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(FDDI) A 100 Mbit/s ANSI standard local area networkarchitecture, defined in X3T9.5. The underlying medium isoptical fibre (though it can be copper cable, in which caseit may be called CDDI) and the topology is adual-attached, counter-rotating token ring.FDDI rings are normally constructed in the form of a "dualring of trees". A small number of devices, typicallyinfrastructure devices such as routers and concentratorsrather than host computers, are connected to both rings -these are referred to as "dual-attached". Host computersare then connected as single-attached devices to therouters or concentrators. The dual ring in its mostdegenerate form is simply collapsed into a single device. Inany case, the whole dual ring is typically contained within acomputer room.This network topology is required because the dual ringactually passes through each connected device and requireseach such device to remain continuously operational (thestandard actually allows for optical bypasses but these areconsidered to be unreliable and error-prone). Devices such asworkstations and minicomputers that may not be under thecontrol of the network managers are not suitable forconnection to the dual ring.As an alternative to a dual-attached connection, the samedegree of resilience is available to a workstation through adual-homed connection which is made simultaneously to twoseparate devices in the same FDDI ring. One of theconnections becomes active while the other one isautomatically blocked. If the first connection fails, thebackup link takes over with no perceptible delay.Usenet newsgroup: news:comp.dcom.lans.fddi.