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 yMake efficient use of shared transmission facilities yMultiplexing yCongestion control zInterfacing yA device must interface with the transmission system zSignal Generation ySignals must be xCapable of being propagated through transmission system xInterpretable as data at the receiver

Key Communications Tasks zSynchronization yReceiver must know xWhen a signal begins to arrive and when it ends xDuration of a signal zExchange Management yBoth devices may transmit simultaneously or take turns yAmount of data sent at one time yData format yWhat to do if an error occurs

Key Communications Tasks zError detection and correction yRequired when errors cannot be tolerated zFlow Control yAssure that source does not overwhelm destination by sending data faster than can be processed zRecovery yRequired when an information exchange is interrupted due to a fault in the system yResume activity at point of interruption or yRestore system state to condition prior to beginning of exchange

Key Communications Tasks zAddressing and routing yA source system must indicate the identity of intended destination yTransmission system maybe a network through which various paths (routes) must be chosen zMessage formatting yBoth parties must agree on form of data to be exchanged zSecurity ySender assured only intended receiver receives data yReceiver assured data not altered in transit and actually came from purported sender zNetwork Management yConfigure system, monitor its status, react to failures and overloads.

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 zCommunication networks classified into yWide area networks (WANs) yLocal area networks (LANs)

Simplified Network Model

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

Simple Switched Network

Circuit Switching zDedicated communications path established for the duration of the conversation ye.g. telephone network zThe path is a connected sequence of physical links between nodes zInefficient yChannel capacity dedicated for duration of connection yIf no data, capacity wasted zSet up (connection) takes time zOnce connected, transfer is transparent zDeveloped for voice traffic (phone)

Packet Switching zStation breaks long message into packets sent one at a time to the network zPackets pass from node to node between source and destination zData sent out of sequence zUsed for computer to computer communications

Packet Switching zAdvantages yLine efficiency xSingle node to node link can be shared by many packets over time xPackets queued and transmitted as fast as possible yData rate conversion xEach station connects to the local node at its own speed xNodes buffer data if required to equalize rates yPackets are accepted even when network is busy xDelivery may slow down zPriorities can be used

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 zOriginal packet switching networks designed with a data rate of 64 Kbps zFrame relay designed with a data rate of 2 Mbps

Asynchronous Transfer Mode zATM is evolution of frame relay zFrame relay uses variable length packets called frames zATM uses fixed length packets called cells zLittle overhead for error control zData rate from 10Mbps to Gbps zConstant data rate using packet switching technique

Integrated Services Digital Network zISDN designed to replace public telecom system zDefined by standardization of user interfaces zImplemented as a set of digital switches and paths zEntirely digital domain zSupports voice and non-voice applications zSupport for switched and non-switched applications zReliance on 64 Kbps connections yBasic service: 192 Kbps yPrimary service: Mbps and 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

LAN Topologies

Protocols & Protocol Architecture zIn addition to data path, we need to account for other factors in communication network: ySource must identify the destination to the network ySource must make sure that destination is prepared to accept data ySecurity must be accounted for; data should go to the intended user on the receiver yIncompatible file formats may need to be translated zComputer Communication yExchange of information between computers for cooperative action

Protocols zCommunication must follow some mutually acceptable conventions, referred to as protocol zSet of rules governing the transfer of data between entities zUsed for communications between entities in different systems zCommunicating entities must speak the same language zEntities: anything capable of sending or receiving information yUser applications, facilities, terminals zSystems: physically distinct object that contains one or more entities yComputer, terminal, remote sensor

Key Elements of a Protocol zSyntax yData format and size ySignal levels zSemantics yControl information yError handling yActions to take in response to reception of different messages zTiming ySpeed matching ySequencing yWhen to discard a message, retransmit, give up

Protocol Architecture zProtocols can quickly become very complicated (and thus incorrect) zImplement functionality with several protocols zLayering is a popular way of structuring such a family of network protocols zEach layer represents a new level of abstraction with well defined function zLayer N defined in terms of layer N-1 only, providing total interface to layer N+1

Protocol Architecture

zInterfaces are primitive objects, operations, services provided by one layer to its higher layers zTask of communication broken up into modules zFor example file transfer could use three modules yFile transfer application yCommunication service module yNetwork access module

Protocol Architecture zFile transfer application yTransmitting passwords, file commands, file records yPerform format translation if necessary zCommunication service module yAssure that the two computers are active and ready for data transfer yKeep track of data being exchanged to assure delivery zNetwork access module yInterface and interact with the network

Simplified File Transfer Architecture

A Three Layer Model zCommunications involve three agents: applications, computers, and networks zFile transfer operation: yApplication=>Computer=>Network=> Computer=>Application zCommunication tasks organized into three layers yNetwork access layer yTransport layer yApplication layer

Network Access Layer zExchange of data between the computer and the network zSending computer provides address of destination zMay invoke levels of service such as priority zDependent on type of network used (LAN, packet switched, circuit switching, etc.) zCommunication software above network access layer need not know type of network

Transport Layer zData must be exchanged reliably and in same order as sent zContains mechanisms for reliable data transportation zIndependent of network being used zIndependent of application zProvides services useful to variety of applications zSharing of communication resources

Application Layer zContains logic needed to support various user applications ye.g. , file transfer zSeparate module for each application

Addressing Requirements zEvery entity in overall system must have a unique address 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

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, called a transport protocol data unit, has a transport header added yDestination SAP ySequence number yError detection code zNetwork PDU adds network header ynetwork address for destination computer yFacilities requests like priority level

Protocol Data Units (PDU)

Operation of a Protocol Architecture

Protocol Architectures zTwo protocol architectures as the basis for development of interoperable communications standards yTCP/IP protocol suite yOSI reference model zTCP (Transmission Control Protocol)/IP (Internet Protocol) is the most widely used interoperable architecture zOSI (Open Systems Interconnection) model is the standard model for classifying communications functions

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 zNature of signals zData rates

Network Access Layer zExchange of data between end system and network zDestination address provision zInvoking services like priority zDifferent standards are used for circuit switching, packet switching (X.25), LANs (Ethernet) zMainly concerned with access and routing data between two computers in same network

Internet Layer (IP) zSystems may be attached to different networks zRouting functions across multiple networks zImplemented in end systems and routers zRouters connect two networks and relay data from one network to the other

Transport Layer (TCP) zReliable delivery of data zOrdering of delivery zMost common protocol is the transmission control protocol (TCP)

Application Layer zContains logic to support various user applications zSeparate module for each application ye.g. http, ftp, telnet

TCP/IP Protocol Architecture Model

OSI Model zOpen Systems Interconnection zDeveloped by the International Organization for Standardization (ISO) zA model for computer communications architecture zFramework for developing protocol standards zSeven layers zTCP/IP is the de facto standard

OSI Layers zApplication zPresentation zSession zTransport zNetwork zData Link zPhysical

OSI Layers

zPhysical layer: yTransmits unstructured bit stream over transmission medium yMechanical, electrical, functional, and procedural characteristics to access medium zData link layer: yReliable transfer of information across physical layer ySends blocks/frames with synchronization, error control, and flow control

OSI Layers zNetwork layer: ySeparates data transmission and switching technologies from upper levels yEstablishes, maintains, and terminates connections zTransport layer: yReliable and transparent transfer of data between end points yEnd-to-end error recovery and flow control

OSI Layers zSession layer: yControl structure for communication between applications yEstablishes, maintains, and terminates sessions between cooperating applications zPresentation layer: yMakes applications independent from differences in data presentation zApplication layer: yAccess to the OSI environment yDistributed information services

OSI vs. TCP/IP

Standards zRequired to allow for interoperability between equipment zGovern physical, electrical, and procedural characteristics of communication 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 (International Organization for Standardization) zITU-T (International Telecommunication Union) yformally CCITT (International Telegraph and Telephone Consultative Committee) zATM forum