FDDI provides a number of fault-tolerant features. In particular, FDDI’s dual-ring environment, the implementation of the optical bypass switch, and dual-homing support make FDDI a resilient media technology.

Dual Ring

FDDI’s primary fault-tolerant feature is the dual ring. If a station on the dual ring fails or is powered down, or if the cable is damaged, the dual ring is automatically wrapped (doubled back onto itself) into a single ring. When the ring is wrapped, the dual-ring topology becomes a single-ring topology. Data continues to be transmitted on the FDDI ring without performance impact during the wrap condition. Figure(a) and Figure(b) illustrate the effect of a ring wrapping in FDDI.

                             Figure ; FDDI ring with a (a) broken link, (b) defective station

When a cable failure occurs, as shown in Fig. 5.5.3(a), devices on either side of the cable fault wrap. Network operation continues for all stations. When a single station fails, as shown in Fig. 5.5.3(b), devices on either side of the failed (or powered-down) station wrap, forming a single ring. Network operation continues for the remaining stations on the ring. It should be noted that FDDI truly provides fault tolerance against a single failure only. When two or more failures occur, the FDDI ring segments into two or more independent rings that are incapable of communicating with each other.

Optical Bypass Switch

An optical bypass switch provides continuous dual-ring operation if a device on the dual ring fails. This is used both to prevent ring segmentation and to eliminate failed stations from the ring. The optical bypass switch performs this function using optical mirrors that pass light from the ring directly to the DAS (dual-attachment station) device during normal operation. If a failure of the DAS device occurs, such as a power-off, the optical bypass switch will pass the light through itself by using internal mirrors and thereby will maintain the ring’s integrity.

Figure ; The Optical Bypass switch uses internal mirrors to maintain a network

The benefit of this capability is that the ring will not enter a wrapped condition in case of a device failure. A somewhat similar technique has been discussed in Token ring section (Star Connected Ring- where relays are used to bypass the faulty node). Figure 5.5.4 shows the functionality of an optical bypass switch in an FDDI network. When using the OB, you will notice a tremendous digression of your network as the packets are sent through the OB unit.

Dual Homing:

Critical devices, such as routers or mainframe hosts, can use a fault tolerant technique called dual homing to provide additional redundancy and to help guarantee operation. In dual-homing situations, the critical device is attached to two concentrators.