CS22 DATA COMMUNICATIONS AND NETWORKS

UNIT I:

Introduction: A brief History Applications Computer networks Categories of networks

Standards and Standards Organizations Network architecture Open Systems and OSI models-

TCP/IP Architecture.

Communication Media and Data Transmission:

Fourier Analysis-Analog and digital Data Transmission-Modulation and Demodulation-

Transmission Media-Wireless Communication-Data Transmission Basics-Transmission mode-

Interfacing-Multiplexing.

Data Link Control and Protocol Concepts: Flow Control-error control-Asynchronous Protocols-High

Level Data Link Control (HDLC)

UNIT II:

Local Area Networks: Types of Networks and Topology-LAN Transmission

Equipment-LAN Installation and Performance-Ethernet: IEEE Standard802.3-Token Bus: IEEE

Standard 802.2-Token Ring: IEEE Standard 802.5-Fiber Distributed Data Interface (FDDI)-

Distributed Queue Dual Bus (DQDB)-: IEEE Standard 802.6-LAN Operating Systems and

Protocols-Ethernet Technologies.

Wide Area Network: WAN Transmission Methods-WAN Carrier Types-WAN Transmission

Equipments-WAN Design and Multicast Considerations-WAN Protocols.

UNIT III:

Integrated Services and Routing Protocols: Integrating Services-ISDN Services-ISDN

Topology-ISDN Protocols-Broadband ISDN-Asynchronous Transfer Mode (ATM)-Principal

characteristics of ATM-Frame Relay-Comparison of ISDN, ATM and Frame Relay.

Wireless LANs:

WLAN Applications-Wireless LAN Requirements-Planning for Wireless

LANs-Wireless LAN Architecture-IEEE 802.11 Protocol Layer-IEEE 802.11 Physical layer-

Designing the Wireless LAN Layout-WAP Services.

UNIT IV:

Internetworking: Principles of Internetworking-Routing Principles-Internet work

Protocols (IP)-Shortcut of IPv4-IP Next Generation.

TCP Reliable Transport Services: Transport protocols-The Services TCP provides to Applications-

End-to-End Services and Datagram-Transmission Control Protocol-User Datagram Protocol

UNIT V:

Network Applications: Client Server Model-Domain Name System (DNS)-Telnet-

File Transfer and remove file access-Electronic Mail-World Wide Web (WWW).

Network Management: Goal of Network management-Network Management

Standards-Network Management Model-Infrastructure for Network Management-Simple Network

Management Protocol (SNMP).

Text Book:

Data Communication and Computer Networks Second Edition Brijendra Singh PHI,

2006.

Reference:

1. Computer Networks, Andrew S.Tanenbaum, 4th Edition.

2. Data Communication and computer Networks-Prakash C.Gupta, Prentice Hall of

India.

3. Data and computer communications, William Stallings, PHI, 2007

4. Data Communication and Networking Behrouz A, Forouzan, TMH, 2005

5. Data Communications and Computer Networks, Brijendra Singh, PHI, 2006

6. Data Communications and Networks-Achyut .S.Godbole, Tata McGraw Hill,

2005

INTRODUCTION

Data communication is the exchange of data between two devices some

form of transmission medium (such as wire cable). Communicating devices must be

part of a communication system made up of a combination of hardware and software.

The effectiveness of data communication system depends on the three characteristics:

delivery, accuracy, and timeliness.

There are five components in data communication systems as shown in

figure 1.1.

1. Message : Message is the information (data message).

2. Sender : Device which sends the data message. 3. Receiver : Device which receives the data message. 4. Medium : Physical path by which a message travels from the sender to the

receiver.

5. Protocol : Set of rules that govern data communication.

A BRIEF HISTORY The fields of communications are certainly not new: people have been

communication. Since the early days when humans grunted and scratched pictures

on cave walls, which are form of communication based on the auditory and visual

senses, where you either hear some one speaking or see letters and symbols that

and symbols that define a message.

Communications changed drastically in 1837, after the invention of the

telegraph by Samuel mores. Telegraph invention made possible to send information

using electrical impulses over a copper wire.

In 1937, Howard A. Alien of Harvard University began work in the design

of a fully automatic calculating machine using the concepts of Babbage and those

used in punch cards in collaboration with the IBM. Seven years later in January

1994. The design became a reality and was named MARKI.

Another event important to communications occurred in 1945 with the

invention of the first electronic computer. ENIAC (electronic numerical integrator

and calculator). It contains vacuum tubes, registers, capacitors and switches and

was faster than MARK I.

The relation between computers and communications began to emerge after

the invention of transistor of 1947 allowing smaller and cheaper computers to be

built. The new generation of computer that emerged during the 1960s made new

applications such as processing and routing telephone calls economically feasible.

Another mile stone in computer-networking occurred with the development

of ARPANET. It was developed by the US department of defense.

The 1970s and 1980s saw the merger of the fields of computer sciences and

data communications that profoundly changed the technology, products and

companies of the now companied-communication industries.

The 1990s saw the emergence of the world wide web, an application that

makes information from around the world easily accessible from ones desk.

Computers and communication have progressed to the point where most

businesses or schools can no longer function without them.

APPLICATIONS

Transferring data between computers is just one area of communications. Data

communication networks have become an indispensable part of business, industry

and entertainment. Some of the network application in different fields are the

following

Electronic Messaging

Probably the most widely used network application is electronic mail (e-mail).

With e-mail). With e-mail, it is possible to send a message to remote locations from

the privacy of your own home.

Facsimile Machine (Fax)

A fax machine creates an electronic equivalent of an image on a sheet of paper

and then sends the image over Telephone lines. A fax machine at the other and

creates the original papers image.

Teleconferencing

Teleconferencing allows conference to occur without the participants being in

the same place. Teleconferencing includes.

*Text conferencing, where participants communicate through their keyboards and

computer monitors.

*Voice conferencing, where participants at a number of locations communicate

simultaneously over the phone.

*Video conferencing, where participants can see as well as talk to one another.

Cellular Telephone

Hitherto, two parties wishing to use the services of a telephone company had to

be linked by a fixed physical connection.

Information services

Information services include bulletin boards and banks. Bulletin boards allow

the free exchange of some software, files or other information.

Financial Services

Financial services include credit history searches, foreign exchanges and

investment services, and electronic fund transfer.

Marketing and sales

Computer networks are used extensively in both marketing and sales

organization.

COMPUTER NETWORKS

A network is a set of devices connected by media links. A node can be a

computer printer, or any other capable of sending and receiving data generated by

other nodes on the network. The links connecting the devices are called

communication channels.

A computer network may be defined as an interconnected collection of

autonomous computer.

Topology is the layout of the collection formed between computer. To some

extent, the reliability and efficiency of a network is determined by its structure.

Bus Topology

All computers attached to the cable can sense an electrical signal any computer

can send data to any other computer.

Advantages:

Connecting a computer or peripheral to a linear bus is easy.

This topology requires least amount of cabling to connect the computers and

therefore, less expensive than other cabling arrangements.

It is easy to extend a bus since two cables can be joined into one longer cable

with a connector.

Disadvantage:

Entire network shuts down if there is a failure in the backbone.

Heavy traffic can slow down a bus because computers on such networks do

not coordinate with each other to reserve time to transmit.

Star Topology

The star topology is the oldest communications design method, with roots in

telephone switching systems. However, the advance in network technology have made

the star technology a good option for modern networks.

A hub is a central device that joins single cable segments or individual LANs

into one network.

A typical hub consists of an electronic device that accepts data from sending

computer and delivers it to the appropriate destination.

Advantage:

Star topology is easy to install and wire.

The network is not disrupted even if a node fails or is removed from the

network.

Fault detection and removal of faulty parts easier in star topology.

Disadvantage:

It requires a longer length of cable.

If the hub fails, node attached to it or disabled.

The cost of the hubs makes the network expensive as compared to bus and

ring topology.

Ring Topology

The ring topology is a continuous path for data with no logical beginning or

ending points and thus no terminators. Workstations and file servers are attached to

the cable at points round the rings.

The ring topology is easier to manage than the bus because the equipment used

to build the ring makes it easier to locate a defective node or cable problem.

Advantages:

Ring topology is easy to install and reconfigure.

Every computer is given equal access to the ring, hence no single computer

can monopolise the networks.

Disadvantages:

Failure in any cable or node breaks the loop and take down the entire

network.

Maximum ring length and number of nodes are limited.

Tree Topology

A tree topology is a variation of star. As in star, nodes in a tree are linked to a

central hub that controls the traffic to the network. However, not every device plugs

directly into the central hub.

Advantages:

The distance to which a signal can travel increases as the signal passes

through a chain of hubs.

Tree topology allows isolating and prioritizing communications from

different nodes.

Disadvantages:

If the backbone line breaks, the entire segment goes down.

It is more difficult to configure wire than other topologies.

Mesh Topology

The mesh topology has a direct connection between every pair of devices in the

network. This is extreme design. Communications becomes very simple because there

is no common line.

Advantages:

The use of large number of links eliminates network congestion.

If one link becomes unusable, it does not disable the entire system.

Disadvantages:

The amount of required cabling is very large.

The amount of hardware required in this type of topology can make it

expensive to implement.

Combined Topologies

Many computer networks use combinations of the various topologies. It has a

common bus , sometimes called the backbones, which allows user to access main

frames and high volume or frequency accessed storage.

Categories of Networks

There are two types of networks based on transmission technology

1. Broadcast networks

2. Point to point networks

Broadcast network have a single communication channel that is shared by all

the machines on the networks. Short messages- called packets. When a packet with

this code is transmitted, it is received and processed by every machine on the

network. This mode of operation is called broadcasting.

Point to point network consist of many connections between individual pairs of

machines.

Local Area Network (LAN)

LANs are used to interconnect distributed communities of computer based data

terminal equipment located within a single building or localized group of building.

LAN interconnect work station distributed around offices within a single building such

as university campus, factory or hospital campus.

Metropolitan Area Network (MAN)

MAN is basically a bigger version of a LAN and normally uses similar

technology. MAN is designed to extend over an entire city.

Wide Area Network (WAN)

A WAN is at the far end of the spectrum because it is for reaching system of

networks that form a complex whole. One WAN is composed of two or more LANs

that are connected across a distance of more than 30 miles.

Multimedia Network

The term multimedia is used to indicate that the information/data being

transferred over the network may be composed of one or more of the following media

types:

Text

Images

Audio

Video

There are 5 basic types of communication network that are used to provide multimedia

communication services:

1. Telephone networks

2. Data networks

3. Broadcast television networks

4. Integrated services digital networks

5. Broadband multi service networks

STANDARDS AND STANDARD ORGANISATIONS

The Need for Standards

The standards used in the computer industry by the various international bodies

were concerned primarily with either the internal operation of a computer or the

connection of a local peripheral devices.

Several national and international agencies play a strong roll in establishing

network standards that ensure a common ground for communications and network

equipments. Key among these agencies are:

American National Standard Institute (ANSI)

International Electro Technical Commission (IEC)

International Telecommunication Unions (ITU)

Institute Of Electrical And Electronics Engineers (IEEE)

International organization For Standardization (ISO)

Internet society and the associated Internet Engineering Task Force (IETF)

Electronic Industries Alliance (EIA) and the associated Telecommunications

Industry Association (TIA)

American National Standard Institute (ANSI)

ANSI is a private non-government agency where members are manufactures

users and other interested companies.

International Electro-Technical Commission (IEC)

IEC is a non governmental agency devising standards for data processing and

inter connections and safety in office equipment.

International Telecommunications Union (ITU)

ITU is an agency of the United Nations and has three sectors:

1. ITU-R deals with radio communication.

2. ITU-D is a development sector.

3. ITU-T deals with telecommunication.

Institute of Electrical and Electronics Engineering (IEEE)

The IEEE is the largest professional organization in the world and consists of

computing and engineering professionals.

International Organizations for Standardizing (ISO)

The International Organization is a non-governmental organization based in

Geneva, Switzerland, in which over 100 countries participate.

Internet society and the associated Internet Engineering Task Force (IETF)

IETF focuses on technical internet issues. Important contributions include the

development of simple network Management Protocol (SNMP).

Electronic Industries Alliance (EIA) and the associated Telecommunications

Industry Association (TIA)

TIA was created as a separate body within a EIA to develop

telecommunications and cabling standards.

NETWORK ARCHITECTURE

Network designers have developed general blue-print-usually called a network

architecture that guides the design and implementation of network.

Designing a network to meet these requirements is no small tasks.

The essential elements of network architecture are:

Digital transmission lines for the transfer of streams of binary information

between equipment.

Exchange of frames of information between adjacent equipment ; those

frames contain delineation information as well as check bits for error control.

Address to identify points of attachment to a network or a inter network.

Exchange of packets of information between packet switches in a network.

Congestion control mechanism may be used to prevent congestion inside the

network.

Inter networking provides connectivity across multiple, possibly dissimilar,

networks by using gateways or routers.

A multiplicity of applications that built on the transfer of messages between

computers.

OPEN SYSTEMS AND OSI MODEL

A set of protocols that would allow any two different systems to communicate

regardless of their underlying architecture is called an open system.

Layers of OSI Model

The OSI Model consists of the following layers:

1. Application layer

2. Presentation layer

3. Session layer

4. Transport layer

5. Network layer

6. Data link layer

7. Physical layer

Application layer protocols have been developed for file transfer e-mail, network

management and other applications.

The presentation layer is responsible for presenting data in the format the user

can understand.

The session layer allows application on two different computers to establish a

session or logical connections.

The transport layer is responsible to ensure that data is sent reliable from the

sending node to the destination node.

The network layer deals with routing strategies, which are responsible for

delivery of a packet from source to destination.

The data link layer provides for the transfer of frames across a transmission link

that directly connects two nodes.

The physical layer deals with the transfer of bits over a communication channel,

for example, copper wire pairs, coaxial cable, radio or optical fiber.

TCP/IP ARCHITECTURE

When the OSI model was developed in 1978, many felt that it would replace

proprietary, vendor-specific architecture such as IBMs system network architecture,

which was in wide spread use in the late 1970s and 1980s.

In 1970s US Department of Defence wanted to interconnect computer and

networks it had acquired from the different vendors.

The government Advanced Research Project Agency (ARPA) developed a set

of protocols called the TCP/IP to enable the interconnection.

TCP/IP is also the protocol of choice for most medium and large sized

networks. It is one of the oldest protocols and is a proven technology that is used by

millions of computer users around the globe.

The TCP/IP protocol suite is made of 5 layers:

Application layer

Transport layer

Internet layer

Network access layer

Physical layer

The first four layers provide physical standards, network interface, internet working,

and transport functions that corresponds to the first 4 layers of the OSI model.

A number of applications have been standardized to operate on top of TCP/IP.

There of the most common here.

Simple Mail Transfer Protocol (SMTP)

Simple Mail Transfer Protocol (SMTP) provides a basic electronic mail facility.

It provides a mechanism for transferring messages among separate hosts. Features of

SMTP include mailing lists, return receipts and forwarding.

File Transfer Protocol (FTP)

FTP is used to send files from one system to another under user command. Both

text and binary files are accommodated, and the protocol provides features for

controlling user access.

The file is transferred over the data connection, without the overhead of any

headers of control information at the application level.

TELNET (Terminal Network)

The main task of the internet and its TCP/IP protocol suite is to provide services

for users.

TELNET is an abbreviation of TErminaL NETwork. Client-server application

program is called TELNET.

Communication Media and Data Transmission

Fourier Analysis:-

A Famous Mathematician, JeanBaptiste Fourier developed a theory

that any periodic function can be expressed as an Infinite series of sums of sine and

cosine function of varying amplitude, frequency and phase shift this series is called

Fourier series.

F(t)=1/2C + an sin(2nft)+ an cos(2nft)

F=1/T is called fundamental frequency.

The number of times a signal oscillates per unit time is called its frequency

It measurements are cycle per second or equivalently ,Hrtz(HZ).

T

sin (2kft) sin (2nft) dt{0 for k=n}

0 { T/2 for kn}

Only one term at summation survives an the bn summation vanishes completely

C=2/T F(t)dt.

Bandwidth limited signals:-

The transmission of the ASCII character b is encoded in an 8 bit byte.

Transmitted is 01100010.

The Maximum Data rate of a channel:-

The maximum data rate =2hLog2 v bits/sec

Maximum number of bits/sec=hLog2h s/n)

Analog And digital Data Transmission:-

An example of analog data is the human voice.

Some one speaks; an analog wave is created in the air.

An example on digital data is data stored in the computer in the form of 0

and 1.

Analog Signals:-

Analog signals can be classified simple or composite.

A composite analog signal is composed of multiple sine wave.

S (t) =A sin (2ft+)

S->Instaneous amplitude

A->peak amplitude

f->frequency

->phase

Frequency refers to the number at periods in one second

Frequency is the inverse of periods f=1/T.

Unit Equivalent

Second(s) 1s

Millisecond(ms) 10power-3s

Microsecond(s) 10power-6s

Nanosecond(ns) 10power-9s

Picosecond(ps) 10power-12s

Unit Equivalent

Hertz(HZ) 1HZ

kilo Hertz (KHZ) 10power3HZ

mega Hertz(MHZ) 10power6HZ

giga Hertz(GHZ) 10power9HZ

Tera Hertz(THZ) 10power12HZ

The position of the waveform relative to time zero.

The phase is measured in degrees or radians.

The phase shift of 360 degree corresponds to a shift of completed period.

180 degree corresponds to a shift one half of the period.

90 degree corresponds to a shift quarter of the period.

Digital Signals:-

Data can be represented by a digital signal.

1can be encoded as a positive voltage

0can be encoded as a negative voltage

Analog transmission can use a Band pass channel.

Modulation and Demodulation:-

Computer are Digital devices,computer communication such as

terminal to computer or computer to disk transmission use digital signals.

Modulation means Digital signal into analog signal.

Example:

PC and a telephone,PC and a digital signal via its modem port Modem convert digital

signal to analog signal .

Telephone lines carry frequencies between 300 and 3300 HZ.its used for voice

transmission.

Digital to Analog Conversion:-

1.frequency modulation

2.amplitude modulation

3. phase modulation

Frequency modulation:-

Its also called the frequency shift keying it assign the frequency range 0 and 1

Amplitude modulation:-

Its also called the amplitude shift keying it similar to frequency

shift keying. it assign the range 0 and 1.

Phase modulation:-

Its also called the phase shift keying it similar to frequency shift keying. it assign the

range 0 and 1.

Analog to Digital conversion:-

Reverse of modulation method. Incoming signals are frequency amplitude

phase shifting.

Transmission Media:-

It can classified as guided or unguided .

Guided media provide physical path for example twisted pair coaxial

cable, optical fibers.

Unguided media employee an antenna for transmitting air, vacuum or

water

Twisted Pair:-

The Twisted pair is a Telephone System, telephones are connected to the

telephone company office by a twisted pair

Twisted pair can run several kilometers and repeater are needed.

The wires are connected in helical form.

Its used for either analog to digital transmission

Electronic Industries association has developed the standards for twisted pair

cables. It comes in several types.

1) category 3 twisted pair

1) category 5 twisted pair

Base band Coaxial Cable:-

It carry signals of higher frequency .it consistence of shift copper wire.

It have outer conductor of metal foil, braid.

It contains the high bandwidth and excellent noise immunity.

Two kinds of cables 50 ohm(digital) 75 ohm (analog)

Base band Coaxial cable:-

It comes from telephone world. 4 KHZ used in transmission. It used analog

transmission. It is also called the brand band.

Base band differ from broad band it need analog amplifiers to strengthen.

Two types of broadband system.

Optical Fiber:-

Optical fiber uses light, not electricity, to transmit information.

An optical fiber cable has a cylindrical shape and consists of three sections.

1) The core

2) The Cladding

3) The jacket

Core is the innermost section and consists of fiber mode of glass or plastic.

Core has a diameter in the range 8 to 100 m

The jacket is composed at plastic and other material layered to protect against

moisture

Single mode fiber

Multi mode fiber

Single mode fiber is manufacture in much smaller diameter than that of multi mode

fiber and with substantially lower density.

Optical transmission system has three components

1)Light source

2)Transmission media

3) Detector

Wireless Communication:-

Wireless communication transport electromagnetic waves without using

physical conductor.

They are several wireless media available for transmitting network package.

1)Radio waves.

2) Infrared signals.

3) Micro waves

Radio Waves:-

Radio network transmission, signal is transmitted one or multiple direction,

depending on the type of antenna

Electromagnetic waves having frequency between 3KH Z 1GHZ are called

radio waves.

Advantages:-

Inexpensive, option for portable communication.

Disadvantages:-

Not feasible, when higher speed communication are need.

Microwaves:-

Electromagnetic waves having frequency is between 1 and 300 GHZ are called

microwaves.

It is used to link two are more ground based microwave transmitted or receiver

know as earth station or ground station.

It handles two links uplink and downlink. uplink is used to receives

transmission on one frequency band .

Down link is used to transmit it on another frequency band.

Single orbiting satellite operate on a number frequency called transponders

channels.

Infrared :-

Can also be used as a media for network communication.

This communication used in remote control device using in television and

stereo.

Does not penetrate walls.

Infrared transmission may not be feasible when high speed communication is

needed.

Data Transmission Basics:-

Data into a computer via a keyboard.

Each selected key elements an alphabetic or numeric

The two most widely used codes that have been adopted for this function are

the Extended Binary Coded Decimal Interchange Code(EBCDIC).

The ASCII American Standards Committee for Information Interchange.

EBCDIC is an 8 bit code .manufactured by IBM.

ASCII is an 7 bit code manufactured by ITUT.

Transmission Mode:-

They are three types of transmission mode.

1) Simplex

2) Half duplex

3) Full duplex

Simplex mode the communication is unidirectional as one way street.

Only one transmit and receiver.

Half duplex mode communication both transmit and receivers.

Full duplex mode communication both transmit and receivers simultaneously.

There are two categories of transmission:-

1) parallel transmission.

2) serial transmission

parallel transmission:-

binary data consisting on 1 and 0 may be organized into group of n bits each.

Parallel transmission means that a group of bits is transmitted simultaneously by

using separate line.

Parallel transmission are commonly used when the distance between the two

devices are short.

Example communication between computer and peripheral devices.

Serial transmission:-

It means that a group of bits is transmitted one by one using one line for all bits.

There are two types of serial communication:

1.asynchronous transmission.

2. synchronous transmission.

Asynchronous transmission means that bits are divided into small groups.

Interfacing :-

Data terminal equipments do not connect to a network directly. The DTE, DCE

interface has 4 important categories :

1.mechanical

2.electronical

3.functional

4.procedural

The mechanical characteristics pertain to the actual physical connection of the DTE

and DCE.

DTE and DCE are connected by pin conductors . Functions can be classified into

the broad categories of data control timing and electrical ground.

Procedural specify the sequence of events for transmitting data based on the

functional .

The difference between x.21 and the RS standers is that x.21 was defined as the

digital signaling interface.

x.21 is useful both as an interface to connect digital computer to analog devices

Such as ISDN.

MULTIPLIXING:-

Multiplexing is the techniques by which simultaneously transmission of multiple

signals through data link is possible.

Multiplexing techniques are:-

1) Frequency division multiplexing.

2) Time division multiplexing.

Frequency division multiplexing:-

It used with analog signals. Perhaps its most common use is in television and

radio transmission.

It accept signals from multiple sources.

It has a specified bandwidth, the signals are combined into another, more

complex signal with large bandwidth.

MUX extracts and separates the individual components its carries frequencies .

Time division multiplexing:-

It has many input signals are combined and transmitted to another its used with

digital signals. Multiple transmission can occupy a single linked for specific time .

Each source are transmission is authorized .

Two basic forms are TDM are synchronous TDM and asynchronous TDM.

synchronous TDM :-

The multiplexer allocates the exactly the same timeslot to each transmission

devices at all times.

Time slots are grouped into frames. A frame consists of one complete cycle of

time slots.

Asynchronous TDM :-

Asynchronous TDM are also called statistical time division multiplexing it

avoid this type of waste .

The number of times slot is an Asynchronous TDM frame

To transmit any given point. Each slot is available to any of the attached input

lines that has data to send.

DATALINK CONTROL AND PROTOCOL CONCEPTS

Protocol is a set of rules that governs the operation of functional units to

achieve communication. The type of protocol used to establish a link between two

stations in accordance with the second layer of OSI model is known as data link

protocol.

Data link protocol can be divided into two small subgroups: asynchronous

and synchronous protocols.

Asynchronous protocols treat each character in a bit stream independently.

Synchronous protocols take the whole bit stream and chop it into characters of equal

size.

FLOW CONTROL:

Flow control is the technique which implies on the data link layer that tells

the sender how much data it can transmit before it must wait for an acknowledgement

from the receiver. Any receiving device has a limited amount of memory in which to

store incoming data. So, the sending station must not send frames at a rate faster then

the receiving station can absorb them.

Two techniques are developed to control the flow of data across

communication links: stop-and-wait flow control and sliding window flow control.

STOP-AND-WAIT FLOW CONTROL:

The sender waits for an acknowledgement from the receiver after every

frame, which it transmitted by the source. It indicates the willingness of the receiver to

accept another frame by sending back an acknowledgement to the sender. The source

must wait until it receives the acknowledgement before sending the next frame. The

destination can thus stop the flow of data simply by withholding acknowledgement.

ADVANTAGES:

Simplicity.

DISADVANTAGE:

Inefficiency.

SLIDING WINDOW FLOW CONTROL:

In the previous case, only one frame at a time can be in transmitting and the

sender waits for an acknowledgement from the receiver after every frame. In sliding

window, multiple frames can be transmitted at a time.

Suppose two stations A and B are connected via a full-duplex link. Station B

allocates buffer space for F frames. Thus station B can accept F frames, and station A

is allowed to send F frames without waiting for any acknowledgement. To keep track

of which frames can be acknowledged, each is labeled with a sequence number of the

next frame expected.

This acknowledgement also implicitly announces that station B is prepared

to receive the next frame, beginning with the number specified. The frames are

numbered modulo-n, which means they are numbered from 0 to n-1.For example, if

n=8,the frames are numbered 0,1,2,3,4,5,6,7.

They have sender sliding window and receiver sliding window. The

sliding window of the sender expands to the right when acknowledgements are

received. .The sliding window of the receiver expands to the left when

acknowledgements are received.

ERROR CONTROL:

Error control in the data link layers is based on automatic repeat

request(ARQ)which means retransmission of data in three cases:

1. Damaged frame

2. Lost frame

3. Lost acknowledgement

DAMAGED FRAME:

A recognizable frame does arrive, but some of the bits are in error.

LOST FRAME:

A frame fails to arrive at the other side. for example, a noise burst may

damage a frame to the extent that the receiver is not aware that frame has been

transmitted.

LOST ACKNOWLEDGEMENT:

An acknowledgement fails to arrive at the source. The sender is not

aware that acknowledgement has been transmitted from the receiver.

The purpose of ARQ is to turn an unreliable data link into a reliable one.

Three versions of ARQ have been standardized.

1. stop-and-wait ARQ

2. Go-back-N ARQ

3. selective-reject ARQ

STOP-AND-WAIT ARQ:

Stop-and-wait ARQis based on the stop-and-wait flow control technique. The

sender transmits a single frame and then must await an acknowledgement. No other

data frames can be sent until the receivers reply arrives at the source station.

The sender sends a single frame to the receiver. There is a chance that a

frame that arrives at the destination is damaged. The receiver detects this by using the

error detection technique. To avoid such type of error, the source station is equipped

with a timer. The sender waits for an acknowledgement for a specified timing after

transmitting the frame. If no acknowledgement is received by the time that the timer

expires, then the same frame is transmitted again.

GO-BACK-N ARQ:

The station A is sending frames to station B. After each

transmission, station A sets an acknowledgement timer for the frame just transmitted.

Suppose that station B has previously successfully received frame

(i-1) and A just frame i.e. We will illustrate go-back-n technique based on damaged

frame, lost frame and lost acknowledgement.

SELECTIVE-REJECT ARQ:

In selective-reject ARQ, only the specific damaged or lost frame is

retransmitted. If a frame is corrupted in transit, a NACK is returned and the frame is

retransmitted out of sequence. The receiving device must be able to send the frames it

has and insert the retransmitted frame into its proper place in its sequence. We will

illustrate selective-reject ARQ based on damaged frame, lost frame and lost

acknowledgement.

ASYNCHRONOUS PROTOCOLS:

Asynchronous protocols-used primarily in modems-feature start and

stop bits and variable-length gaps between characters. A variety of Asynchronous

protocols have been developed. That is

1. X-modem

2. Y-modem

3. Z-modem

4. Blocked Asynchronous transmission (BLAST)

X-MODEM:

File transfer communication for telephone line communication between

PCs, designed by Ward Christiansen in 1979, is known as X-modem. It is a half

duplex stop-and-wait ARQ protocol. In this protocol, transmission begins with the

sending of a NACK frame from the receiver to the sender. each time the sender sends

a frame, it must wait for an acknowledgement before the next frame can be sent. A

frame can be resend either if response is not received by the sender after a specified

period of time or if NACK is received by the sender.

Y-MODEM:

Y-modem is a protocol similar to X-modem, with the following major

differences:

1. The data unit is 1024 bytes.

2. ITU-T CRC-16 is used for error checking

3. Multiple files can be sent simultaneously.

Z-MODEM:

Z-modem is a protocol which combines features of both X-modem and Y-

modem.

BLAST:

BLAST is more powerful than X-modem. It is full duplex with sliding window

flow control. It allows the transfer of data and binary files.

SYNCHRONOUS PROTOCOLS:

Synchronous protocols can be divided into two classes. That is

1. Character oriented protocols

2. Bit-oriented protocols.

CHARACTER ORIENTED PROTOCOLS:

Character oriented protocols also called byte-oriented protocols interpret a

transmission frame or packets as a succession of characters, each usually composed of

one byte. These are in use in both point-to-point and multipoint applications. They are

characterized by the selected transmission control characters used to perform the

various transmission control functions associated with link management, flow control,

error control and data transparency.

In all data link protocols, control information is inserted into the data stream

either as separate control frames or as additions to existing data frames.

In character oriented protocols, this information is in the form code words

taken from existing character sets such as ASCII or EBCDIC. The binary Synchronous

communication protocol often referred to as BSC or character-oriented data link

protocol was developed by IBM. It is used with Synchronous, half-duplex

communications and uses a stop-and-wait flow control. Binary Synchronous

communication protocol (BSC) does not support full-duplex communication or sliding

window protocol.

BIT-ORIENTED PROTOCOLS:

In 1975, IBM pioneered the development of bit-oriented protocols with

Synchronous data link control and lobbied the ISO to make SDLC the standard. ANSI

modified SDLC and it became ADCCP and subsequently ISO modified ADCCP to

HDLC. All of these protocols are based on the same stuffing for data transparency.

Since 1981,ITU-T has developed a series of protocols called link access protocols

such as:

1. Link access procedures, balanced (LAPB)

2. Link access procedures, D-channel (LAPD)

3. link access procedures, modem(LAPM)

All link access protocols are based on HDLC. All bit oriented protocols are related to

the HDLC bit-oriented protocol published by ISO.

HIGH LEVEL DATALINK CONTROL:

PRIMARY STATION (CONTROL STATION):

Primary station is responsible for controlling the operation of the link.

It means, the station manages dataflow by issuing commands to other stations and

acting on their responses.

SECONDARY STATION (GUEST STATION):

This station operates under the control of primary station. It means the

secondary station responds to commands issued by a primary station.

COMBINED STATION:

This station combines the features of primary and secondary station.

UNBALANCED CONFIGURATION:

Consists of one primary and one or more secondary stations ans supports

both full duplex and half duplex transmissions.

BALANCED CONFIGURATION:

Consists of two combined stations and supports both full duplex and half

duplex transmissions. HDLC defines three data transfer modes:

1. Normal response mode

2. Asynchronous response mode

3. Asynchronous balanced mode

It is used with a balanced configuration. ABM is used in configuration connecting

combined stations.

CHAPTER -6

LOCAL AREA NETWORKS (LAN)

Introduction:

Local area networks which we normally refer to simply as LANs.

Local area networks or LANs are used to interconnect distributed

communities of computer-based DTE is located within a single building.

For example: University campus,

All the equipment is located within a single establishment; LANs are

normally installed and maintained by the organization.

LAN standards have been developed by the IEEE 802 committee of the

Institute Electronic Engineers (IEEE) and accredited in the area of LAN by

the American National Standards Institute (ANSI).

The following are the examples of requirements that call for higher-speed

LANs:

Centralized server farms:

Many applications are a need for user, systems to be able to draw a

huge amount of data from multiple centralized servers called server farms.

An example is a color publishing operation, in which servers contain tens of

gigabytes of image data that must be downloaded to imaging work stations.

The servers themselves has increased, the bottleneck has shifted to the

network. Switched Ethernet alone would not solve this problem because of

the limit of 10 Mbps on a single link to the client.

Power workgroups:

These groups typically consist of a small number of cooperating

users who need to draw massive data files across the network. Examples are

a software development group that runs tests on a new software version or a

computer-aided design (CAD). In such cases, large amounts of data are

distributed to several workstations, processed, and updated at very high

speed for multiple iterations.

High-speed local backbone:

As the processing demand grows, LANs proliferate at a site, and

high-speed interconnections are necessary.

6.1:TYPES OF NETWORKS AND TOPOLOGY :-

Each types of network require cabling, network equipment, file

servers, workstations, software and training. Some types of networks have

low start-up costs, but are expensive to maintain or upgrade. The topology

is the physical layout of a network combined with its logical characteristics.

The logical side of the network is the way the signal is transferred from

point-to-point along the cable. The layout may be centralized, with each

station physically connected to a central device that dispatches packets

from workstation to workstation. Centralized layouts are like a star with

workstations as its points.

There are three main topologies:

Bus topology, Ring topology, Star topology.

Client/server applications generate a medium to high level of network

traffic, depending on the client/server software design. Networks on which

there is frequent exchange of database information, scientific programs and

publications software generate high levels of traffic. The impact of hosts

and servers on a network is closely linked to the type of software

applications that are used. The network topology for a small business.

Heavily trafficked networks need high-speed data transmission capabilities.

Security, i.e. the protection of data so that only authorized persons have

access, is another issue that influences network design. It may also use data

encryption, which encodes packets and allows only authorized computers to

decode them. High-security networks use fiber-optic cable.

When a new LAN is installed, there are several factors that affect its design,

including the following:

*Anticipated network traffic

*Redundancy requirements

*User movement

*Future growth

*Security consideration

*WAN connectivity

6.2: LAN Transmission Equipment:

LAN Transmission Equipment is used to connect devise on a single

network, to create and connect multiple networks or sub-networks, and to

set up a campus enterprise.

These are included the followings:

1. Network Interface Card 2. Repeaters 3. Hubs 4. Bridges 5. Routers 6. Brouters 7. Switches 8. Gateway

6.2.1:Network Interface Card

It is used to enable a network device, such as a computer equipment, to

connect to the network. The network connection requires four components:

*An appropriate connector for the network medium.

*A transceiver

*A controller to support the Media Access Control data link protocol

*Protocol control firmware

The connector and its associated circuits are designed for a specific type of

medium, for example, coax, and twisted pair or optical fiber. The cable

connector is attached to the transceiver, which may be external to the NIC

or built into it.

The MAC controller unit and the firmware work together to correctly

encapsulate source and destination address, the data to be transported and

the CRC information into the service data unit.

The MAC controller and firmware are customized for particular type of

network transport, which can be any one of the following:

Ethernet

Fast Ethernet

Gigabit Ethernet

Token ring

Fast token ring

Fast Distributed Data Interface

Asynchronous Transfer Mode Ethernet, Fast Ethernet, high speed communication. It is able to handle both

half and full duplex transmission.

6.2.2:Repeaters:

It is an electronic device that operates on only the physical layer of

the OSI model. It connects one or more cable segments and retransmits any

incoming signal to all other segments.

For example the maximum distance that a single can travel on an Ethernet

cable segment is 500 meters but one repeater cans double the effective

length of an Ethernet to 1,000 meters.

Repeaters are not capable of connecting two dissimilar network

technologies.

6.2.3:Hub:

It is a central network device that connects network nodes. It

contains the star topology. Hub may be referred to as a connector, and is a

device that can have multiple inputs and outputs, all active at one time.

Provide a central unit from which to connect multiple nodes into one

network.

Permit large numbers of computers to be connected on single or multiple LANs.

Reduce network congestion by centralizing network design.

Provide multi-protocol services.

Consolidate the network backbone.

Enable high speed communication.

Provide connections for several different media types.

Enable centralizes network management. It also called multistation access unit.

Operating as a central hub an MAU functions at the OSI physical and data

link layers.

There are different kinds of hubs:

1. Passive hub(acts as path way) Data to follow from one device to another.

2. Intelligent hub: It can detect errors and provide assistance to a technician when

attempting to locate a failing component.

3. Active hub: Regenerate and process signals.

6.2.4:Bridge:

It is a network device that connects one LAN segment to another. It

is high efficiency and security. It performs error detection, frame

formatting, frame routing.

Bridges are used:

Extend a LAN when the maximum connection limit such as the 30-node limit on an Ethernet segment, has been reached.

Extend a LAN beyond the length limit, for example beyond 185metres with thin-net Ethernet.

Segment LANs to reduce data traffic bottlenecks. Prevent unauthorized access to a LAN.

If the bridge knows that the destination of a frame is on the segment as

the source of the frame, it drops the frame because there is no need to

forward it.

If the bridge does not know the destination segment, the bridge

transmits the frame to all segments except the source segments, a process

that is called flooding.

Bridges can greatly enhance the performance of a network because

they offer the ability to segment network traffic, limiting traffic to those

networks where it belongs.

A firewall is software or hardware that sources data from being

accessed outside a network and that can also prevent data from leaving the

network through an inside source.

6.2.5:ROUTER:

A router performs some of the same function as a bridge.

Routers connect LANs at the network layer of the OSI model, which

enables them to interpret more information from packet traffic than bridges

can.

In general, routers are used to:

Efficiently direct packets from one network to another, reducing excessive traffic.

Join neighboring or distant network. Connect dissimilar networks. Prevent network bottlenecks by isolating portions of a network. Secure portions of a network from intruders.

The logic that routers use to determine how to forward data is called a

routing algorithm.

6.2.6:BROUTER:

A bridge router (brouter) performs both the functions of a bridge (OSI

layer 2) and a router (OSI layer 3) in a single device.

A brouter is a network device that acts as a bridge in one circumstance and

as a router in another.

Brouters are used to:

Handle packets efficiently on a multiprotocol network that includes some protocols that can be routed and some that cannot be.

Isolate and direct network traffic to reduce congestion. Join networks. Secure a certain portion of a network by controlling who can access it.

6.2.7:SWITCHES:

A switch is a device that connects two or more network segments and

allows different nodes to communicate smoothly with each other as if they

are the only two connecting at the time.

Switches provide bridging capacity Along with the ability to increase the

bandwidth on existing networks.

Switches used on LANs are similar to bridges.

A switch may act as a multiport bridge to connect devices or segments

in a LAN.

A store-and-forward switch stores the frame in the input buffer until

the whole packet has arrived. A cut-through switch on the other hand.

Forwards the packet to the output buffer as soon as the destination address

is received.

6.2.8:GATEWAYS:

Gateways usually operate at OSI layer 4 or higher, and basically

translate the protocols to allow terminals on two dissimilar networks to

communicate.

Gateways can be either/or combinations of hardware and software.

An internet service provider (ISP), which connects users in a home to the

Internet, is a gateway.

Gateways can suffer from slow performance.

A dedicated computer acting as a gateway, if it is of reasonable speed,

usually eliminates any performance problems.

For examples, you might use a gateway to:

Convert commonly used protocols (e.g. TCP/IP) to a specialized protocol (for example, an SNA: System Network Architecture).

Convert message formats from one format to another. Translate different addressing schemes. Link a host computer to a LAN. Provide terminal emulation for connections to a host computer. Direct electronic mail to the right network destination. Connect networks with different architectures.

6.3:LAN Installation and Performance:

Once the LAN has been selected based on the requirements of the

organization, it must be installed by the people within the organization.

Several suppliers of LAN hardware or software may be contacted in the

course of evaluating and selecting the LAN and these companies may offer

installation and maintenance services. Important tasks of installation a LAN

are given below:

Install: New workstations NICs on existing workstations

Wiring or cabling Server hardware Bridges, routers, brouters, or gateways LAN software Determine the access and capability required by each user Document the LANs hardware and software configuration Train the users Using the LAN and its new capabilities Troubleshoot any startup problems

The performance of LAN is based on several factors, including the

protocol that is used, the speed of the transmission, the amount of traffic,

the error rate, the efficiency of the LAN software, and the speed of server

computers and disks.

6.4:ETHERNET: IEEE STANDARD 802.3

IEEE 802.3 supports a LAN standard originally developed by Xerox

and later extended by a joint venture between Digital Equipment

Corporation, Intel Corporation and Xerox. This was called Ethernet.

Ethernet has a bus topology. Stations contended for the segment using

a form of the CSMA/CD contention protocol. It is commonly used to

connect PCs, workstations, and printers and file servers and even

mainframes.

Specifically, the data link layer is responsible for accurate

communication between two nodes in a network. This involves frame

formats, error checking a d flow control. Data link layer is further divided

into two sub layers:

1. Logical Link Control(LIC)

2. Medium Access Control(MAC)

Medium access sub layer

Network can be divided into two categories: point-to-point network

and broadcast network.

Broadcast channels are sometimes referred to as multi-access

channels or random access channels. The protocols used to determine who

goes next on multi-access channel belong to a sub layer of the data link

layer called the multiple access protocols. Many algorithms for allocating

multiple access channels are known. Some of these are:

Pure Aloha

Slotted Aloha

Carrier Sense Multiple Access (CSMA)

CSMA with Collision Detection.

6.4.1:Pure Aloha

In pure Aloha, frames are transmitted at completely arbitrary times.

We have made the frames all of the same length because the throughput of

Aloha systems is maximized by having a uniform frame size rather than

allowing variable length frames.

Whenever two frames try to occupy the channel at the same time,

there will be a collision and both will be confused. If the first bit of a new

frame overlaps with just the last bit of a frame almost finished, both frames

will be totally destroyed and both will have to be transmitted later.

6.4.2:Slotted Aloha

Slotted Aloha has double the capacity of an Aloha system. In slotted

Aloha, time is divided into discrete intervals, each interval corresponding to

one frame. This approach requires the users to agree on slot boundaries.

In slotted Aloha, as against in pure Aloha, a computer is not permitted

to send whenever a carriage return is typed. Instead, it is required to wait

for the beginning of the next slot. Thus the continuous pure Aloha is turned

into a discrete one.

6.4.3:Carrier Sense Multiple Access (CSMA) Protocols

Protocols in which stations listen for a carrier (transmission) and act

accordingly are called carrier sense protocols.

The first carrier sense protocol is 1-persistent CSMA. When a station

has data to send, it first listens to the channel to see if anyone else is

transmitting at the moment. If the channel is busy, the station waits until it

detects an ideal channel.

The second carrier sense protocol is non-persistent CSMA. In this

protocol, a conscious attempt is made to be less greedy than in the previous

one. Before sending, a station senses the channel. If no one else is sending,

the station begins doing so itself.

6.4.4:CSMA with Collision Detection (CSMA/CD)

Persistent and non-persistent CSMA protocols are clearly an

improvement over Aloha because they ensure that no station begins to

transmit when it senses the channel busy.

The interference between two signals is called a collision.

Technically, monitoring a cable during transmission is known as collision

detection (CD), and the Ethernet mechanism is known as Carrier Sense

Multiple Access with Collision Detection (CSMA/CD).

The access mechanism used in an Ethernet is called Carrier Sense

Multiple Access with Collision Detection (CSMA/CD standardized in IEEE

802.3).

6.5:Token Bus: IEEE Standard 802.4:

LAN have a direct application in factory automation and process

control, where the nodes are computers controlling the manufacturing

process. It is a real time processing with minimum delay is needed.

Ethernet is not suitable for this purpose because the number of collisions is

not predictable. It support for factory automation and process control

application that required real time process. It combines features of Ethernet

and collisions free. It is physical bus that operates as a logical ring using

tokens. For example A-B-C-D. A send to D then it passes the information

through B and C. The bus token specify the destination address in the

source. A station receives a token from its predecessor and sends a token to

its successor.

Token bus is limited to factory automation and process control and

has no commercial application in data communication.

6.6:Token Ring: IEEE Standard 802.5:

It is defined by the IEEE standard 802.5. The token ring is a MAC

protocol sitting between the Logical Link Control and the physical layer in

the OSI model.

Station on a token ring LAN is connected in a ring using a NIC.

All the stations are connected to the NIC and then connected to another

one. The network access mechanism used by Ethernet is not infallible and

may result in collision.

6.7:FIBRE DISTRIBUTED DATA INTERFACE (FDDI)

The fiber distributed data interface (FDDI) standard for a 100Mbps

fiber optic LAN was developed during the mid-1980s by a subcommittee of

ANSI and was completed in 1990. LANs based on the IEEE 802 standards

reached capacity, optical fiber LANs based on the FDDI standard became

an alternative growth path. FDDI LANs were used to provide high-speed

backbone connections between distributed LANs

Two types:

Single mode fiber (SMF) and Multimode fiber (MMF)

Single mode fiber: it can deliver connectivity over longer distances,

with higher performance than MMF.

Multimode fiber is usually used to connect devices within a building or

a small geographically contained area.

FDDI has implemented over twisted pair copper wire. The copper

distributed data interface (CDDI) called uses only shielded twisted pair or

unshielded twisted pair category 5 cabling but supports distances of 100

meters and data rates of 100 Mbps. FDDI network contains two complete

rings one that is used to send data when everything is working correctly,

and another that is used only when the first ring fails.

6.8:DISTRIBUTED QUEUE DUAL BUS (DQDB): IEEE

STANDARD 802.6

Local area networks are usually restricted to a single site.

Metropolitan Area Network (MAN) expands network coverage to include

several buildings or sites within a limited area.

IEEE standard 802.6 defines the Distributed Queue Dual Bus (DQDB)

which resembles a LAN standard. It is designed to be used in MAN.

DQDB uses a dual bus configuration.

Each device in the system connects to two backbone links.

Access to these links is granted not by contention (as in 802.3) or token

passing (as in 802.4 and 802.5) but by a mechanism called distributed

queues.

This protocol specifies a dual-bus topology to carry data in forward

and reverse directions.

The forward direction bus carries data while the reverse direction

handles queuing and control information.

For example:

Two unidirectional buses are labeled Bus A and Bus B. Two unidirectional buses (cables) to which all computers are

connected.

Each bus has a head-end. Each bus connects to the stations directly through input and output

ports; no drops lines are used. To send data on one bus, a station must

use the other bus to make a reservation.

6.9:LAN OPERATING SYSTEM AND PROTOCOLS

Several LAN operating systems are associated with specific protocols

that are transported within Ethernet or Token ring.

LAN operating systems include the following:

Novell Netware Windows NT LAN Manage and LAN Server Apple Talk

A local area network can transport several network protocols

individually or in combinations of two, three or more protocols.

For example routers, are often set up to automatically configure themselves

by recognizing the different protocols.

A single Ethernet LAN might host one protocol for a mainframe

computer, a different protocol for Novell servers, and still another protocol

for Windows NT servers.

Internet Packet Exchange (IPX) protocol is designed for use with

Netware. The advantage of IPX over some other early protocols is that it

can be routed, meaning that it can transport data over multiple networks in

an enterprise.

Novell implemented a comparison protocol called Sequence Packet

Exchange (SPX)

The native protocol for Windows NT is NetBEUI, which was

developed for LAN manager and LAN server before the creation of

Windows NT. NetBEUI was developed when computer networking

primarily meant local area networking for a relatively small number of

computers, from just a few to as many as 200.

The advantage of having multiple LAN protocols on a network is that

such a network can perform many different functions on the same LAN,

such as enabling Internet access and access to mainframe computers and

servers.

The disadvantage is that some protocols operate in broadcast mode,

meaning that they frequently send out packets to identify devices on the

network causing a significant amount of redundant network traffic.

The properties of a LAN protocols are similar to those of other

communication protocols, but some LAN protocol were developed in the

early days of networking.

In general, LAN protocols must provide the following:

Reliable network Links Relatively high speeds Source and destination node address handling Adherence to network standards, particularly the IEEE, 802 standards.

6.10:Ethernet Technologies:

Most common Ethernet Technologies are,

10 Base-2 Ethernet Base-5 Thick Ethernet Technology 10 Base-7 and 100 Base-T Gigabit Ethernet

6.10.1:10 Base-2 Ethernet:

Popular and more flexible coaxial cable only 0.25inches in diameters. It is used in PC LANs. In 10 Base-2, 10 is stands for 10Mbps and 2 is denote for 200

meters.

It use bus topology and is the approximate maximum distance between any two nodes.

A dozen PCs need 12 Ethernet cards. It is a cheap-net.

6.10.2:Base-5 thick Technology:

It is using standard coaxial cable, which is 0.4inch in diameter. A speed of 10Mbps using base band transmission for a maximum

distance of 500 meters. It is frequently called thick Ethernet.

6.10.3:10 Base-T and 100 Base-T:

10Base-T transmits at 10Mbps and 100 Base-T Ethernet transmits at 100Mbps.

100Base-T is also commonly called fast Ethernet. It uses a star topology. It has a central office device called a hub. Each adapter on each node has a direct, point-to-point connection to

the hub.

It is used in different building in the same campus.

6.10.4:Gigabit Ethernet:

It is highly successful 10Mbps and 100Mbps Ethernet standards. It offering a raw data of 1000Mbps. It referred to as IEEE802.3z

standard.

It uses CSMA/CD for shared broadcast channels. his allows a full-duplex operation at 1000Mbps in both directions for

point-to point channels. It has a star topology with a hub or switch at its centre. It is a backbone of the interconnecting multiple 10Mbps and 100Mbps

Ethernet LANs.

Local Area Networks

Introduction:

Local Area Networks (LANs), discussed in Chapter 6, typically cover small

geographical areas. They are designed around relatively simple bus or ring topologies.

Some networks such as Wide Area Networks (WAN) however, cover much larger

areas, sometimes panning several continents. In such cases the LAN protocols are

inappropriate and new ones must be defined.

LAN uses include file transfer, electronic mail and file servers just as for WAN.

WAN can be used for remote log-ins also (An application in which a user in one

location logs into a computer at another). WAN protocols must distinguish between

various applications.

There is a difference in routing between LAN and WAN. Routing strategies are

more complex in WAN than in LAN. The fact that there are many ways to go from

one point to another by itself makes the situation more complex.

To add to the complexity, sometimes a link in a chosen route experiences a

failure. What does the network protocol do with all the date traveling in that route? In

some cases a route may prove to be so popular that too much data travels over it. The

result is congestion and sometimes failures. Can network protocols avoid such

situations? If they can not, what can they do to minimize their effects? When data is

delayed due to failures and congestion, it must be stored somewhere while WAN

protocols decide what to do with it. Network nodes must be equipped with software

and buffers to do this.

LANs are controlled and managed by a single organization or department. If a

problem occurs, users know whom to call. Some WANs such as the Internet have

evolved mainly due to voluntary efforts of universities and government agencies.

Consequently, there is no central authority responsible for fixing problems or updating

protocols so that problems do not recur. The success of such network operations

depends on the cooperation of the organizations that use them.

7.1 WAN TRANSMISSION METHODS

WAN transmission methods use different switching techniques. Switching

techniques are use to create one or more data paths called channels for transmitting

data. The channels may be created using one communication cable or using several

cables that offer a range of paths along which data can be transmitted. Switching can

enable multiple nodes to simultaneously transmit and receive data or it can enable data

to be transmitted over different routes to achieve maximum efficiency in terms of

speed and cost. The following are the common switching techniques used in WANs:

Time Division Multiple Access (TDMA)

Frequency Division Multiple Access (FDMA)

Statistical Multiple Access

Circuit Switching

Message Switching

Packet Switching

7.1.1. Time-Division Multiple Access (TDMA)

TDMA divides the channels into distinct time slots. Each time slot is

designated for a particular networks node, as if it were a dedicated line. The WAN

switching deice rotates from time slot to time slot for each channel. This is similar to

a 24-hour television programming, where the time has been specified for a particular

program. TDMA does not guarantee the most efficient use of the network medium

since transmission occurs only via one channel at a time. The timing of node

transmission is also important, since a node may transmit at an interval that is out of

synchronization with its time slot.

7.1.2. Frequency Division Multiple Access (FDMA)

FDMA divides the channels into frequencies instead of time slots. Each

channel has its own broadcast frequency and bandwidth. The switching device

switches from frequency to frequency as it sends data. This is similar to four listeners

with headsets sharing a radio modified to have four channels. The first listener might

be listening to a classical station, the send to a talk show the third to a base ball game

and thr fourth to the news. Each listener is at a different frequency. The radio inputs

to each channel so quickly that none can tell it is quickly switching from channel to

channel as it receives the signal on each frequency.

7.1.3. Statistical Multiple Access (SMA)

Statistical multiple access or statistical multiplexing, is sued by many WAN

technologies, such as X.25, ISDN and frame relay. This method is more efficient than

TDMA and FDMA, because the physical medium bandwidth is dynamically allocated

according to the application need. The switching device continuously monitors each

channel to determine the communication requirements. For example, at one moment a

channel may need to transmit a large graphics file, and then be quiet. Algorithms on

the switch determine the bandwidth needed to transmit the file. After the file is

transmitted, the switch reallocates bandwidth to another channel. This might be

compared to the way in which a workstation operating system automatically decides

how much memory to give to three applications running at the same time. It might

give 15 KB for an active word processing file, 7 MB for an image from a scanner and

1.2MB for printing a graphic.

7.1.4. Circuit Switching

Circuit switching involves creating a dedicated physical circuit between the

sending and receiving nodes. This acts as a straight channel on which to send data

back and forth without interruption, similar to a telephone call between two parties.

The transmission channel remains in the service until the two nodes disconnect.

Communication via circuit switching implies that there is a dedicated communication

path between two stations. The path is a connected sequence of links between

network nodes. On each physical link, a logical channel is dedicated to the

connection. Communication via circuit switching involves there phases.

Phase I: Circuit establishment.

Before any signals can be transmitted an end-to-end (station to station) circuit

must be established. For example, station A sends a request to node4, requesting a

connection to Station E. Typically, the link from A to 4 is a dedicated line, so that part

of the connection already exists. Node 4 must find the next leg in a route leading to

node 6 based on routing information and measures of availability and perhaps cost.

Node 4 selects the link to node 5, allocates a free channel (using FDM or TDM) on

that link and sends a message requesting connection to E. So far, a dedicated path has

been established from A through 4 to 5. Because a number of stations may attach to 4,

it must be able to establish internal paths from multiple stations to multiple nodes.

The remainder of the process proceeds similarly. Node 5 dedicates a channel to node

6 and internally ties that channel to the channel from node 4. Node 6 completes the

connection to E. In completing the connection, a test is made to determine if E is busy

or is prepared to accept the connection.

Phase II: Data Transfer.

Information can now be transmitted from A through the network to E. The data

may be analog or digital, depending on the nature of the network. As the carriers

evolve the fully integrated digital networks, the use of digital (binary) transmission for

both voice and data is becoming the dominant method. The path is: A-4 link, internal

switching through4; 4-5 channel, internal switching through 5; 5-6 channel, internal

switching through 6; 6-E link. Generally the connection is fully duplex.

Phase III: Circuit disconnect.

After some period of data transfer, the connection is terminated, usually by the

action of one of the two stations. Signals must be propagated to nodes 4,5 and 6 to de-

allocate the dedicated resources.

Note that the connection path is established before data transmission begins.

Thus, channel capacity must be reserved between each pair of nodes in the path and

each node must have available internal switching capacity to handle the request

connection. The switches must have the intelligence to make these allocations and to

devise a route through the network.

Circuit switching can be rather inefficient. Channel capacity is dedicated for the

duration of a connection, even if no data are being transferred. For a voice

connection, utilization may be rather high, but it still does not approach 100 per cent.

For a terminal-to-computer connection, the capacity may be ideal during most of the

time of the connection. In terms of performance, there is delay prior to signal transfer

for call establishment. However, once the circuit is established, the network is

effectively transparent to the users. Information is transmitted at a fixed data rate with

no delay other than the propagation delay through the transmission links. The delay at

each node is negligible.

Circuit switching was developed to handle voice traffic but is now also used for

data traffic. The best known example of a circuit-switching network is the public

telephone network. This is actually a collection of national networks intern-connected

to form the international service. Although originally designed and implemented to

service analog telephone subscribers, it handles substantial data traffic via modem and

is gradually being converted to a digital network. Another well known application of

circuit switching is the private branch exchange (PBX) used to interconnect telephones

within a building or office

7.1.5 Message Switching

Message switching uses a store-and-forward communication method to transmit

data from the sending to the receiving node. The data is send from one node to

another, which stores is temporarily until a route towards the datas final destination

becomes available. Several nodes along the route store and forward the data until it

reaches the destination node. Message switching is used for example, when you send

an e-mail message on an enterprise network with file servers acting as post offices.

The message goes from one post office to the next until it reaches the intended

recipient.

7.1.6 Packet Switching

Circuit switching was designed for voice communication. In a telephone

conversation, for example once a circuit is established it remains connected for the

duration of the session. Circuit switching creates temporary (dialed) or permanent

(leased) dedicated links that are well suited to this type of communication.

A key characteristic of circuit-switching networks is that resources within the

network are dedicated to a particular call. For voice connections, the resulting circuit

will enjoy a high percentage of utilization because most of the time, one party or the

other is talking. However, as the circuit-switching network began to be used

increasingly for data connections, two shortcomings became apparent.

1) In a typical user/host data connection (for example, a personal computer user

logged on to a database server) much of the time the line is idle. Thus, with data

connections, a circuit-switching approach is inefficient.

2) In a circuit-switching network, the connection provides for transmission at a

constant data rate. Thus, each of the two devices that are connected must transmit and

receive at the same data rate as the other. This limits the utility of the network in

interconnecting a variety of the host computers and workstations.

To understand how packet switching addresses these problems, let us briefly

summarize the packet-switching operation. Data are transmitted in short packets. A

typical upper bound on packet length is 1000 octets (bytes). If the source has no

longer message to send, the message is broken up into a series of packets as shown in

Figure 7.2 Each packet contains a portion (or all for a short message) of the users

data, plus some control information that the network requires to be able to route the

packet through the network and deliver it to the intended destination. At each node en

route, the packet is received, stored briefly and passed on to the next node.

Now assume that Figure 7.2 depicts a simple packet-switching network.

Consider a packet to be send from Station A to Station E. The packet includes control

information that indicates that the intended destination is E. the packet is send from A

to node 4 stores the packet, determines the next leg of the route (say 5) and queues

the packet to to-out on that link (the 4-5 link). When the link is available, the packet is

transmitted to node 5, which forwards the packet to node 6 and finally to E. This

approach has a number of advantages over circuit switching:

Line efficiency is greater, because a single node-to-node link can be

dynamically shared by many packets over time. The packets are queued up and

transmitted as rapidly as possible over the link. By contrast, with circuit switching,

time on a node-to-node link is pre-allocated using synchronous time-division

multiplexing. Much of the time, such a link may be ideal because a portion of its time

is dedicated to a connection that is ideal.

A packet-switching network can perform date-rate conversion. Two stations of

different data rates can exchange packets because each connects to its node as its

proper data rate.

When traffic becomes heavy on a circuit-switching network, some calls are

blocked; that is, the network refuses to accept additional connection requests until the

load on the network decreases. On a packet-switching network, packets are still

accepted, but delivery delay increases.

Priorities can be used. Thus, if a node has a number of packets queued for

transmission; it can transmit the higher-priority packets first. These packets will

therefore experience less delay than lower-priority packets.

If the station has a message to send through a packet-switching network that is of

length greater than the maximum packet size, it breaks the message up into packets

and sends these packets, one at a time, to the network. A question arises as to how the

network will handle this stream of packets as it attempts to route them through the

network and deliver them to the intended destination. There are two approaches that

are used in contemporary networks: datagram and virtual circuit

In the datagram approach to packet switching, each packet is treated

independently from all others. Even when one packet represents just a piece of a multi

packet transmission, the network (and network layer functions) treats it as though it

existed alone. Packets in this technology are referred to as datagrams. Figure 7.1

shows how the datagram approach can be used to deliver three packets from Station A

to Station E. In this example, all the three packets (or datagram)belong to the same

message, but may go by different paths to reach their destination. This approach can

cause the datagrams of transmission to arrive at their destination out of order. It is the

responsibility of the transport layer in most protocols to reorder the datagrams before

passing them on to the destination port. The link joining each pair of nodes can

contain multiple channels. Each of these channels is capable, in turn, of carrying

datagrams either from several different sources or from one source. Multiplexing can

be done using TDM or FDM.

In the virtual circuit approach to packet switching. The relationship between all

packets belonging to a message or session is preserved. A single route is chosen

between the send and receiver at the beginning of the session. When the data are sent,

all packets of the transmission travel one after another along that route. The difference

from the datagram approach is that, with virtual circuits, the node need not make a

routing decision for each packet. It is made only once for all packets using the virtual

circuit.

7.2 WAN CARRIER TYPES

There are several physical signaling or carrier methods for transporting data on

WANS. Some of the most common include the following:

Point to point

T-carrier

SONET

ISDN

Wireless

7.2.1 Point-to-Point

Point-to-point carrier communications through public dial-up lines and leased

telephone lines represent the most basic WAN carrier communications. For example,

a simple WAN is established every time you employ a modem to make a modem-to-

modem to make a modem-to-modem connection over a dial-up line. The modem at

the other end may be connected to a network or to a computer that is a few miles away

or a few thousand mi