Source : Free On-Line Dictionary of Computing

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 {router}s and {concentrator}s
        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
        {router}s or {concentrator}s.  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
        {workstation}s and {minicomputer}s that may not be under the
        control of the {network manager}s 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)