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ANSI FDDI Standard


    Fiber Distributed Data Interface (FDDI) is a high-speed LAN protocol designed by ANSI for use with optical fiber transmission media. It is capable of achieving data rates in order of 100 mbps, and a network size in order of 1000 stations. Furthermore, with the high reliability of optical fiber, station distances in order of kilometers and a geographic spread of the LAN in order of hundreds of kilometers are feasible.


Topology


    FDDI utilizes a ring topology that involves two counter-rotating rings and two classes of stations.

    Class A stations are connected to both rings. Each class A station also has a bypass switch (i.e., short circuit) which when activated causes the station to be excluded from the ring without affecting the ring continuity.

    Class B stations are only connected to one of the rings via a concentrator. The concentrator also provides a bypass switch. Furthermore, because each of the B stations is independently connected to the concentrator, it can be switched off without affecting the other stations. Typically class A represents the more important stations (e.g., servers) and class B represents the less significant stations (e.g., infrequently-used PCs and terminals).

FDDI Ring Topology

Token Ring Protocol


    The FDDI protocol is specified by the ANSI X3T9 standard. It is a token ring protocol similar to IEEE 802.5 but with an important difference: stations can transmit even if the token they receive is busy. When a station receives a frame, it examines the address of the frame to see if it matches its own address, in which case it copies the data. In either case, if the station has nothing to transmit, it passes the frame to the next station. If it does have a frame to transmit, it absorbs the token, appends its frame(s) to any existing frames and then appends a new token to the result. The whole thing is then sent to the next station in the ring. As with earlier token ring protocols, only the originating station is responsible for removing a frame. The FDDI frame structure, which is almost identical to the IEEE 802.5 frame structure, except for the different physical size of the fields, and that it includes a Preamble and excludes the Access Control field.

    The FDDI protocol requires that each station maintains three timers for regulating the operation of the ring:

* The token holding timer determines how long a transmitting station can keep the token. When this timer expires, the station must cease transmission and release the token.

* The token rotation timer facilitates the normal scheduling of token rotation between stations.

* The valid transmission timer facilitates recovery from transmission errors.