FDDI is a LAN protocol standardised by ANSI and ITU-T. It supports data rates of 100 Mbps and provides a high-speed alternative to Ethernet and token ring. When FDDI was designed, the data rate of 100 Mbps required fibre-optic cable. The access method in FDDI is also called token passing. In a token ring network, a station can send only one frame each time it captures the token. In FDDI, the token passing mechanism is slightly different in that access is limited by time. Each station keeps a timer which shows when the token should leave the station. If a station receives the token earlier than the designated time, it can keep the token and send data until the scheduled leaving time. On the other hand, if a station receives the token at the designated time or later than this time, it should let the token pass to the next station and wait for its next turn.
FDDI is implemented as a dual ring. In most cases, data transmission is confined to the primary ring. The secondary ring is provided in case of the primary ring’s failure. When a problem occurs on the primary ring, the secondary ring can be activated to complete data circuits and maintain service.
