The Fiber Distributed Data Interface (FDDI) is end of the 1980er years developed 100 MBit/s standardized network architecture for local of networks (ANSI standard X3T9.5). As medium optical waveguide cables in a double, ring moving in opposite directions with token access mechanism are used. 1994 were extended the FDDI standard and the transmission also over protected (STP) and ungeschirmte (UTP type 5) twisted copper lines standardized (CDDI, C for copilot by).

Standard

• ANSI X3T9.5, Physical Media Dependent (PMD) specification, access to medium (LWL, copper)
• ANSI X3T9.5, Physical (PHY) specification, coding of the data with clock information
• ANSI X3.139, Media Access Control (MAC) specification, token Passing, Frame format, structure of ring
• ANSI X39.5, station management (SMT) specification, connecting and structure of ring, error recognition and removal, station management.

The following characteristics distinguish FDDI of networks:

• Medium: Glass fiber 1300 Nm
• Frequency band: Baseband
• Data rate: 100 MBit/s/1000 MBit/s
• Topology: Double ring (data ring and reserve ring)
• Arbitrierung: Token
• Error tolerance: max. 1 station (additional bypass possibility)
• Distance between neighbouring stations: max. 2 km
• Ring length: max. 100-200 km
• Stations per ring: max. 1000

FDDI of rings are normally as "„double ring with trees "“developed. A small equipment's number (rout and concentrators) is attached to both rings (binarily attached). Normal computers rout then on or attached concentrators over simple cables.

FDDI ring with routing and concentrators

Normally only one ring is used. A token goes through all stations of the ring. It must of each station, which receives it is passed on. If a station wants to send, it waits for token, sends the pending data and attaches tokens.

If a station on the ring precipitates, then the second (reserve) ring is used in opposite direction. Before and behind the incorrect station the data are sent back, so that a single ring develops. If a further station precipitates, it comes to the Separation of the network. The standard plans an optical bypass, in practice functions this however not always reliably.

Loss of a station in the FDDI ring

FDDI was into the 1990er years of the designate successors for old 10 Mbit the Ethernet. New developments such as Gigabit Ethernet and AT were however faster to begin substantially more economically and more easily. FDDI attained meaning however within another range: Because of its high range and reliability is often used as central LAN structure (Backbone), by which several Ethernet or token loop networks are interconnected.

In order to be able to operate over FDDI networks at least to small extent also Multimedia applications, limited the real timable FDDI version 2 was created. Beside "„the Shared Media available for all stations "“- range for it were defined which are reserved such as video or audio applications for isochronous applications. The transmission time within these data channels amounts to 125

Connection types

FDDI devices are divided in two classes. Devices of the Class A can be merged directly into the ring, this can rout, concentrators or also work stations with two connections to be. Relevant criterion are here at least two available connections. Devices with only one FDDI interface will be able as Class B devices designated and not directly into the ring to be merged.

In order to be able to attach now devices of the Class B, it requires devices of the Class A, which make additional connections available for Class B of devices, these devices calls one concentrators. Only the use of concentrators makes it possible to form and connect tree and Ringstrukturen.

Concentrators

Concentrators are the backbone of each FDDI system, it serve as distributors and merge single Attached station (SAS) into the FDDI ring. A loss of a FDDI concentrator or its disconnection interrupts the ring and leads to a new configuration. The loss or switching a SAS off at the concentrator, has no influence on the primary FDDI double ring, here separates the concentrator is attached simply the station from the ring and bridges the connection inside the concentrator. Similar to the above organization into devices of the Class A or Class B concentrators are divided in two classes:

• Class A concentrators are called dual Attached of Concentrators (DAC)
• Class B concentrators are called single Attached of Concentrators (SAC)

Single Attached station

Single Attached station (SAS) are stations with only one network connection, it can into the double ring be brought and are not devices of the Class B. typical SAS are servers or simple concentrators. A loss does not entail a reconfiguration of the double ring, but in the superordinate equipment by a bypass is intercepted. From a network with pure SAS can the largest expansion with most stations be formed, however at the price of the largest loss risk.

Binarily Attached station

Binarily Attached station () are stations directly into the FDDI double ring be brought can, but inevitably in the double ring to be merged not have, them belong the Class A. Typical THOSE are rout, concentrators or important servers, which permit only short Wartungsintervalle. If one falls THAT, which is brought in in the double ring out or a such station switched off, it comes to a reconfiguration of the ring, with the secondary ring been used. If a further loss arises, it comes to a separation of the ring and to the education of two separate rings. Since the loss of a connection does not lead to the break, THAT is used everywhere, where a increased availability is needed.

Binarily Homing

A third kind of the binding represents to dual the Homing, here not at one, but at two concentrators is tied up. This special kind of the binding represents the highest safety stage in FDDI systems and permits it surely losses of concentrators or network interfaces to intercept. This kind of the binding is selected for important servers with a maximum availability.

Maximum expansion

In the relevant literature data are like 500 to 1000 stations and 100 km to 200 km range. This apparently quite generously measured clearance explains itself by the two kinds of connection single Attached station (SAS) and dual Attached station () and the limiting tokens rotation Time, which is to move on the average between 4 to 165 ms. As limiting factor here the propagation speed of the light wave signal comes in the line medium to carrying with the maximum ring length.

This means that only from developed FDDI ring can reach maximally 500 stations and a total ring length of up to 100 km. SAS rings can have 1,000 stations and be long 200 km.

This circumstance leads to the fact that a the ring reconfigures itself in the event of an error in such a way that the secondary ring is used as backward channel and the overall length of the ring thereby nearly doubles itself. With a SAS the disturbed station is taken simply by the net and the ring shortens.

See also

• Time token rotation Protocol
• Fiber Distributed Data Interface 2 (FDDI-2)

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