Fiber Distribution Data Interface. A token ring network technology based on fiber optics. FDDI specifies a 100 Mbps data rate using 1300 nanometer light wavelength, and limits networks to approximately 200 km in length, with repeaters every 2 km or less.

Additional information concerning FDDI:

Fiber Distributed Data Interface (FDDI) uses fiber optics with a token-passing scheme. (Refer to the Ring Topology section of network topologies.) The token-passing used in FDDI is more complex.

The FDDI system actually incorporates two counter-rotating rings, whereas normal Token Ring LANs use one. The dual rings can prevent one FDDI node from bringing the entire network down.

                      //     \\
                     //       \\
                   node2     node4
                     \\       //
                      \\     //

If node1 broke, or was physically removed from the FDDI ring, the data from node3 trying to get to node2 would not be lost because of a 'black hole' where node1 used to be. The FDDI LAN card at node4 would automatically place the data onto the secondary ring, which travels back to node2 via node3. Node2 would also sense the problem with node1 and also route any information required through the secondary ring. In effect, the network will appear to the nodes to be:

                        \   / 
                         \ /  

Note that this only shows the apparent path, not the physical path. This ability to continue functioning with a major malfunction in a node makes the FDDI network very reliable. The costs for a FDDI system are very high, and require a skilled technician to work with the fiber optic cables.

An FDDI System Example

I worked on an FDDI system installation in Bangkok, Thailand. The new SkyTrain (basically an elevated train for commuters) uses an FDDI network for communications between the stations. A master controlling server oversees all 26 stations, each of which includes an Ethernet 10baseT sub-network. Each station has an automated fare collection system and a ticket office where people can buy tickets (much like the MetroCards used in the New York subway system). All of the revenue information is passed on to the master server for accounting. All of this technology must work together, and the FDDI system was chosen as the backbone. Should the FDDI system become inoperative, each station can operate autonomously. If a train jumped the tracks and destroyed the Mo Chit station, the rest of the system would still function because of the FDDI networks' ability to bypass network failures.

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