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Kỹ thuật truyền số liệu 13

Tham khảo tài liệu 'kỹ thuật truyền số liệu 13', giáo dục - đào tạo, cao đẳng - đại học phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | HAPTER 13 Metropolitan Area Networks i A metropolitan area network MAN is a network designed to extend over an entire city. When local area networks LANs in close proximity need to exchange data they can be connected privately using cable and routers or gateways. When LANs of a single r 1 enterprise are distributed over a larger area such as a city or large campus however privately owned connecting infrastructure is impractical. Most organizations find that 1 even if they could get permits to lay cable on public land a better alternative is to use i. the services of existing utilities such as the telephone company. One of these services is switched multimegabit data services SMDS which nor-s mally uses another protocol called distributed queue dual bus DQDB . In this chapter . we first discuss DQDB and then concentrate on SMDS. 13.1 IEEE 802.6 DQDB In addition to the protocols discussed in Chapter 12 another protocol in the IEEE Project 802 IEEE 802.6 is distributed queue dual bus DQDB . Although DQDB resembles a LAN standard it is designed to be used in MANs. Access Method Dual Bus As its name implies DQDB uses a dual bus configuration Each device in the system connects to two backbone links. Access to these links is granted not by contention as in 802.3 or token passing as in 802.4 and 802.5 but by a mechanism called distrib- uted queues. 1 1 Figure 13.1 shows a DQDB topology. In this illustration the two unidirectional buses are labeled Bus A and Bus B. Five numbered stations connect to the buses as shown. Each bus connects to the stations directly through input and output ports no drop lines are used. 413 414 CHAPTER 13 METROPOLITAN AREA NETWORKS Directional Traffic - Each bus supports traffic in only one direction. The direction of traffic on one bus is the opposite of traffic on the other. In Figure 13.1 for example where the beginning of each bus is represented by a square and the end by a triangle Bus A traffic moves from right to left. The bus itself .