FDDI was developed by the IEEE and adopted as an ANSI standard through the X3T9.5 committee. Although this is a relatively new standard (first introduced in 1992), work on it has been under way for some time. The concept for a fiber optics network utilizing a ring topology was first introduced in 1982.

FDDI is similar to Token Ring in that it uses a token to identify an idle network, eliminating the possibility of data collisions. Where FDDI differs is in its handling of the token. FDDI uses a timed token, allowing nodes to hold tokens for a predetermined time before they are forced to relinquish control of the token. FDDI also uses a dual ring topology. This ring is counterrotating, which means that data is passed in both directions. This adds to the integrity of the network. As seen in Fig. 7.1, each node has two inputs and two outputs, each moving in opposite directions. If the ring breaks between two nodes, the nodes pass data back to the other ring, forming one ring rather than two.

Figure 7.1: Counterrotating FDDI Ring.

FDDI relies on other protocols above its own MAC layer, such as Ethernet MAC (802.2). In reality, where FDDI works best is as a backbone network used to move data from one LAN to another. FDDI provides 100-Mbps bandwidth, easily handling the capacity of Ethernet and Token Ring networks that may be attached. This is important for any backbone technology because the aggregate bandwidth from multiple networks...