As its name implies, FDDI runs on fiber-optic cable, and thus combines high-speed performance with the advantages of the token-passing ring topology. FDDI runs at 100 Mbps, and its topology is a dual ring. The outer ring is called the primary ring and the inner ring is called the secondary ring.

Normally, traffic flows only on the primary ring. If it fails, then the data automatically flows onto the secondary ring in the opposite direction. When this occurs, the network is said to be in a wrapped state. This provides fault tolerance for the link. The Figures and illustrate how FDDI works.

Computers on a FDDI network are divided into two classes:

• Class A – Computers connected to the cables of both rings.
• Class B – Computers connected to only one ring.

A FDDI dual ring supports a maximum of 500 nodes per ring. The total distance of each length of the cable ring is 100 kilometers, or 62 miles. A repeater (device that regenerates signals) is needed every 2 kilometers, which is why FDDI is not considered to be a WAN link.

The specifications described so far refer to a FDDI that is implemented over fiber-optic cable. It is also possible to use the FDDI technology with copper cabling. This is called Copper Distributed Data Interface (CDDI). The maximum distances for CDDI are considerably lower than those for FDDI.

Advantages of FDDI
FDDI combines the advantages of token passing on the ring topology with the high speed of fiber-optic transmission. The dual ring topology provides redundancy and fault tolerance. The fiber-optic cable is not susceptible to EMI and noise, and it is more secure than copper wiring. It can send data for greater distances between repeaters than Ethernet and traditional Token Ring.

Disadvantages of FDDI
As always, high speed and reliability come with a price. FDDI is relatively expensive to implement, and the distance limitations, though less restrictive than those of other LAN links, make it unsuitable for true WAN communications.