Fiber Distributed Data Interface

(FDDI) A 100 Mbit/s ANSI standard local area network

architecture, defined in X3T9.5. The underlying medium is

optical fibre (though it can be copper cable, in which case

it may be called CDDI) and the topology is a

dual-attached, counter-rotating token ring.

FDDI rings are normally constructed in the form of a "dual

ring of trees". A small number of devices, typically

infrastructure devices such as routers and concentrators

rather than host computers, are connected to both rings -

these are referred to as "dual-attached". Host computers

are then connected as single-attached devices to the

routers or concentrators. The dual ring in its most

degenerate form is simply collapsed into a single device. In

any case, the whole dual ring is typically contained within a

computer room.

This network topology is required because the dual ring

actually passes through each connected device and requires

each such device to remain continuously operational (the

standard actually allows for optical bypasses but these are

considered to be unreliable and error-prone). Devices such as

workstations and minicomputers that may not be under the

control of the network managers are not suitable for

connection to the dual ring.

As an alternative to a dual-attached connection, the same

degree of resilience is available to a workstation through a

dual-homed connection which is made simultaneously to two

separate devices in the same FDDI ring. One of the

connections becomes active while the other one is

automatically blocked. If the first connection fails, the

backup link takes over with no perceptible delay.

Usenet newsgroup: news:comp.dcom.lans.fddi.

(1994-12-13)