1. Computer Networks Chapter 1 Introduction

2. A Communications Model ÑSource Ñgenerates data to be transmitted ÑTransmitter ÑConverts data into transmittable signals ÑTransmission System ÑCarries data ÑReceiver ÑConverts received signal into data ÑDestination ÑTakes incoming data

3. Simplified Communications Model - Diagram

4. Key Communications Tasks Ñ Transmission System Utilization Ñ Interfacing Ñ Signal Generation Ñ Synchronization Ñ Exchange Management Ñ Error detection and correction Ñ Addressing and routing Ñ Recovery Ñ Message formatting Ñ Security Ñ Network Management

5. Simplified Data Communications Model

6. Networking ÑPoint to point communication not usually practical ÑDevices are too far apart ÑLarge set of devices would need impractical number of connections ÑSolution is a communications network

7. Simplified Network Model

8. Wide Area Networks ÑLarge geographical area ÑCrossing public rights of way ÑRely in part on common carrier circuits ÑAlternative technologies ÑCircuit switching ÑPacket switching ÑFrame relay ÑAsynchronous Transfer Mode (ATM)

9. Circuit Switching ÑDedicated communications path established for the duration of the conversation Ñe.g. telephone network

10. Packet Switching ÑData sent out of sequence ÑSmall chunks (packets) of data at a time ÑPackets passed from node to node between source and destination ÑUsed for terminal to computer and computer to computer communications

11. Frame Relay ÑPacket switching systems have large overheads to compensate for errors ÑModern systems are more reliable ÑErrors can be caught in end system ÑMost overhead for error control is stripped out

12. Asynchronous Transfer Mode ÑATM ÑEvolution of frame relay ÑLittle overhead for error control ÑFixed packet (called cell) length ÑAnything from 10Mbps to Gbps ÑConstant data rate using packet switching technique

13. Integrated Services Digital Network ÑISDN ÑDesigned to replace public telecom system ÑWide variety of services ÑEntirely digital domain

14. Local Area Networks ÑSmaller scope ÑBuilding or small campus ÑUsually owned by same organization as attached devices ÑData rates much higher ÑUsually broadcast systems ÑNow some switched systems and ATM are being introduced

15. Protocols ÑUsed for communications between entities in a system ÑMust speak the same language ÑEntities ÑUser applications Ñe-mail facilities Ñterminals ÑSystems ÑComputer ÑTerminal ÑRemote sensor

16. Key Elements of a Protocol ÑSyntax ÑData formats ÑSignal levels ÑSemantics ÑControl information ÑError handling ÑTiming ÑSpeed matching ÑSequencing

17. Protocol Architecture ÑTask of communication broken up into modules ÑFor example file transfer could use three modules ÑFile transfer application ÑCommunication service module ÑNetwork access module

18. Simplified File Transfer Architecture

19. A Three Layer Model ÑNetwork Access Layer ÑTransport Layer ÑApplication Layer

20. Network Access Layer ÑExchange of data between the computer and the network ÑSending computer provides address of destination ÑMay invoke levels of service ÑDependent on type of network used (LAN, packet switched etc.)

21. Transport Layer ÑReliable data exchange ÑIndependent of network being used ÑIndependent of application

22. Application Layer ÑSupport for different user applications Ñe.g. e-mail, file transfer

23. Addressing Requirements ÑTwo levels of addressing required ÑEach computer needs unique network address ÑEach application on a (multi-tasking) computer needs a unique address within the computer ÑThe service access point or SAP

24. Protocol Architectures and Networks

25. Protocols in Simplified Architecture

26. Protocol Data Units (PDU) ÑAt each layer, protocols are used to communicate ÑControl information is added to user data at each layer ÑTransport layer may fragment user data ÑEach fragment has a transport header added ÑDestination SAP ÑSequence number ÑError detection code ÑThis gives a transport protocol data unit

27. Network PDU ÑAdds network header Ñnetwork address for destination computer ÑFacilities requests

28. Operation of a Protocol Architecture

29. TCP/IP Protocol Architecture ÑDeveloped by the US Defense Advanced Research Project Agency (DARPA) for its packet switched network (ARPANET) ÑUsed by the global Internet ÑNo official model but a working one. ÑApplication layer ÑHost to host or transport layer ÑInternet layer ÑNetwork access layer ÑPhysical layer

30. Physical Layer ÑPhysical interface between data transmission device (e.g. computer) and transmission medium or network ÑCharacteristics of transmission medium ÑSignal levels ÑData rates Ñetc.

31. Network Access Layer ÑExchange of data between end system and network ÑDestination address provision ÑInvoking services like priority

32. Internet Layer (IP) ÑSystems may be attached to different networks ÑRouting functions across multiple networks ÑImplemented in end systems and routers

33. Transport Layer (TCP) ÑReliable delivery of data ÑOrdering of delivery

34. Application Layer ÑSupport for user applications Ñe.g. http, SMPT

35. TCP/IP Protocol Architecture Model

36. OSI Model ÑOpen Systems Interconnection ÑDeveloped by the International Organization for Standardization (ISO) ÑSeven layers ÑA theoretical system delivered too late! ÑTCP/IP is the de facto standard

37. OSI Layers ÑApplication ÑPresentation ÑSession ÑTransport ÑNetwork ÑData Link ÑPhysical

38. OSI v TCP/IP

39. Standards ÑRequired to allow for interoperability between equipment ÑAdvantages ÑEnsures a large market for equipment and software ÑAllows products from different vendors to communicate ÑDisadvantages ÑFreeze technology ÑMay be multiple standards for the same thing

40. Standards Organizations ÑInternet Society ÑISO ÑITU-T (formally CCITT) ÑATM forum

41. Further Reading ÑStallings, W. Data and Computer Communications (6th edition), Prentice Hall 1999 chapter 1 ÑWeb site for Stallings book Ñwww.shore.net/~ws/DCC6e.html ÑWeb sites for IETF, IEEE, ITU-T, ISO ÑInternet Requests for Comment (RFCs) ÑUsenet News groups Ñcomp.dcom.* Ñcomp.protocols.tcp-ip