A token-passing mechanism allows orderly access to a network. Apart from token ring the most commonly used token-passing network is the fibre distributed data interchange (FDDI) standard. This operates at 100 Mbps and, to overcome the problems of line breaks, has two concentric token rings, as illustrated in Figure 10.1. Fibre optic cables have a much higher specification than copper cables and allow extremely long interconnection lengths. The maximum circumference of the ring is 100 km (62 miles), with a maximum 2 km between stations (in FDDI nodes are known as stations). It is thus an excellent mechanism for connecting networks over a city or a campus. Up to 500 stations can connect to each ring with a maximum of 1000 stations for the complete network. Each station connected to the FDDI highway can be a normal station or a bridge to a conventional local area network, such as Ethernet or token ring.
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