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 xData rates, duration, and spacing zSignal Generation zSynchronization xSynchronous or Asynchronous transfers zExchange Management xCooperation between sender and receiver zError detection and correction xDue to signal distortion

Key Communications Tasks zAddressing and routing xHow to deliver data to another entity outside my network zRecovery xIn case of any failure? How do communicating entities recover zMessage formatting xWhat is the format of the message to be understandable by the receiver zSecurity zNetwork Management xStatus, overloads, failures, future growth

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

Internet model local ISP company network regional ISP router workstation server mobile

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 xnetwork resources (e.g., bandwidth) divided into “pieces” xpieces allocated to calls xresource piece idle if not used by owning call (no sharing)

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 each end-end data stream divided into packets z Two packets share network resources z each packet uses full link bandwidth z resources used as needed

Packet switching: Internet case A B C 1.5 Mbs D E statistical multiplexing queue of packets waiting for output link Sequence of A & B packets does not have fixed pattern  statistical multiplexing.

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 yPacket switching 64kbps yFrame Relay 2Mbps

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 yDigital switches zWide variety of services y Supports a broad range of traffic type zEntirely digital domain yNarrowband ISDN 64kbps yBroadband ISDN 100s of Mbps

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

Computer communication zInformation exchanged between two computers for the purpose of cooperative actions is referred to as computer communications. zTwo computers are interconnected yComputer Networks

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

Protocols Hi Got the time? 2:00 Hi time Got the time?

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 xFile password, file command, file records. yCommunication service module xMake sure that the file data is sent in reliable fashion yNetwork access module xResponsible for sending data

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 yPasses the data to the network, regardless of its type zIndependent of application yData can be a web page, an , the transport layer does not know the difference

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

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 xAccess to user + distributed services zPresentation xIndependence from application processes, data same format zSession xResponsible of establishing, managing, terminating sessions between two communicating applications. zTransport xReliable transport, end-to-end error recovery, flow control zNetwork xProvides switching capabilities to connect between system xResponsible for establishing, maintaining, terminating connections zData Link xResponsible of sending packets, synchronizing, error control, flow control zPhysical xCommunicating unstructured bit stream, mechanical, electrical, physical access

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 yInternet, xIETF (Internet Engineering Task Force) xIAB (Internet Active Board) xIESG (Internet Engineering steering Group) xRFC zISO (international Organization for Standardization) zITU-T International communication Union (formally CCITT) zATM forum