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FDDI
Work on the Fiber Distributed Data Interface (FDDI) began in 1982 by both
standards bodies (ANSI) and vendors (Digital, Control Data, Sperry Rand, and
others) out of the need for a higher-speed LAN protocol. In 1990, FDDI became an
ANSI X3 standard. It specifies a 100Mbps token-passing (similar in operation to
Token Ring) shared transmission medium LAN.
The original FDDI standard specified a passive, counter-rotating dual-ring topology
over fiber optic cable. In a dual-ring topology, each end-station is a
Dual-Attachment Station (DAS). One ring is the primary, on which data flows in one
direction, and it is used for data transmission. The secondary ring is backup. The
same data that is transmitted on the primary ring is transmitted on the secondary
ring, but it flows in the opposite direction. The secondary ring remains idle during
normal operation. In the event of a station failure or break in the ring, the primary
and secondary rings wrap back on themselves, creating a single ring. In the event
that more than one break occurs, the ring will segment itself into separate rings
until the damaged state is resolved. In addition to the dual-attachment topology,
FDDI can utilize a dual-attachment concentrator, which is part of the dual ring, but
provides the capability for FDDI Single Attachment Stations (SAS) to connect to the
primary ring (they move to the secondary ring in the event of a failure). A
dual-attachment implementation of the FDDI protocol utilizes copper cabling, called
Copper Distributed Data Interface (CDDI), developed by Crescendo
Communications (now part of Cisco Systems). The CDDI specification was added to
the ANSI FDDI PHY specification.
FDDI has enjoyed a lot of success as a LAN backbone and high-speed server
technology. Typically, a FDDI backbone is constructed and bridges are used to
interface low-speed (Ethernet and so on) LAN hubs and concentrators. Then,
through the use of DAS NIC and DAS concentrators, high-speed servers are
attached to the network backbone. Figure 4.23 illustrates a FDDI backbone topology
utilizing this approach.