Fiber Distributed Data Interface

The Fiber Distributed Data Interface ( FDDI, colloquially fiber optic metro rings ) is a late 1980s developed standardized 100-MBit/s-Netzwerkstruktur for local area networks (ANSI Standard X3T9.5 ). As a medium, fiber optic cables are used in a dual counter-rotating ring with token - access mechanism. In 1994, the FDDI standard was expanded and the transmission over shielded (STP) and unshielded (UTP type 5) twisted-pair copper cables standardized ( CDDI, C for copper).

Standards

  • ANSI X3T9.5, Physical Media Dependent (PMD ) specification, access to media (optical fiber, copper)
  • ANSI X3T9.5, Physical (PHY ) specification, coding of data with timing 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 assembly, error detection and correction, station management.

The following characteristics distinguish FDDI networks:

  • Medium: glass fiber 1300 nm
  • Frequency Band: baseband
  • Topology: Double ring ( data ring and reserve ring )
  • Arbitration: Token
  • Fault tolerance: max. 1 station (additional bypass capability )
  • Distance between adjacent stations: max. 2 km
  • Ring length: max. 100-200 km
  • Stations with a simple ring: max. 1000
  • Stations with dual ring: max. 500

FDDI rings are normally constructed as a " dual ring of trees ". A small number of devices (routers and concentrators ) is connected to both rings (dual attached). Normal hosts are then connected to the router or concentrators via simple cable.

Typically only one ring is used. A token will be passed through the ring. It needs to be forwarded from each station, it receives. If a station wants to send, it waits for the token sends the pending data and depends again on a token.

The failure of a station on the ring, then the second (reserve ) ring is used in the opposite direction. Before and behind the faulty station, the data is sent back, so that a single ring is formed. If one more station, it comes to the separation of the network. Although the standard provides for an optical bypass, but in practice this does not always reliable.

FDDI was the designated successor for the old 10 -Mbit Ethernet in the 1990s. New developments such as Gigabit Ethernet and ATM, however, were faster, much cheaper and easier to use. FDDI has gained but in another area of ​​importance: It is because of its high range and reliability often as a central LAN structure (backbone ) used to be interconnected via the multiple Ethernet or Token - Ring networks.

To over FDDI networks to a lesser extent to at least operate multimedia applications, the limited real-time FDDI Version 2 was created. In addition to the "shared -media " bandwidth available for all the stations are defined for 64-kbit/s-Datenkanäle reserved for isochronous applications such as video or multimedia applications. The transmission time within each of these channels is 125 microseconds.

Connectivity

FDDI devices are divided into two classes. Class A devices can be integrated directly into the ring, this can routers, concentrators or workstations with two connections to be. The decisive criterion here are at least two available ports. Devices with only one FDDI interface are referred to as Class B digital device, and can not be directly involved in the ring.

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

Concentrators

Concentrators are the backbone of any FDDI system, they serve as a distributor and bind Single Attached Stations ( SAS) in the FDDI ring a. A failure of a FDDI concentrator, or switching off interrupts the ring and results in a reconfiguration. The failure or disconnection of a SAS, which is connected to the concentrator, has no effect on the primary FDDI dual ring, in this case, the concentrator separates easily from the station ring and bridged the connection inside the concentrator. Be divided into two classes concentrators analog of the above division into units of Class A or Class B:

  • Class A - concentrators are called Dual Attached Concentrator (DAC )
  • Class B concentrators are called Single Attached Concentrator (SAC )

Single Attached Stations

Single Attached Stations ( SAS) are stations with only one network connection, they can not be introduced into the double ring and devices are Class B. Typical SAS are server or simple concentrators. A failure has intercepted no reconfiguration of the double ring themselves, it is in the Parent device through a bypass. From a pure braid with SAS, the largest expansion can be made ​​with most stations, but at the cost of the greatest risk of default.

Dual Attached Stations

Dual Attached Station (DAS) are stations that can be introduced directly into the FDDI dual ring, but need not necessarily be involved in a double ring, they belong to the class A at. Typical DAS, routers, concentrators or important servers that allow only short maintenance intervals. If a DAS, which is introduced in a double ring or such a station is turned off, there is a reconfiguration of the ring, in which the secondary ring becomes operational. An additional failure occurs in, there is a separation of the ring and the formation of two separate rings. Since the failure of a connection of a DAS does not lead to the break, DAS be used anywhere where an increased availability is required.

Dual Homing

A third type of connection represents the dual homing, this one DAS is not connected to one but to two concentrators. This particular type of connection represents the highest level of security in FDDI systems and allows failures of concentrators or network interface trap safely. This type of connection is chosen for critical servers with a maximum availability.

Maximum expansion

In the relevant literature there are data such as 500 to 1000 stations and 100 to 200 km range. This seems quite generous margin is explained by the two types of connection SAS and DAS and the limiting token rotation time that is supposed to move in the middle between 4-165 ms. A limiting factor for the maximum loop length is in this case the speed of propagation of the optical signal in the cable medium for carrying.

This means that a built-up, only from The FDDI ring may reach maximum of 500 and a total length of the ring of up to 100 km. SAS rings can have 1000 stations and be 200 km.

This circumstance leads to a DAS - ring so reconfigured in case of error, that the secondary ring is used as a return channel and the total length of the ring thereby almost doubled. In a SAS the disturbed station is simply removed from the network and the ring is shortened.

Specifications

  • RFC 1188, A Proposed Standard for the Transmission of IP Datagrams over FDDI Networks
Technical standard Asynchronous Transfer Mode B channel
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