asthegloballeaderinintegratedcasinomanagementservices,ProgressiveGaming’sgoalistoproducereliable feature-rich gaming operations for its customer casinos.
Whileworkingwitheverymajorcasinointheworld,ProgressiveGamingusesmultiplesoftware’stohelpthosecasinos keep their customers coming and staying longer. Through progressive jackpot technology, an expanding range of slot and table games and advanced player tracking technology, Progressive Gaming not only improves the casino visitor experience, but also helps casinos maintain the integrity of their customers.
ProgressiveGaming’s suite of casinomanagement products supports every facet of a gaming operation andconsolidates all gaming management from slots to table management to cashier functionality into one fully integrated system.
Technical Skills Ubuntu Operating System, C and C++, JavaScript, Serial I/O
The Task: To Enhance LCD in-machine displaysProgressive Gaming came to Right Hand with a request to upgrade the communications protocol on an electronic LCD panel used in one of their casino games. Progressive Gaming is often looking for ways to apply their networked slot gaming systems to multiple environments. The purpose of upgrading the protocol was to allow for the deployment in additional customer networks and show jackpot pool information on the LCD as well as display celebrations when these jackpots are won.
The Right Hand Solution: Adding additional communication protocols for messagingRight Hand responded to the task by simply adding additional communication protocols proprietary to gaming. These message-parsing protocols enhanced the LCD display by processing messages sent across the network. The enhanced LCD screen can be used in additional networks to show jackpot pool information and displays jackpot celebrations on a winning machine. Right Hand worked with the client to incorporate the new protocol parser into the existing platform code, and then delivered it for testing in a live gaming environment.
(Source: The Company – Progressive Gaming International Corporation. [Online]Availabe at: <http://www.righthandtech.com/enhanced-LCD-display.php> [Accessed 15 September 2011])
QuestionsWhat is the goal of Progressive Gaming International Corporation?1. AnswerProgressiveGamingisaleading,diversifiedsupplierofgamingequipmentandsystemstotheinternationalgamingindustry.Asthegloballeaderinintegratedcasinomanagementservices,ProgressiveGaming’sgoalisto produce reliable feature-rich gaming operations for its customer casinos.
What is the task performed by Progressive Gaming?2. AnswerProgressive Gaming came to Right Hand with a request to upgrade the communications protocol on an electronic LCD panel used in one of their casino games. Progressive Gaming is often looking for ways to apply their networked slot gaming systems to multiple environments.
Data Communications
130/JNU OLE
What is the role of Right Hand Solutions?3. AnswerRight Hand responded to the task by simply adding additional communication protocols proprietary to gaming. These message-parsing protocols enhanced the LCD display by processing messages sent across the network. The enhanced LCD screen can be used in additional networks to show jackpot pool information and displays jackpot celebrations on a winning machine. Right Hand worked with the client to incorporate the new protocol parser into the existing platform code, and then delivered it for testing in a live gaming environment.
131/JNU OLE
Case study II
Abuzz Technologies Abuzz Technologies designs and manufactures interactive touch-screen kiosks. Founded in 1995 as a design agency,Abuzzproduceditsfirstkiosksasaninnovativewaytopresentitsportfolioofdesignsat tradeshows.The overwhelming response to these prototypes convinced the company to shift its focus to interactive touch-screen products in 1997. A kiosk is a self-service terminal comprising a secure enclosure that houses hardware and software. Kiosks usually incorporate a touch-screen monitor, printer, speakers and keyboard as well as a custom-developed software application. The company is now one of the largest kiosk manufacturers in Australia. Based in Sydney, with approximately 50 staff, Abuzz exports to 13 countries around the world, including Germany, the Netherlands, the United Kingdom and the United States.
Abuzz’simpressiveclientlistincludesAMP,BMW,ColesMyer,GreaterUnion,Hoyts,Nokia,Toyota,Westfieldand Westpac. Abuzz designs and assembles the kiosks and develops specialised applications to ensure each kiosk meetstheneedsofeachcustomer.Forexample,shoppingcentremanagementcompanyWestfielduseskioskstoenable shoppers to search for and locate the stores they need, while cinema chain Greater Union uses them for self-service ticket sales. Banks such as the Commonwealth Bank of Australia and Westpac use Abuzz kiosks as personnel directories. Business challengeAbuzzwasexperiencingrapidrevenuegrowthofabout50percenteachyear,butthecompany’sdataandtelephonysystemsdidnothavethescaleorflexibilityrequiredtomeetitsevolvingbusinessneeds.ItsoutdatedPABXphonesystem was proving expensive in terms of support and maintenance.
“Everytimewewantedtodoadds,movesorchanges,weneededtocallanexternalconsultant,”explainedMorganDrew,ManagingDirectorofAbuzzTechnologies.“Thisslowedusdownandwasexpensive.Asasmallbusinessweneedtobeagileandflexible,andourphonesystemwasholdingusback.
“Inaddition,weweremissingalotofcallsifphonesweren’tmanuallydiverted.Forexample,ifouradministrationpersonleftthefrontdeskanddidn’tdivertthephone,thecallswouldgostraighttoanansweringservice.Thiswasnot giving our customers the right levels of professionalism and customer service.” However,Abuzz’s telephony systemwasn’t its onlyproblem.Thecompanywas alsodissatisfiedwith its datanetwork. Staff working remotely was unable to connect to critical business applications. Network security was also a concern.Abuzz’sdisconnectedsystemsmeantithadtomanagemultiplevendors–oneforitsPABXphonesystem,anotherforLinuxfirewallandathirdforitsdatanetwork.Thisincreasedthecostofmanagingandmaintainingthecompany’stechnologysystems.
“Wewantedatelephonyenvironmentthatwouldenableustomanagecallflowsourselvesandthatwouldintegratewithourcustomerrelationshipmanagementsystem,”saidDrew.“WealsowantedtoallowstaffintheofficetoconnectremotelytokiosksaroundAustralia.Withourexistingdataandtelephonysystems,thiswasn’tgoingtohappen.”
Secure, reliable network“InstallingtheCiscoVPNhasensuredournetworkisverysecure,”saidDrew.“Thishasmadeabigdifferencefor us. Now, when staff are overseas, they can log in as if they were part of the network here in Sydney. This has boostedproductivityandourcustomerservicelevelsdon’tdropifstaffisoutoftheoffice.”Asecurenetworkhasalso made it easier for Abuzz to remotely maintain kiosks at client sites around Australia.
“Thesecure,low-costVPNconnectionbetweenourofficeandremotesitesmeanswecanmeasurestatistics,monitorsystem health, provide 24x7 support and update content remotely,” said Drew.
Data Communications
132/JNU OLE
“WethoughtCiscowouldbeasafebetasit’sapremiumproductthatoffersveryhighlevelsofsecurity.That’swhatthe brochures say, and this has certainly been our experience.”
Integration with CRM systemTheadvancedfeaturesofCiscoUnifiedCommunicationsallowthesystemtointegrateseamlesslywithAbuzz’sCRM system, which is hosted by an external provider.
“IntegratingourtelephonyenvironmentwiththeCRMsystemhasreallyboostedproductivity,”saidDrew.“Nowwecan look up customer details and use the click-to-dial feature to contact them directly, which is so much easier.”
Creative solutions for Abuzz customers The knowledge Abuzz gained from its own implementation allowed it to develop an advanced meeting room solutionforoneofitscustomersinmid-2006.ThecompanyhadcolourCisco[7970]UnifiedIPPhonesineachofits meeting rooms. However, it still struggled to overcome problems such as staff not checking room availability and meeting times clashing.
Abuzz designed and developed a software system that uses the Cisco IP Phones as a platform. Abuzz built a server thatcollectscalendardatafromthecompany’scalendarapplicationanddistributesthisinformationtotheCiscoIPPhones. The phones in rooms reserved for a meeting display a red screen, whereas those in available rooms are green. The screen also displays information such as the time left until the next meeting and who has booked the room.
“With289meeting roomsand thousandsof staff,havinganefficientbooking system formeeting rooms isofparamount importance,”saidDrew.“Ourclient isnowusingmoremeetingroomsformore timeandavoidinginterrupted meetings. The solution has really increased productivity and the effective utilisation of space. In fact, it isinterestedinrollingoutthesolutiontoitsotherofficesaroundthecountry.” Source: (AbuzzTechnologies.[Online]Availableat:<http://www.efficientdata.com.au/Clients.aspx?id=1&m=4 > [Accessed 15 September 2011])
QuestionsWhich are the waves in the EM spectrum that are visible to human?1. How is the chaos solved when there occurs a multiple access of the network?2. How will you communicate data to a large audience?3.
133/JNU OLE
Case Study III
Ornskoldsviks Municipality
Safe communication in the water and wastewater systemsIn Ornskoldsviks municipality there are both mountains and valleys in an area of 62 x 62 square kilometers. The handlingofthewaterandwastewaterisdividedintothefiveareasBjasta,Bredbyn,Bjorna,HusumandCentralorten.The scattered areas are coordinated by the common surveillance system ABB PC Manager. The fringe areas have machinists who start from the one and the same control system and work with both the water & wastewater systems. Theyarerelocatedinthefringeareasandshareanofficewithothermunicipalactivityinthatspecificarea.
“Thisorganisationisduetothefactthattheyoncewereseparatemunicipalities,”StenLjungberg,processengineerat Ornskoldsviks municipality, explains. To be able to communicate within these areas Ornskoldsviks municipality has chosen Radius digital radio communication solution - Radius PDR. When it comes to water it could be equipment to rise pressure, water plants, measuring points and reservoirs that need a safe communication, and with wastewater itcouldbepurificationfacilitiesandpumpstations.
Separate but synchronised systemsEach area has its own surveillance system, but the machinists can get connected to the other systems to control the otherareas’facilities.Theordinaryprocedureisthatanalarmishandledattheoperationalcontrolcenters,buteachareaisconnectedtothemunicipality’semergencycenterKAC.InKAC,afewdesignatedalarmsthathavebeenprioritised by the operational management arrive.
Different criterions, for example what time of the day it is, which day at the week it is, control which alarms go directlytoKACandwhichonescouldbeputonholduntilregularworkinghours.Fromthefirestationinthecentreof Ornskoldsvik, the alarms are supervised and on call-duty personnel are sent out to all of the areas.
Both VHF and UHF in the same systemThere are PDR masters in the different areas that connect the facilities to the SCADA system. Usually, a master unit is handlingthearea’sslaveunits,butinBjalstatherearetwomasterunits;oneintheoperationalcontrolcenterandoneinSidensjo,19milesaway,thatareconnectedviaafixedline.Thereareabout120PDRslavesinthemunicipality.In most cases Ornskoldsviks municipality uses UHF, but in Galnas, that is part of Centralorten, VHF is also used. “Sincethefarthestfacilityismorethan19milesaway,itisbettertouseVHF”StenLundbergexplains.
Data Communications
134/JNU OLE
(Source: Safe communication in the water & wastewater systems. [Online] Available at:< http://www.radius.net/Default.aspx.LocID-01onew02r.RefLocID-01o00c00400a.Lang-EN.htm> [Accessed 15 September 2011]) Questions
What are the Guided and Unguided media?1. How the transmission media affect the communication?2. How the unguided media do connects different systems without adhering to the geographical distance?3.
135/JNU OLE
Bibliography
ReferencesAhmad, A., 2003. • Data communication principles for fixed and wireless networks, Springer.Amutha Jeevakumari, S.A., 2008. • Elements of Data Communication and Networks, Laxmi Publications, Ltd.An Internet Encyclopedia, • An Internet Encyclopedia [Online] Available at: <http://www.freesoft.org/CIE/Topics/26.htm> [Accessed 24 August 2011].Banzal, S., 2007, • Data and Computer Network Communication, Firewall Media.Beyda, J. W., 1996. • Data communications: from basics to broadband, Prentice Hall.Blanchet, M., 2006. • Migrating to IPv6: a practical guide to implementing IPv6 in mobile and fixed networks, John Wiley and Sons.Clerk, E. G., 2009. • Comptia Network+Cert.St.Gd, Tata McGraw-Hill Education.Dhotre, A. I. and Bagad, S. V., 2008. Data Communication, Technical Publications.•Diablo Team, • Guided Media [Online] Available at: <http://www.sabah.edu.my/cc044.wcdd/guided.html> [Accessed 15 September 2011].Dr. Benthien, W. G., 2007. • Digital Encoding and Decoding.[pdf] Available at: <http://gbenthien.net/encoding.pdf> [Accessed 14 September 2011].Error Detection, Correction, and Related Topics• [Online] Available at: <http://www.cs.nmsu.edu/~pfeiffer/classes/573/notes/ecc.html> [Accessed 15 September 2011].Fundamentals of Data and Signals• [pdf] Available at: <http://facstaff.swu.ac.th/watcharachai/course/ee462-chap2-notes.pdf> [Accessed 14 September 2011].General telecom, • Multiplexing Techniques [Online] Available at: < http://telecom.tbi.net/mux1.html>. [Accessed 14 September 2011].Godbole, S. A., 2002. • Data communication and network, Tata McGraw-Hill Education.IIT Kharagpur, • Data Communication [Video Online] Available at: <http://freevideolectures.com/Course/2278/Data-Communication/19> [Accessed 15 September 2011].IIT Kharagpur, • Data Communication [Video Online] Available at: < http://freevideolectures.com/Course/2278/Data-Communication/24> [Accessed 15 September 2011].Irvine, J. Harle, D., 2002. • Data communications and networks: an engineering approach, John Wiley and SonsKessler, C. G., 2011. • An Overview of Cryptography [Online] Available at: < http://www.garykessler.net/library/crypto.html> [Accessed 16 September 2011].Khurana, M., 2009. • Data Communication System, Laxmi Publications, Ltd.Kundu, S., 2005. • Fundamentals of Computer Networks, 2nd ed., Prentice-Hall.LearnNetworking, 2008. • Carrier Sense Multiple Access Collision Detect (CSMA/CD) Explained [Online] Available at: <http://learn-networking.com/network-design/carrier-sense-multiple-access-collision-detect-csmacd-explained> [Accessed 15 September 2011].Parr, B., 2011. • IPV4 & IPV6: A Short guide [Online] Available at: < http://mashable.com/2011/02/03/ipv4-ipv6-guide/> [Accessed 16 September 2011].Prof. Gallager, R. & Prof. Zheng, L., 2006. • Principles of Digital Communications I [Video Online] Available at: <http://freevideolectures.com/Course/2376/Principles-of-Digital-Communications-I/24> [Accessed 15 September 2011].Prof. Ghosh, S., 2005. • Lecture 19: Ethernet - CSMA/CD [Video Online] Available at: <http://freevideolectures.com/Course/2276/Computer-Networks/19> [Accessed 15 September 2011].Prof. Lewis, R. H., 2005. • Lecture 27: Public key cryptography [Video Online] Available at: <http://freevideolectures.com/Course/2656/CSCI-E-2-Bits/27> [Accessed 16 September 2011].
Data Communications
136/JNU OLE
Prof. Pal, A., 2005. • Lecture 11:Multiplexing I [Video Online] Available at: <http://freevideolectures.com/Course/2278/Data-Communication/11> [Accesses 15 September 2011].Prof. Pal, A., 2005. • Lecture 12:Multiplexing II [Video Online] Available at: <http://freevideolectures.com/Course/2278/Data-Communication/12> [Accesses 15 September 2011]. • Prof. Pal, A., 2005. Lecture 16: Error Detection and Correction [Video Online] Available at: <http://freevideolectures.com/Course/2278/Data-Communication/16> [Accessed 15 September 2011].Prof. Pal, A., 2005. • Lecture 16: Error Detection and Correction [Video Online] Available at: <http://freevideolectures.com/Course/2278/Data-Communication/17> [Accessed 15 September 2011].Prof. Pal, A., 2005. • Lecture 8: Transmission of Digital Signal – II [Video Online] Available at: <http://freevideolectures.com/Course/2278/Data-Communication/8> [Accessed 15 September 2011].Prof. Pal, A., • Lecture 40: Secured Communication – I [Video Online] Available at: <http://freevideolectures.com/Course/2278/Data-Communication/40> [Accessed 16 September 2011].Prof. Pal, A., Transmission Impairments and Channel Capacity [Video Online] Available at: <http://•freevideolectures.com/Course/2278/Data-Communication/4> [Accessed 15 September 2011].Prof. Pal, A., Transmission Impairments and Channel Capacity [Video Online] Available at: <http://•freevideolectures.com/Course/2278/Data-Communication/3> [Accessed 15 September 2011].Prof. Prasad, S., • Pulse Code Modulation [Video Online] Available at:Rappaport, S. T., 2009. • Wireless Communications - Principles And Practice, 2nd ed., Pearson Education India.Singal, T. L., 2010. • Wireless Communications, Tata McGraw-Hill Education.Skullbox.net, 2011. • Multiplexing [Online] (Updated 6 June 2011) Available at: <http://www.skullbox.net/multiplexing.php>. [Accessed 14 September 2011].Stallings,W., 2007. Business Data Communication, 5th ed., Pearson Education India.•Sudha, M., 2010. • Data and Signals [Online] Available at: <http://www.scribd.com/doc/26493680/Data-and-Signals> [Accessed 14 September 2011].Tanenbaum, S. A., 2003. Computer networks, 4th ed., Prentice Hall PTR.•Tom Sheldon and Big Sur Multimedia, • CSMA/CD (Carrier Sense Multiple Access/Collision Detection) [Online] Available at :<http://www.linktionary.com/c/csma.html> [Accessed 15 September 2011].Toolbox.com, 1998. • TCP/IP |Protocols-l| Tcp-ip- [Online] (Updated 17 July 2008). Available at: <http://it.toolbox.com/wiki/index.php/Layers_in_TCP/IP_model> [Accessed 6 September 2011].TrainSignalInc• , 2007. Train Signal Training: Intro to TCP/IP [Video Online] Available at: <http://www.youtube.com/watch?v=gJ5h4_0mllI&feature=related> [Accessed 6 September 2011].VegasRage, 2008. • Subnetting in 6 easy steps - part 1 [Video Online] Available at: <http://www.youtube.com/watch?v=wl5_J0UtINg&feature=related> [Accessed at 6 September 2011].Wetteroth, D., 2002. • OSI reference model for telecommunications, McGraw-Hill.www.pulsesupply.com, 2011. High Level Data Link Control [Online] Available at: <http://www.pulsewan.com/•data101/hdlc_basics.htm>. [Accessed 15 September 2011].www.tpub.com, Pulse code modulation, [Online]. Available at: <http://www.tpub.com/neets/book12/49l.htm>. •[Accessed 14 September 2011].Yvan Pointurier, 2011. • Virtual circuit packet switching [Online] Available at :<http://www.cs.virginia.edu/~mngroup/projects/mpls/documents/thesis/node8.html>. [Accessed 15 August 2011].
137/JNU OLE
Recommended ReadingAhmad, A. 2003. • Data communication principles: for fixed and wireless networks, Springer.Beyda, J. W., 1996. • Data communications: from basics to broadband, 2nd ed., Prentice Hall.Blanchet, M., 2006. • Migrating to IPv6: a practical guide to implementing IPv6 in mobile and fixed networks, John Wiley and Sons.Buehrer, M. and Buehrer, M. R., 2006. Code Division multiple access, Morgan & Claypool Publishers.•Dodd, Z. A., 2002. • The essential guide to telecommunications, 3rd ed., Prentice Hall Professional.Gupta, C. P., 2006. Data • Communications and Computer Networks, PHI Learning Pvt. Ltd.Held, G., 2002. • Understanding Data Communications, 7th ed., Pearson Education India.James, E.G. and Phillip, T. R., 2004. • Applied Data Communications: A Business-orientedApproach, • John Wiley & Sons.Kundu, S., 2005. • Fundamental of Computer Networks, 2nd, ed., PHI Learning Pvt. Ltd.Michael, D. and Richard, R., 2003. • Data Communications and Computer Networks: For Computer Scientists and Engineers. Prentice Hall.Phillips, 2008. • Signals, Systems and Transforms, 4th ed., Pearson Education India.SBanzal, S., 2007. • Data and Compute Network Communication, Firewall Media.Schweber, L. W., 2009. • Data Communications, Tata McGraw-Hill Education.Sharma, R., 2008. • Data & Computer Communication, Laxmi Publications, Ltd.Smith, W. S., 2003. • Digital signal processing: a practical guide for engineers and scientists, Newnes.Spohn, L. D., 2003. • Data Network Design, 3rd ed., Tata McGraw-Hill Education.Stallings, W., 2010. • Cryptography and network security: principles and practice, 5th ed., Prentice Hall.Tomasi, W., 2007. • Introduction to Data Communication and Networking, 3rd ed., Pearson Education India.Viterbi, J. A., 1995. • CDMA: principles of spread spectrum communication, Addison-Wesley Pub. Co.White, C., 2010. • Data Communications and Computer Networks: A Business User’s Approach, Cengage Learning.William, S., 2003.• Data and Computer Communications Pearson Education, 7th ed.
Data Communications
138/JNU OLE
Self Assessment Answers
Chapter Ib1. a2. d3. a4. c5. a6. c7. d8. a9. b10.
Chapter II
a1. b2. b3. d4. c5. a6. c7. b8. a9. d10.
Chapter III
a1. d2. a3. c4. d5. d6. b7. c8. a9. b10.
Chapter IV
b1. d2. c3. a4. b5. c6. d7. b8. c9. c10.
139/JNU OLE
Chapter Vb1. a2. a3. c4. a5. d6. b7. b8. c9. d10.
Chapter VI
b1. a2. d3. b4. c5. a6. b7. b8. a9. a10.
Chapter VII
a1. c2. a3. d4. b5. a6. c7. a8. a9. d10.
Chapter VIII
a1. c2. b3. a4. c5. d6. a7. c8. a9. c10.