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  1. 1. 1-1© 2001 by Prentice HallLocal Area Networks, 3rd EditionDavid A. StamperPart 1: Introduction to DataCommunications and Local AreaNetworksChapter 1Introduction to Datacommunications
  2. 2. © 2001 by Prentice HallChapter PreviewIn this chapter you will study:• The requirements for communication• What constitutes a network• The various types of networks incommon use today• The OSI Reference Model• Some of the basic terminology ofdata communications and networks
  3. 3. © 2001 by Prentice HallEssential Elements ofCommunication• A message• A sender• A receiver• A medium
  4. 4. © 2001 by Prentice HallUnderstanding the Message• In computer systems, data can berepresented by any of severaldifferent codes, the two mostcommon being:– the American Standard Code for InformationInterchange (ASCII)– the Extended Binary Coded Decimal InterchangeCode (EBCDIC)
  5. 5. © 2001 by Prentice HallSecurity• Sensitive data like your credit cardnumber or other secret data should besafeguarded during transmission. Themost common mechanism for protectingdata during transmission is encryption.– Encryption transforms plain text into an (presumably)undecipherable form called cipher-text.
  6. 6. © 2001 by Prentice HallTwo Types of Networks• Terminal Network– consists of a single host computer with attachedterminals– the host computer does all or most of the processing,and the terminals imply act as input/output (I/O) devicesthrough which a person gains access to the host’sapplications.• Network of Computers– two or more nodes connected by a data communicationsmedium.– individual nodes may have terminals attached to them– a single node on this network can look just like theterminal network
  7. 7. © 2001 by Prentice HallThree Network Subtypes• Local Area Network (LAN)• Metropolitan Area Network (MAN)• Wide Area Network (WAN)
  8. 8. © 2001 by Prentice HallLAN/MAN/WAN ComparisonLimited—typically upto 2,500 meters or 2milesHigh—typically inexcess of 10 Mbps—10,100 and 1,000 arestandardLocally owned—twisted-pair wires,fiber optic cable,wireless (not satellite)Can be any, but mostare desktop computersLimited—typically upto 200 kilometers or100 milesHigh—typically100 MbpsLocally owned andcommon carrier—twisted-pair wires,fiber optic cableCan be any, but mostare desktop computersand minicomputersUnlimitedSlower—usually 1.5MbpsLocally owned andcommon carrier—twisted-pair wires,coaxial cable, fiberoptic cable, wireless toinclude satelliteCan be any, but mostare desktop computersDistanceSpeedMediaNodesLAN MAN WAN
  9. 9. © 2001 by Prentice HallThe OSI Reference Model• The problem of network interconnection isso important that the ISO created the OSIReference Model that describes thefunctions a generic network needs toprovide.• The OSI Reference Model has become thebasis for many data communicationsstandards.– Because these standards are placed in the public domain, they arecalled open standards and lead to open systems.
  10. 10. © 2001 by Prentice HallOSI Peer Layer CommunicationApplicationApplicationPresentationPresentationSessionSessionTransportTransportNetworkNetworkData LinkData LinkPhysicalPhysicalApplicationApplicationPresentationPresentationSessionSessionTransportTransportNetworkNetworkData LinkData LinkPhysicalPhysicalProcessor 1 Processor 2Logical Path Physical Path
  11. 11. © 2001 by Prentice HallOSI Reference Model FormattingTrans-Id Data Date=dd/mm/yyTrans-Id Data Date=mm-dd-yyyyID Length Trans-Id Data Date=mm-dd-yyyyTSAP ChkSum ID Length Trans-Id Data Date=mm-dd-yyyyAddress Seq Nbr TSAP ChkSum ID Length Trans-Id Data Date=mm-dd-yyyyHeader Address Seq Nbr TSAP ChkSum ID Length Trans-Id Data Date=mm-dd-yyyy Chksum(a) Application Layer(b) Presentation Layer(c) Session Layer(d) Transport Layer(e) Network Layer(f) Data Link Layer
  12. 12. © 2001 by Prentice HallThe OSI Model at Work• Application Layer– The application on Node A builds a record with a transactionidentifier, the number of the account to be updated, the date andtime of the transaction, and the amount to be deducted.• Presentation Layer– The presentation layer is responsible for translating from one formatto another.• Session Layer– The session layer’s major functions are to set up and perhapsmonitor a set of dialogue rules by which the two applications cancommunicate and to bring a session to an orderly conclusion.
  13. 13. © 2001 by Prentice HallThe OSI Model at Work (cont.)• Transport Layer– The transport layer is the first of the OSI layers responsible foractually transmitting the data.• Network Layer– The network layer provides accounting and routing functions.• Data Link Layer– The data link layer is responsible for data delineation, errordetection,and logical control of the link.• Physical Layer– The physical layer does not append anything to the message. Itsimply accepts the message from the data link layer and translatesthe bits into signals on the medium.
  14. 14. © 2001 by Prentice HallReceiving the Message1. The message is passed over the link connecting Nodes Aand X.2. The message is passed to the data link layer in Node X. Themessage is checked for transmission errors, the PDUinformation applied by A’s data link layer is removed, andthe message is sent to X’s network layer.3. X’s network layer records the accounting information forthe message and then strips off the network layer protocoldata and examines the destination address. The destinationis not Node X in this case, so the network layer consults itsnetwork routing table and determines the next link on thepath to Node M. X’s network layer affixes the propernetwork layer protocol data and sends the message toNode X’s data link layer.
  15. 15. © 2001 by Prentice HallReceiving the Message (cont.)4. Node X’s data link layer creates its PDU and sends themessage to Node M.5. Node M’s data link layer receives the message, strips offNode X’s data link layer protocol data, checks fortransmission errors, and passes the data up to Node M’snetwork layer.6. Node M’s network layer gathers accounting data, strips offthe network layer protocol data, and fins that the messageis destined for an application in this node.7. The message is passed up to M’s transport layer, where thesequence number is checked to ensure that no messageshave been lost. The transport layer protocol data isremoved.
  16. 16. © 2001 by Prentice HallReceiving the Message (cont.)8. The message arrives at the session layer, whererelevant protocol data is examined and remove.9. The message arrives at Node M’s presentationlayer, where appropriate action is taken.10.The message arrives at the application, whereit is acted on.
  17. 17. © 2001 by Prentice HallGeneral Network Implementations:LANs• A LAN services a limited geographic area at high speeds—usually 10 million bits per second or higher. Allcomponents of the LAN are commonly owned by theorganization that uses it.• The nodes in many of today’s LANs are desktop systemslike personal computers. Henceforth, we will also use theterms workstations, clients, and servers in referring to LANnodes.• A workstation is used here to represent a LAN user’scomputer; other terms used in referring to a workstationare client and node.• A server is a network node that is dedicated to providingservices to client nodes.
  18. 18. © 2001 by Prentice HallGeneral Network Implementations:MANs• A MAN (metropolitan area network) is a high-speed network covering wider distances thanLAN.• A MAN spans distances of approximately 100miles; therefore, it is suitable for connectingdevices and LANs in a metropolitan area.• MAN speeds are typically 100 Mbps or higher.• The most commonly implemented MAN is thefiber distributed data interface (FDDI). It operatesat 100 Mbps over fiber optic cable for distancesup to 200 kilometers.
  19. 19. © 2001 by Prentice HallGeneral Network Implementations:WANs• A WAN is the oldest type of network.• WANs generally span a wide geographic area likea state, province, country, or multiple countries.However, some WANs are confined to a limitedgeographic area, like a LAN.• A WAN in a limited geographic area could beeasily extended over a wide area using the sametechnologies. The same is not true of a LAN.
  20. 20. © 2001 by Prentice HallGeneral Network Implementations:VANs• A VAN is a network owned by a communicationsutility that sells the services of the network toother companies.• A communications utility that owns a VANprovides connectivity to multiple locations.• The value added by the communications utility isthe maintenance and management of thecommunications circuits.
  21. 21. © 2001 by Prentice HallGeneral Network Implementations:Enterprise Networks• An enterprise network is an organization’scomplete network.• With the advent of LANs, many companiesinstalled departmental LANs to improve theproductivity of work groups.• Soon, these companies realized that there was abenefit to having users on one LAN communicatewith users or applications on other LANs or onthe WAN, and the various networks wereconnected together to form one corporate-widenetwork, the enterprise network.
  22. 22. © 2001 by Prentice HallGeneral Network Implementations:The Internet• An internet (with a lowercase ‘i’) is theinterconnection of two or more networks. Anenterprise networks just described is an exampleof an internet.• The Internet (with an uppercase ‘I’) is a specificinstance of an internet.• The Internet is a global network of networks. TheInternet is made up of hundreds of networks,thousands of nodes, and millions of usersthroughout most countries of the world.
  23. 23. © 2001 by Prentice HallGeneral Network Implementations:Intranets• An intranet is an organization’s private Web.• Companies have found that WWW capabilitiescan improve the information flow and availabilityin a company.• Companies may use an intranet rather thanpublishing on the Internet because theinformation being provided is intended forcorporate use only and not for the public at large.