Fiber Distributed Data Interface

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FDDI in the TCP / IP protocol stack :
application HTTP IMAP SMTP DNS ...
transport TCP UDP
Internet IP ( IPv4 , IPv6 )
Network access FDDI

The Fiber Distributed Data Interface ( FDDI , colloquially also fiber optic metro ring ) is a standardized 100 Mbit / s network structure for local networks ( ANSI Standard X3T9.5) developed in the late 1980s . Fiber optic cables are used as the medium in a double, counter-rotating ring with a token access mechanism. In 1994, the FDDI standard was expanded and transmission was standardized via shielded (STP) and unshielded (UTP type 5) twisted copper cables ( CDDI , C for copper). FDDI was gradually replaced by inexpensive Ethernet technology. Market-leading manufacturers of network components no longer offer FDDI support for their products, so that the technology is considered obsolete.


  • ANSI X3T9.5, Physical Media Dependent (PMD) specification, access to medium ( fiber optic , 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, ring structure
  • ANSI X39.5, Station Management (SMT) specification, connection and ring structure, error detection and elimination, station management.

FDDI networks have the following properties:

  • Medium: glass fiber 1300 nm
  • Frequency band : baseband
  • Data rate : 100 Mbit / s / 155 Mbit / s / 1000 Mbit / s
  • Topology : double ring (data ring and reserve ring)
  • Arbitration : Token
  • Error tolerance: max. 1 station (additional bypass option )
  • Distance between neighboring stations: max. 2 km
  • Ring length: max. 100-200 km
  • Stations with a single ring: max. 1,000
  • Stations with double ring: max. 500
FDDI ring with routers and concentrators

FDDI rings are usually designed as a "double ring with trees". A small number of devices ( routers and concentrators ) are connected to both rings ( dual attached ). Normal computers are then connected to routers or concentrators via simple fiber optic cables.

Usually only one ring is used. A token goes through all stations of the ring. It must be forwarded by every station that receives it. If a station wants to send, it waits for the token, sends the outstanding data and appends a token again.

Failure of a station in the FDDI ring

If one station fails on the ring, the second (reserve) ring is used in the opposite direction. The data is sent back in front of and behind the faulty station, creating a single ring. If another station fails, the network is separated. Although the standard provides for an optical bypass, this does not always work reliably in practice.

In the 1990s, FDDI was the designated successor to the old 10 Mbit Ethernet. However, new developments such as Gigabit Ethernet and ATM were faster, much cheaper and easier to use. However, FDDI has gained importance in another area: Due to its high range and reliability, it is often used as a central LAN structure (backbone) via which several Ethernet or token ring networks are connected to one another.

In order to be able to operate multimedia applications at least to a small extent via FDDI networks , the limited real-time FDDI version 2 was created. In addition to the “shared media” bandwidth available for all stations, 64 kbit / s data channels were defined for this purpose , which are reserved for isochronous applications such as video or audio applications. The transmission time within these data channels is 125 µs.


Connection types at a glance

FDDI devices are divided into two classes. Class A devices can be integrated directly into the ring; these can be routers, concentrators or workstations with two connections. The decisive criterion here is at least two available connections. Devices with only one FDDI interface are referred to as class B devices and cannot be integrated directly into the ring.

In order to be able to connect Class B devices, Class A devices are required that provide additional connections for Class B devices; these devices are called concentrators. Only the use of concentrators allows tree and ring structures to be formed and connected.


Concentrators are the backbone of every FDDI system, they serve as distributors and integrate Single Attached Stations (SAS) into the FDDI ring. A failure of an FDDI concentrator or its shutdown interrupts the ring and leads to a reconfiguration. The failure or disconnection of a SAS connected to the concentrator has no effect on the primary FDDI double ring; the concentrator simply disconnects the station from the ring and bridges the connection inside the concentrator. Analogous to the above division into Class A or Class B devices, concentrators are divided into two classes:

  • Class A concentrators are called Dual Attached Concentrators (DAC)
  • Class B concentrators are known as Single Attached Concentrators (SAC)

Single attached stations

Single Attached Stations (SAS) are stations with only one network connection; they cannot be integrated into the double ring and are Class B devices . Typical SAS are servers or simple concentrators. A failure does not result in a reconfiguration of the double ring, but is intercepted in the higher-level device by a bypass. The largest expansion with most stations can be formed from a network with pure SAS, but at the price of the greatest risk of failure.

Dual attached stations

Dual Attached Stations (DAS) are stations that can be incorporated directly into the FDDI double ring, but do not necessarily have to be integrated into the double ring; they belong to Class A. Typical DAS are routers, concentrators or important servers that only allow short maintenance intervals. If a DAS installed in the double ring fails or such a station is switched off, the ring is reconfigured and the secondary ring is used. If another failure occurs, the ring will separate and two separate rings will be formed. Since the failure of a DAS connection does not result in the connection being terminated, DAS are used wherever increased availability is required.

Dual homing

A third type of connection is dual homing , in which a DAS is not connected to one, but to two concentrators. This special type of connection represents the highest level of security in FDDI systems and allows failures of concentrators or network interfaces to be safely intercepted. This type of connection is chosen for important servers with maximum availability.

Maximum expansion

The relevant literature provides information such as 500 to 1000 stations and a range of 100 to 200 km. This apparently quite generous scope is explained by the two connection types SAS and DAS and the limiting token rotation time , which should be between 4 and 165 ms on average. The speed of propagation of the lightwave signal in the line medium is the limiting factor for the maximum ring length.

This means that an FDDI ring composed only of DAS can reach a maximum of 500 stations and a total ring length of up to 100 km. SAS rings can have 1000 stations and be 200 km long.

This fact leads to the fact that in the event of a fault a DAS ring reconfigures itself in such a way that the secondary ring is used as a return channel and the total length of the ring is almost doubled. With a SAS, the disturbed station is simply removed from the network and the ring is shortened.


  • Manfred Burke: Computer Networks. Concepts and techniques of data transmission in computer networks, BG Teubner Verlag, Stuttgart 1994, ISBN 978-3-519-02141-4 .
  • Dieter Conrads: data communication. Procedure - Networks - Services. 2nd edition, Friedrich Vieweg & Sohn Verlag, Wiesbaden 1993, ISBN 3-528-14589-7 .
  • Bernhard Albert: FDDI and FDDI-II. Architecture - Protocols and Performance, Artech House, London 1994, ISBN 978-0-89006-633-1 .
  • Andrew Mills: Understanding FDDI. A 100Mbps Solution for Today's Corporate LANs, Prentice Hall, 1995, ISBN 978-0-13219-973-5 .

See also

Web links

Individual evidence

  1. Manufacturer information from Cisco. May 1, 2017, Retrieved May 1, 2017 .