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Computing (FOLDOC) dictionary
Fiber Distributed Data Interface
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(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.