Token Bus & FDDI

Token Bus

Token Bus technology uses the same mechanism as token ring for media access control. But it is physically wired in a bus topology using coaxial cable. The devices form a virtual ring and passes token between them.

Token Bus was standardized as IEEE 802.4 and was used in industrial applications such as automated production environments. In such applications there is a need for real time communication. i.e., some messages have to be passed urgently and cannot wait for tokens for long time. Hence a variant of token controlled medium access called timed token is used.

There are two types of messages - synchronous and asynchronous. Synchronous messages are urgent real time messages. Asynchronous messages are non-time critical, and these are passed on a priority basis. There is a total cycle time limit for the rotation of tokens. When a station seizes a token, time critical messages are always passed, but asynchronous messages are passed only if the token is early.

Token Bus technology was not very good at handling device failures and was difficult to upgrade and soon became obsolete.


FDDI stands for Fibre Distributed Data Interface and as the name suggests the medium used is fibre optic. It uses ring topology but media access is controlled by a timed token like in token bus. Hence the standard is closer to IEEE 802.4

The main advantage of FDDI is the long distances that it can cover (about 200km) and hence it can be used not only for LAN but also for MAN (Metropolitan Area Network).

FDDI offers dual ring, one primary and secondary in case one ring fails. This network offers a 100Mbps LAN. In the past when Ethernet was only 10Mbps and token ring was only 4 or 8Mbps, this was considered good speed. But with the arrival of Fast Ethernet (100Mbps) with switching and Gigabit Ethernet (1Gbps) FDDI became obsolete. High speed Ethernet variants have low cost as well as more speed.

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