Data Communications

Chapter 1Introduction

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A Communications ModelSource

generates data to be transmitted

Transmitter Converts data into transmittable signals

Transmission System Carries data

Receiver Converts received signal into data

Destination Takes incoming data

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Simplified Communications Model - Diagram

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Key Communications Tasks 1/3Transmission System Utilization

Efficient use of channel Multiplexing Congestion Control

Interfacing Connection to physical interface / media

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Key Communications Tasks 2/3Signal Generation

Form & intensity Propagation Recovery of data

Synchronization Timing

Exchange Management Setup Data size Error contingencies

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Key Communications Tasks 3/3Error detection and correction

Parity, ECCFlow ControlAddressing and routingRecovery

Recovery from fault in txMessage formatting

Binary codeSecurityNetwork Management

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Simplified Data Communications Model

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NetworkingPoint to point communication not usually

practical Devices are too far apart Large set of devices would need impractical

number of connections

Solution is a communications network Wide Area Networks Local Area Networks

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Simplified Network Model

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Wide Area NetworksLarge geographical areaCrossing public rights of wayRely in part on common carrier circuitsAlternative technologies

Circuit switching Packet switching Frame relay Asynchronous Transfer Mode (ATM)

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Circuit SwitchingDedicated communications path

established for the duration of the conversation

Physical connection is actually connectedConnection cannot be sharede.g. telephone network

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Packet SwitchingData may be sent out of sequenceSmall chunks (packets) of data at a timePackets passed from node to node

between source and destinationUsed for terminal to computer and

computer to computer communicationsBasic rate of 64 kbps

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Frame RelayPacket switching systems have large

overheads to compensate for errorsModern comms systems are more reliableErrors can be caught in end systemMost overhead for error control is stripped

outVariable packet lengthData rate up to 2 Mbps

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Asynchronous Transfer ModeATM (Cell Relay)ATM is partly evolution of frame relay

Little overhead for error control Fixed packet (called cell) length

Partly evolution of circuit switching Multiple virtual channels Dynamic channel bandwidth of channel

Can offer constant data rate using packet switching technique

Anything from 10Mbps to Gbps

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Integrated Services Digital NetworkISDNDesigned to replace public telecom

systemWide variety of servicesEntirely digital domainTwo Generations

Narrowband ISDNFrame Relay

Broadband ISDNATM

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Local Area NetworksSmaller scope

Building or small campus

Usually owned by same organization as attached devices

Data rates much higherUsually broadcast systemsNow some switched systems and ATM are

being introduced

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Protocols

Used for communications between entities in a system

Entities must speak the same language Entities

User applications e-mail facilities terminals

Systems Computer Terminal Remote sensor

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Key Elements of a ProtocolSyntax

Data formats Signal levels

Semantics Control information Error handling

Timing Speed matching Sequencing

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Protocol ArchitectureTask of communication broken up into

modulesFor example file transfer could use three

modules File transfer application Communication service module Network access module

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Simplified File Transfer Architecture

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A Three Layer ModelNetwork Access LayerTransport LayerApplication Layer

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Network Access LayerExchange of data between the computer

and the networkSending computer provides address of

destinationMay invoke levels of service

Priority, through put

Software will be dependent on type of network used (LAN, packet switched etc.)

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Transport LayerReliable data exchange

Data arrival Data order

Independent of network being usedIndependent of application

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Application LayerSupport for different user applicationse.g. e-mail, file transfer

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Addressing RequirementsTwo levels of addressing requiredEach computer needs unique network

addressEach application on a (multi-tasking)

computer needs a unique address within the computer The service access point or SAP Port

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Protocol Architectures and Networks

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Protocols in Simplified Architecture

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Protocol Data Units (PDU) At each layer, protocols are used to communicate Control information is added to user data at each

layer Transport layer may fragment user data to make

it more manageable Each fragment has a transport header added

Destination SAP (DSAP) Sequence number Error detection code

This header combined with the application layer data make up a transport protocol data unit

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Network PDUAdds network header

network address for destination computer (DHOST)

Facilities requestsPriorityThroughput

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Operation of a Protocol Architecture

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TCP/IP Protocol ArchitectureDeveloped by the US Defense Advanced

Research Project Agency (DARPA) for its packet switched network (ARPANET)

Used by the global InternetNo official model but a working one.

Application layer Host to host or transport layer Internet layer Network access layer Physical layer

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Physical LayerPhysical interface between data

transmission device (e.g. computer) and transmission medium or network

Characteristics of transmission mediumSignal levelsData ratesetc.

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Network Access LayerExchange of data between end system

and networkDestination address provisionInvoking services like priority

In some models the Network Access layer and the Physical layer are combined into a single layer

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Internet Layer (IP)Systems may be attached to different

networksRouting functions across multiple

networksImplemented in end systems and routers

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Transport Layer (TCP)Reliable delivery of dataOrdering of delivery

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Application LayerSupport for user applicationse.g. http, SMPT

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TCP/IP Protocol Architecture Model

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OSI ModelOpen Systems InterconnectionDeveloped by the International

Organization for Standardization (ISO)Seven layersA theoretical system delivered too late!TCP/IP is the de facto standard

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OSI LayersApplicationPresentationSessionTransportNetworkData LinkPhysical

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Application LayerThe application layer represents the level

at which applications access network services. This layer represents the services that directly support applications such as software for file transfers, database access, and electronic mail.

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Presentation layerThe Presentation layer translates data

from the Application layer into an intermediary format. This layer also manages security issues by providing services such as data encryption, and compresses data so that fewer bits need to be transferred on the network.

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Session LayerThe Session layer allows two applications

on different computers to establish, use, and end a session. This layer establishes dialog control between the two computers in a session, regulating which side transmits, plus when and how long it transmits. The services provided are dialogue discipline – full & half duplex communication; Grouping – grouping of data and Recovery – use of check points in the data transmission.

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Transport Layer The Transport layer handles error recognition and

recovery. It also repackages long messages when necessary into small packets for transmission and, at the receiving end, rebuilds packets into the original message. The receiving Transport layer also sends receipt acknowledgments.

A connection oriented transport service ensures that data is delivered error free, in sequence with no losses or duplications.

The transport layer is concerned with optimising the network and providing Quality of Service parameters to session entities. These may specify acceptable error rates, maximum delay priority and security.

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Network Layer The Network layer addresses messages and

translates logical addresses and names into physical addresses.

It determines the route from the source to the destination computer and manages traffic problems, such as switching, routing, and controlling the congestion of data packets.

It relieves higher layers from knowing about underlying data transmission and switching technologies used to interconnect systems. This layer may use packet switched and circuit switched networks.

The packet level of the X.25 standard is a network layer standard.

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Data Link LayerThe Data Link layer packages raw bits

from the Physical layer into frames (logical, structured packets for data). This layer is responsible for transferring frames from one computer to another, without errors. After sending a frame, it waits for an acknowledgment from the receiving computer. Examples of standards at this layer are HDLC, LAPB,LLC and LAPD.

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Physical LayerThe Physical layer transmits bits from one

computer to another and regulates the transmission of a stream of bits over a physical medium. This layer defines how the cable is attached to the network adapter and what transmission technique is used to send data over the cable.

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OSI v TCP/IP

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StandardsRequired to allow for interoperability

between equipmentAdvantages

Ensures a large market for equipment and software

Allows products from different vendors to communicate

Disadvantages Freeze technology May be multiple standards for the same thing

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Standards OrganizationsInternet SocietyISOITU-T (formally CCITT)ATM forum

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Internet SocietyInternet Architecture Board

Defines overall architecture of internet

Internet Engineering Task Force (IETF) Protocol engineering & development

Internet Engineering Steering Group Technical management of IETF & Internet

standards process

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Internet RFC Publication Process

Internet Draft: Up to 6 monthsProposed Standard 6 monthsDraft Standard 4 months

RFC’s are the Proposed Standards

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International Organisation for standardisation (ISO)Standardisation steps:

Proposal: Proposal to technical committee Preparatory:Working draft of technical solution Committee:Concensus obtained for submission

of Draft International Standard (DIS) Enquiry:5 month period of enquiry resulting in

a Final DIS Approval: 2 month for yes/no approval Publication

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ITU-T (formally CCITT)International Telecommunications Union

Telecommunications Standardisation Section

United Nations specialised agencyPrimary objective is to standardise

techniques & operations to enable end to end compatibility of international telecommunication connections, regardless of the countries of origin of the source or destination.

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ATM ForumInternational non profit body with 600

member companiesAccelerated development of standards wrt

ATMStandards based on majority as opposed

to concensusEffort feeds into the ITU-T standardisation

effort

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Further Reading

Stallings, W. Data and Computer Communications (6th edition), Prentice Hall 1999 chapter 1

Web site for Stallings book www.shore.net/~ws/DCC6e.html

Web sites for IETF, IEEE, ITU-T, ISO Internet Requests for Comment (RFCs) Tanenbaum, Computer Networks, chapter 1

TCP/IP model Usenet News groups

comp.dcom.* comp.protocols.tcp-ip