Fiber Distributed Data Interface

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Introduction
FDDI (Fiber-Distributed Data Interface) is a standard for data transmission on fiber optic lines in that can extend in range up to 200 km (124 miles). The FDDI protocol is based on the token ring protocol. In addition to being large geographically, an FDDI local area network can support thousands of users.
An FDDI network contains two token rings, one for possible backup in case the primary ring fails. The primary ring offers up to 100 Mbps capacity. If the secondary ring is not needed for backup, it can also carry data, extending capacity to 200 Mbps. The single ring can extend the maximum distance; a dual ring can extend 100 km (62 miles).
FDDI is a product of American National Standards Committee X3-T9 and conforms to the open system interconnect (OSI) model of functional layering. It can be used to interconnect LANs using other protocols. FDDI-II is a version of FDDI that adds the capability to add circuit-switched service to the network so that voice signals can also be handled. Work is underway to connect FDDI networks to the developing Synchronous Optical Network.

Full Support of FDDI ANSI Standards

Fiber Distributed Data Interface (FDDI) is an American National Standards Institute
(ANSI) standard definition of a high-speed (100 megabits per second) local area network
(LAN) based on optical fiber technology. Clear Path HMP NX and A Series servers can
access an FDDI LAN through either of the following channel adapters:
1. FDDI channel adapter style CA601-FDI or CA602-FDI.This channel adapter fully supports the ANSI FDDI standard. It provides a fiber optical connection to an FDDI network.
2. CDDI channel adapter style CA604-CDI. This channel adapter supports the ANSI TP-PMD standard for category 5 (data grade) unshielded twisted-pair (UTP) connections. It provides an electrical connection to the FDDI network.

How Does an FDDI Network Work?

An FDDI network uses duplex fiber-optic cable for point-to-point connections between a
number of stations to form two closed loops. The two rings, designated as primary and
secondary, operate as counter-rotating rings. Each of these counter-rotating rings
comprises a single logical ring. If a dual ring experiences a problem in the fiber medium or
at a station, the stations on each side of the failure automatically wrap the primary and
secondary rings to form a single logical ring.
Stations on the trunk or primary ring gain access to transmit onto the ring by capturing a
free token off the network. The token is a series of bits indicating whether the ring is
available for use or not. The token is transferred from one station to the next, providing
each station the opportunity to transfer data. Each station regenerates and repeats each
symbol. Once the token is captured, data is transmitted onto the network in formatted
FDDI frames. After all data is transferred onto the network, the token is released to the
network to be picked up by another station.
All stations read each FDDI frame as it passes around the ring. When a station determines
that it is the intended recipient, it extracts data from the frame but still retransmits the
frame to the next station on the ring. When the frame returns to the originating station,
that station “strips” the frame from the network.

FUNCTION OF FDDI

1. Background
The Fiber Distributed Data Interface (FDDI) specifies a 100-Mbps token-passing, dual-ring LAN using fiber-optic cable. FDDI is frequently used as high-speed backbone technology because of its support for high bandwidth and greater distances than copper. It should be noted that relatively recently, a related copper specification, called Copper Distributed Data Interface (CDDI) has emerged to provide 100-Mbps service over copper. CDDI is the implementation of FDDI protocols over twisted-pair copper wire. This chapter focuses mainly on FDDI specifications and operations, but it also provides a high-level overview of CDDI.