1. William Stallings Data and Computer Communications Chapter 1 Introduction

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. Simplified Communications Model - Diagram

4. 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

5. 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

6. 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

7. 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.

8. Simplified Data Communications Model

9. 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)

10. Simplified Network Model

11. 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)

12. Simple Switched Network

13. 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)

14. 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

15. 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

16. 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

17. 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

18. 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: 1.544 Mbps and 2.048 Mbps

19. 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

20. LAN Topologies

21. 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

22. 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, e-mail facilities, terminals zSystems: physically distinct object that contains one or more entities yComputer, terminal, remote sensor

23. 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

24. 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

25. Protocol Architecture

26. 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

27. 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

28. Simplified File Transfer Architecture

29. 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

30. 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

31. 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

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

33. 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

34. Protocol Architectures and Networks

35. 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

36. Protocol Data Units (PDU)

37. Operation of a Protocol Architecture

38. 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

39. 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

40. 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

41. 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

42. 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

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

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

45. TCP/IP Protocol Architecture Model

46. 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

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

48. OSI Layers

49. 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

50. 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

51. 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

52. OSI vs. TCP/IP

53. 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

54. Standards Organizations zInternet Society zISO (International Organization for Standardization) zITU-T (International Telecommunication Union) yformally CCITT (International Telegraph and Telephone Consultative Committee) zATM forum