Mạng và viễn thông P19

Local Area Networks (LANs)We have seen how packet switching has contributed greatly to the efficiency and flexibility of ‘wide area’ data networks, involving a large number of devices spread at geographically diverse locations. Packet switching, however, is not so efficient for smaller scale networks, those limited to linking personal computerswithin an office building; that is the realm of an alternative type of packet-switched-like network called a local area network or LAN for short. In this chapter we discuss the concept of a LAN and the various technical realizations which are available....
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Mạng và viễn thông P19 Networks and Telecommunications: Design and Operation, Second Edition. Martin P. Clark Copyright © 1991, 1997 John Wiley & Sons Ltd ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic) 19 Local Area Networks (LANs) We have seen how packet switching has contributed greatly to the efficiency and flexibility of ‘wide area’ data networks, involving a large number of devices spread at geographically diverse locations. Packet switching, however, is not so efficient for smaller scale networks, those limited to linking personal computerswithin an office building; that is the realm of an alternative type of packet-switched-like network called a local area network or LAN for short. In this chapter we discuss the concept of a LAN and the various technical realizations which are available.19.1 THE EMERGENCE OF LANs LANs emerged in the late 1980s as the most important means of conveying data between different computers and computerperipheral devices (printer, file server, electronic mail server, fax gateway, host gateway, computer printer, scanner, etc.) within a single office, office building or small campus. LANs are constrainedby their mode of operation to a geographically limited area, but areideally suited for shortdistance data transport. A high bit speed LAN can carry high volumes of data with rapid response times. Such performance is crucial for most office applications, and has made them the ideal foundation for the new generation of ‘electronic offices’ comprising electronic work- stations,wordprocessors,sharedprinters,electronic filing cabinets,electronicmail systems and so on. Most LANs conform to one of the different types specified in the Institution of Electrical and Electronic Engineers’ IEEE 802 series of standards. All the types have been developed from proprietary LANs, developed earlier by individual companies or organizations, but have now achieved American and worldwide recognition, as I S 0 8802 standards.19.2 LAN TOPOLOGIESANDSTANDARDS The different types of LAN are characterized by their distinctive topologies. They all comprise a single transmission path interconnecting all the dataterminal devices, with a 367368 (LANS) LOCAL AREA NETWORKS *c> (a) Star ( b ) Ring Figure 19.1 AlternativeLANtopologies [ c ) Bus bit speed typically between 1 and 30 Mbit/s, together with appropriate protocols (called the logical link control and the medium access control ( M A C ) ) to enable data transfer. The three most common topologies are illustrated in Figure 19.1, and are called the star, ring and bus topologies. Slightly different protocol standards apply to the different topologies. For example, IEEE 802.3 defines a physical layer protocol called CSMAjCD (carrier sense multiple access with collision detection) which may be used with a bus or star topology. Used with a bus form medium, such LANs are normally referred to as ethernets. IEEE 802.4 ( I S 0 8802.4) defines an alternative layer-l protocol for a token bus, again suitable for either a bus or star topology. IEEE 802.5 defines a layer 1 protocol suitable for use on a token ring topology. Finally, IEEE 802.2 ( I S 0 8802.2) defines a logicallinkcontrol protocol (equivalent to the OS1 layer 2) that can be used with any of the above. This provides for the transfer of information between any two devices connected to the LAN. The information to transported (i.e. information frameor packet) is submitted be ------ Logicallinkcontrol O S 1 layer 2 ( I E E E 802.21 ------ I P By s io rrkS t a r nh uw c a et s o l CSMAlCD Token (IEEE 802.4 I I ring I Token Figure 19.2 The IEEE 802 LAN standards i Token ring (IEEE 802.51CSMAjCD (IEEE 802.3, I S 0 8802.3): ETHERNET ...