Fiber Distributed Data Interface (FDDI)
Fiber distributed data interface (FDDI) is a computer
network protocol that uses fiber optic cable as the transmission medium to
provide high-speed data transmission service to LANs. FDDI is a token protocol.
The basic transmission rate of FDDI is 100 Mbps. FDDI is commonly used as a
backbone network that interconnects several LANs within a company. The FDDI
specification is IEEE 802.2 and FDDI data transmission speed range from 100 to
200 Mbps. 1000 Mbps and higher FDDI speeds are in development.
FDDI is a LAN architecture that is based on redundant fiber rings
that transmit in opposite directions. One of the rings is the primary ring and
the other ring is the secondary ring. When the primary ring ceases to be
operational (such as a cut cable) the network reconfigures itself (called
“selfhealing”) and it reconfigures the secondary ring as the primary ring.
Both single mode fiber and multimode fiber cable systems can be
used with FDDI. Multimode fibers have a wider optical bandwidth transmission
capability. However, this introduces distortion and limits the maximum distance
for multimode fiber systems to about 2 kilometers. Single mode fiber systems
have maximum range of approximately 60 km.
FDDI is a token passing architecture differing from token ring in
that while a station has a token it can transmit as many frames as possible
before the token expires. Because of this, there can be multiple frames on the
ring at any time.
The interconnection devices in a FDDI network include a dual
attached concentrator (DAC) and dual attached station (DAS). These devices
remove and insert data to the FDDI ring. Each of these devices has dual
transmission capability. If the fiber ring is cut, they can automatically
redirect data onto its other channel (the secondary ring).
The DAC is a concentrator the converts the optical data on the
FDDI system into another format that can be used to connect to other data
networks. This allows one FDDI network node to connect to many other data
communication devices.
Figure 16 shows
FDDI system that uses dual rings that transmit data in opposite directions. This
diagram shows one dual attached station (DAS) and a dual attached concentrator
(DAC). The DAS receives and forwards the token to the mainframe computer. The
DAC receives and token and coordinates its distribution to multiple data devices
that are connected to it.
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