8 - Survivability: Protection and Restoration

Summary

Survivability against failures, including failure recovery, is important in any telecommunications network but is highly critical for high-bandwidth optical networks. As more traffic is concentrated on fewer routes, the number of customers that can be potentially affected by a failure is increased. An analysis of failures in the Public Switched Telephone Network over a two-year period in the 1990s showed that human error, acts of nature, and overloads were the major sources of failure. The impact of the failures was measured in terms of how many times a particular failure occurred, duration of the outage, and number of customers and number of customer minutes affected during that outage. During that period, the average number of customers affected due to cable cuts or cable component failures was 216,690, costing 2,643 million in customer minutes. Similarly, the average number of customers affected by each equipment failure was 1,836,910, costing 3,544.3 million in customer minutes . Cable cuts and hardware/equipment failures account for approximately half of the failures encountered in the network during that period.

Fiber cuts are considered one of the most common failures in fiber-optic networks. Furthermore, the use of WDM over these fibers produces an extremely high volume of traffic on a cable. Commercially available fiber-optic transmission systems can run at 10 Gbps or more per channel with 80 or more channels (wavelengths) per fiber. This translates to more than 800 Gbps per fiber.