Its 100 Mbps bandwidth is 10 timesgreater than that of Ethernet. The extra speed allows FDDI to transmitmore data per unit of time. In addition, FDDI permits a large number ofdevices to exploit the extra transmission speed. It supports multipletokens, which means that many different packets of data can be transmittedover the LAN simultaneously. FDDI is carried over a pair of rings, one primary and the other secondary.
Data flows in opposite directions along each ring.
During normaloperation, in the absence of faults, the secondary ring remains inactive. Devices or LANs can be attached to either one or both rings via FDDInodes. If the nodes are attached to both rings, as in dual-attach, thenode can heal itself if the primary cable fails. The disadvantage of thisis that an extra fiber-optic interface is required, adding to the totalcost. The alternative is the single-attached node connected to just one ofthe rings, which is cheaper but does not offer the same protection againstcable faults.