Mạng và viễn thông P21

Campus and Metropolitan Area Networks (MANs)Metropolitan area networks (MANs) are network technologies similar in nature to local area networks (LANs), but with the capability to extend the reach of the LAN across whole cities or metropolitan areas, rather than being limited to, say, 100-200 metres of cabling. MANS have evolved because of the desire companies to extend LANs throughout company office buildings of spread across a campus or a number of different locations in a particular city.
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Mạng và viễn thông P21 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) 21 Campus and Metropolitan Area Networks (MANs) Metropolitan area networks (MANs) are network technologies similar in nature to local area networks (LANs), but with the capability to extend the reach of the LAN across whole cities or metropolitan areas, rather than being limited to, say, 100-200 metres of cabling. MANS have evolved because of the desire companies to extend LANs throughout company office buildings of spread across a campus or a number of different locations in a particular city. They provide for high speed data transport (at over lOOMbit/s) and are ideal for the interconnection of LANs. There was some effort extend MAN capabilitiesto include the carriage of telephone and video to signalsan as ‘integrated’ network, this has but work largely overtaken ATM been by (asynchronous transfer mode), that the MAN technologies themselves already obsolescent. so are We review here, but only briefly, the most important MAN techniques, FDDI (fibre distributed data interface), and SMDS (switched multimegabit digital service) which is based on the DQDB (distributed queue dual bus) technique.21.1 FIBRE DISTRIBUTED DATAINTERFACE The jibre distributed data interface (FDDI) is a 100 Mbit/s token ring network. It is defined in IEEE 802.8 and IS0 8802.8. FDDI can be used to interconnect LANs over an area spanning up to100 km, allowing high speed data transfer. Originally conceived as a high speed link for the needs of broadband terminal devices, FDDI is now per- ceived as the optimum backbonetransmission system for campus-wide wiring schemes, especially where network management and fault recovery are required. In particular, FDDI became popular in association with the very first optical fibre building cabling schemes, because it provided oneof the first means to connect LANs on different floors of a building or in different buildings on a campus via optical fibre. Unfortunately, due to itsexpensive nature and the rapid development of ATM (asynchronous transfer mode, see Chapter 26) as well as alternative building cabling schemes, FDDI has fallen into decline, no longer being recommended or further developed by most LAN and computer manufacturers. 391392 CAMPUS AND METROPOLITAN (MANS) AREA NETWORKS A second generation version of FDDI, FDDI-2, was developed to include a capability similar to circuit-switching to allow voice and video to be carried reliably in addition to packet data, but these capabilities were never widely used. The FDDI standard is defined in four parts 0 media access control ( M A C ) ,like IEEE 802.3 and 802.5 (see Chapter 19) defines the rules for token passing and packet framing e physical layer protocol ( P H Y ) defines the data encoding and decoding e physical media dependent ( P M D ) defines drivers for the fibre optic components e stationmanagement ( S M T ) definesamulti-layerednetworkmanagementscheme which controls MAC, PHY and PMD The ring of an FDDI is composed of dual optical fibres interconnecting all stations. The dual ring allows for fault recovery even if a link is broken by reversion to a single ring, as Figure 21.l(a) shows. The fault need only be recognized by the CMTs (connec- tion management mechanisms) of the station immediately on either side the break. To of all other stations the ring will appear still to beinitsnormal contra-rotating state (Figure 21.l(b)). When configured as a ring, of the stations said to be in dual-attached connection. each is Alternatively, a fibre ...