William Stallings Data and Computer Communications Chapter 1 Introduction

A Communications Model zSource ygenerates data to be transmitted zTransmitter yConverts data into transmittable signals zTransmission System yCarries data zReceiver yConverts received signal into data zDestination yTakes incoming data

Simplified Communications Model - Diagram

Key Communications Tasks zTransmission System Utilization zInterfacing zSignal Generation zSynchronization zExchange Management zError detection and correction zAddressing and routing zRecovery zMessage formatting zSecurity zNetwork Management

Simplified Data Communications Model

Networking zPoint to point communication not usually practical yDevices are too far apart yLarge set of devices would need impractical number of connections zSolution is a communications network

Simplified Network Model

Wide Area Networks zLarge geographical area zCrossing public rights of way zRely in part on common carrier circuits zAlternative technologies yCircuit switching yPacket switching yFrame relay yAsynchronous Transfer Mode (ATM)

Circuit Switching zDedicated communications path established for the duration of the conversation ze.g. telephone network zDWDM

Packet Switching zData sent out of sequence zSmall chunks (packets) of data at a time zPackets passed from node to node between source and destination zUsed for terminal to computer and computer to computer communications

Frame Relay zPacket switching systems have large overheads to compensate for errors zModern systems are more reliable zErrors can be caught in end system zMost overhead for error control is stripped out

Asynchronous Transfer Mode zATM zEvolution of frame relay zLittle overhead for error control zFixed packet (called cell) length zAnything from 10Mbps to Gbps zConstant data rate using packet switching technique

Integrated Services Digital Network zISDN zDesigned to replace public telecom system zWide variety of services zEntirely digital domain

Local Area Networks zSmaller scope yBuilding or small campus zUsually owned by same organization as attached devices zData rates much higher zUsually broadcast systems zNow some switched systems and ATM are being introduced

Protocols zUsed for communications between entities in a system zMust speak the same language zEntities yUser applications y facilities yterminals zSystems yComputer yTerminal yRemote sensor

Key Elements of a Protocol zSyntax yData formats ySignal levels zSemantics yControl information yError handling zTiming ySpeed matching ySequencing

Protocol Architecture zTask of communication broken up into modules zFor example file transfer could use three modules yFile transfer application yCommunication service module yNetwork access module

Simplified File Transfer Architecture

A Three Layer Model zNetwork Access Layer zTransport Layer zApplication Layer

Network Access Layer zExchange of data between the computer and the network zSending computer provides address of destination zMay invoke levels of service zDependent on type of network used (LAN, packet switched etc.)

Transport Layer zReliable data exchange zIndependent of network being used zIndependent of application

Application Layer zSupport for different user applications ze.g. , file transfer

Addressing Requirements zTwo levels of addressing required zEach computer needs unique network address zEach application on a (multi-tasking) computer needs a unique address within the computer yThe service access point or SAP

Protocol Architectures and Networks

Protocols in Simplified Architecture

Protocol Data Units (PDU) zAt each layer, protocols are used to communicate zControl information is added to user data at each layer zTransport layer may fragment user data zEach fragment has a transport header added yDestination SAP ySequence number yError detection code zThis gives a transport protocol data unit

Network PDU zAdds network header ynetwork address for destination computer yFacilities requests

Operation of a Protocol Architecture

TCP/IP Protocol Architecture zDeveloped by the US Defense Advanced Research Project Agency (DARPA) for its packet switched network (ARPANET) zUsed by the global Internet zNo official model but a working one. yApplication layer yHost to host or transport layer yInternet layer yNetwork access layer yPhysical layer

Physical Layer zPhysical interface between data transmission device (e.g. computer) and transmission medium or network zCharacteristics of transmission medium zSignal levels zData rates zetc.

Network Access Layer zExchange of data between end system and network zDestination address provision zInvoking services like priority

Internet Layer (IP) zSystems may be attached to different networks zRouting functions across multiple networks zImplemented in end systems and routers

Transport Layer (TCP) zReliable delivery of data zOrdering of delivery

Application Layer zSupport for user applications ze.g. http, SMPT

TCP/IP Protocol Architecture Model

OSI Model zOpen Systems Interconnection zDeveloped by the International Organization for Standardization (ISO) zSeven layers zA theoretical system delivered too late! zTCP/IP is the de facto standard

OSI Layers zApplication zPresentation zSession zTransport zNetwork zData Link zPhysical

OSI v TCP/IP

Standards zRequired to allow for interoperability between equipment zAdvantages yEnsures a large market for equipment and software yAllows products from different vendors to communicate zDisadvantages yFreeze technology yMay be multiple standards for the same thing

Standards Organizations zInternet Society zISO zITU-T (formally CCITT) zIEEE zATM forum

Further Reading zStallings, W. Data and Computer Communications (6th edition), Prentice Hall 1999 chapter 1 zWeb site for Stallings book ywww.shore.net/~ws/DCC6e.html zWeb sites for IETF, IEEE, ITU-T, ISO zInternet Requests for Comment (RFCs)