Wednesday, 21 November 2012


A high-speed network technology, conforming to the Open Systems Interconnection (OSI) reference model for networking and the American National Standards Institute (ANSI) standard X3T9, which runs at 100 Mbps over fiber-optic cabling; often used for network backbones in a local area network (LAN) or metropolitan area network (MAN).

         Fiber Distributed Data Interface(FDDI) is usually implemented as a dual token-passing ring within a ring topology (for campus networks) or star topology(within building). The dual ring consists of a primary and secondary ring. The primary ring carries data. The counter-rotating secondary ring can carry data in the opposite direction, but is more commonly reserved as a backup in case the primary ring goes down. This provides FDDI with the degree of fault tolerance necessary for network backbones. In the event of failure on the primary ring, FDDI automatically re-configures itself to use the secondary ring as shown in the illustration.
Faults can be located and repaired using a fault isolation technique called beaconing. However, the secondary ring can also be configured for carrying data, extending the maximum pothential bandwidth to 200 Mbps. Stations connect to one (or both) rings using a media interface connector (MIC). There are two different FDDI implementations, depending on whether stations are attached to one or both rings:

Single-attached stations(Class B stations):

        Connect to either the primary or secondary ring using M ports. Single-attached FDDI uses only the primary ring and is not as commonly deployed for network backbones as dual-attached FDDI. Single-attached stations are used primarily to connect Ethernet LANs or individual servers to FDDI backbones. 

Dual-attached stations(Class A stations):

          Connect to both rings. The A port at which the primary ring enters and the secondary ring leaves; the B port is the reverse. M ports provide attachments points for single-attached stations. Dual-attached FDDI uses both rings; with the secondary ring serving as a backup for the primary.      

Fig: Fiber Distributed Data Interface(FDDI)

                               FDDI uses a timed token-passing FDDI stations generate a token that controls the sequence in which other stations will gain access to the wire. The token passes around the ring, moving from one node to the next. When a station wants to transmit information, it captures the token, transmits as many frames of information as it wants (within the specified access period), and then releases the token. Every node on the ring checks the frames. The recipient station then reads the information from the frames, and when the frames return to the originating station, they are stripped from the ring.
                      There can be up to 500 stations on a dual-ring FDDI network. The maximum circumference for an FDDI ring is 100 Kilometers(or 20 kilometers for both rings combined) and there must be a repeater every e kilometers or less. Bridges or routers are used to connect the FDDI backbone network to Ethernet or token ring