1. Data Communications Chapter 1 Introduction

2. 2 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

3. 3 Simplified Communications Model - Diagram

4. 4 Key Communications Tasks 1/3 zTransmission System Utilization yEfficient use of channel yMultiplexing yCongestion Control zInterfacing yConnection to physical interface / media

5. 5 Key Communications Tasks 2/3 zSignal Generation yForm & intensity yPropagation yRecovery of data zSynchronization yTiming zExchange Management ySetup yData size yError contingencies

6. 6 Key Communications Tasks 3/3 zError detection and correction yParity, ECC zFlow Control zAddressing and routing zRecovery yRecovery from fault in tx zMessage formatting yBinary code zSecurity zNetwork Management

7. 7 Simplified Data Communications Model

8. 8 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 yWide Area Networks yLocal Area Networks

9. 9 Simplified Network Model

10. 10 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)

11. 11 Circuit Switching zDedicated communications path established for the duration of the conversation zPhysical connection is actually connected zConnection cannot be shared e.g. telephone network

12. 12 Packet Switching zData may be 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 zBasic rate of 64 kbps

13. 13 Frame Relay zPacket switching systems have large overheads to compensate for errors zModern comms systems are more reliable zErrors can be caught in end system zMost overhead for error control is stripped out zVariable packet length zData rate up to 2 Mbps

14. 14 Asynchronous Transfer Mode zATM (Cell Relay) zATM is partly evolution of frame relay yLittle overhead for error control yFixed packet (called cell) length zPartly evolution of circuit switching yMultiple virtual channels yDynamic channel bandwidth of channel zCan offer constant data rate using packet switching technique zAnything from 10Mbps to Gbps

15. 15 Integrated Services Digital Network zISDN zDesigned to replace public telecom system zWide variety of services zEntirely digital domain zTwo Generations yNarrowband ISDN xFrame Relay yBroadband ISDN xATM

16. 16 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

17. 17 Protocols zUsed for communications between entities in a system zEntities must speak the same language zEntities yUser applications ye-mail facilities yterminals zSystems yComputer yTerminal yRemote sensor

18. 18 Key Elements of a Protocol zSyntax yData formats ySignal levels zSemantics yControl information yError handling zTiming ySpeed matching ySequencing

19. 19 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

20. 20 Simplified File Transfer Architecture

21. 21 A Three Layer Model zNetwork Access Layer zTransport Layer zApplication Layer

22. 22 Network Access Layer zExchange of data between the computer and the network zSending computer provides address of destination zMay invoke levels of service yPriority, through put zSoftware will be dependent on type of network used (LAN, packet switched etc.)

23. 23 Transport Layer zReliable data exchange yData arrival yData order zIndependent of network being used zIndependent of application

24. 24 Application Layer zSupport for different user applications ze.g. e-mail, file transfer

25. 25 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 yPort

26. 26 Protocol Architectures and Networks

27. 27 Protocols in Simplified Architecture

28. 28 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 to make it more manageable zEach fragment has a transport header added yDestination SAP (DSAP) ySequence number yError detection code zThis header combined with the application layer data make up a transport protocol data unit

29. 29 Network PDU zAdds network header ynetwork address for destination computer (DHOST) yFacilities requests xPriority xThroughput

30. 30 Operation of a Protocol Architecture

31. 31 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

32. 32 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.

33. 33 Network Access Layer zExchange of data between end system and network zDestination address provision zInvoking services like priority In some models the Network Access layer and the Physical layer are combined into a single layer

34. 34 Internet Layer (IP) zSystems may be attached to different networks zRouting functions across multiple networks zImplemented in end systems and routers

35. 35 Transport Layer (TCP) zReliable delivery of data zOrdering of delivery

36. 36 Application Layer zSupport for user applications ze.g. http, SMPT

37. 37 TCP/IP Protocol Architecture Model

38. 38 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

39. 39 OSI Layers zApplication zPresentation zSession zTransport zNetwork zData Link zPhysical

40. 40 Application Layer z The 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.

41. 41 Presentation layer z The 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.

42. 42 Session Layer z The 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.

43. 43 Transport Layer z 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. z A connection oriented transport service ensures that data is delivered error free, in sequence with no losses or duplications. z 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.

44. 44 Network Layer z The Network layer addresses messages and translates logical addresses and names into physical addresses. z 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. z 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. z The packet level of the X.25 standard is a network layer standard.

45. 45 Data Link Layer z The 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.

46. 46 Physical Layer z The 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.

47. 47 OSI v TCP/IP

48. 48 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

49. 49 Standards Organizations zInternet Society zISO zITU-T (formally CCITT) zATM forum

50. 50 Internet Society zInternet Architecture Board yDefines overall architecture of internet zInternet Engineering Task Force (IETF) yProtocol engineering & development zInternet Engineering Steering Group yTechnical management of IETF & Internet standards process

51. 51 Internet RFC Publication Process Internet Draft: Up to 6 months Proposed Standard 6 months Draft Standard 4 months RFC’s are the Proposed Standards

52. 52 International Organisation for standardisation (ISO) zStandardisation steps: yProposal: Proposal to technical committee yPreparatory:Working draft of technical solution yCommittee:Concensus obtained for submission of Draft International Standard (DIS) yEnquiry:5 month period of enquiry resulting in a Final DIS yApproval: 2 month for yes/no approval yPublication

53. 53 ITU-T (formally CCITT) zInternational Telecommunications Union Telecommunications Standardisation Section zUnited Nations specialised agency zPrimary 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.

54. 54 ATM Forum zInternational non profit body with 600 member companies zAccelerated development of standards wrt ATM zStandards based on majority as opposed to concensus zEffort feeds into the ITU-T standardisation effort

55. 55 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) zTanenbaum, Computer Networks, chapter 1 TCP/IP model zUsenet News groups ycomp.dcom.* ycomp.protocols.tcp-ip