The Fiber Distributed Data Interface (FDDI) standard was produced by the ANSI X3T9.5 standards committee in the mid-1980s. During this period, high-speed engineering workstations were beginning to tax the capabilities of existing local-area networks (LANs) (primarily Ethernet and Token Ring). A new LAN was needed that could easily support these workstations and their new distributed applications. At the same time, network reliability was becoming an increasingly important issue as system managers began to migrate mission-critical applications from large
computers to networks. FDDI was developed to fill these needs.
After completing the FDDI specification, ANSI submitted FDDI to the International Organization for Standardization (ISO). ISO has created an international version of FDDI that is completely compatible with the ANSI standard version.
Today, although FDDI implementations are not as common as Ethernet or Token Ring, FDDI has gained a substantial following that continues to increase as the cost of FDDI interfaces diminishes. FDDI is frequently used as a backbone technology as well as a means to connect high-speed computers in a local area.
FDDI specifies a 100-Mbps, token-passing, dual-ring LAN using a fiber-optic transmission medium. It defines the physical layer and media-access portion of the link layer, and so is roughly analogous to IEEE 802.3 and IEEE 802.5 in its relationship to the Open System Interconnection (OSI) reference model.
Although it operates at faster speeds, FDDI is similar in many ways to Token Ring. The two networks share many features, including topology (ring), media-access technique (token passing), reliability features (redundant rings, for example), and others. For more information on Token Ring and related technologies.
One of the most important characteristics of FDDI is its use of optical fiber as a transmission medium. Optical fiber offers several advantages over traditional copper wiring, including security (fiber does not emit electrical signals that can be tapped), reliability (fiber is immune to electrical interference), and speed (optical fiber has much higher throughput potential than copper cable). FDDI defines use of two types of fiber: single mode (sometimes called monomode) and multimode. Modes can be thought of as bundles of light rays entering the fiber at a particular angle. Single-mode fiber allows only one mode of light to propagate through the fiber, while multimode fiber allows multiple modes of light to propagate through the fiber. Because multiple modes of light propagating through the fiber may travel different distances (depending on the entry angles), causing them to arrive at the destination at different times (a phenomenon called modal dispersion), single-mode fiber is capable of higher bandwidth and greater cable run distances than multimode fiber. Due to these characteristics, single-mode fiber is often used for interbuilding connectivity, while multimode fiber is often used for intrabuilding connectivity. Multimode fiber uses light-emitting diodes (LEDs) as the light-generating devices, while single-mode fiber generally uses lasers.
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Fiber Distributed Data Interface
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
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.