Society’s growing use of communication networks in both the public and private sector has led to an increased focus on reliability and survivability. Over the last decade, widely publicized network outages illustrated that a disruption of communications services can be very expensive to businesses and critical services (such as 911 service). The growing vulnerability of the public switched telephone network in the United States was initially discussed in 1989 by the National Research Council which noted a As we become more dependent on network, the consequences of network failure become greater and the need to reduce network vulnerabilities increases commensurately.o With the popularity of the Internet, a variety of network failures has surfaced in the recent years. It is clear that fault management will grow in importance for the foreseeable future. A variety of network failures are possible with typical events resulting in a failure being accidental cable cuts, hardware malfunctions, software errors, natural disasters (e.g., ® re, hurricane, earthquake), human error (e.g., incorrect maintenance), and malicious attack (both hardware and software). Broadly, we can classify the failures as physical-type failures and software-type failures. First we discuss physical failures in this context. Consider, a ® ber cable cut in a selfhealing ring in a metropolitan area telecommunication network; the failure can be restored in milliseconds by changing the direction of ow (with appropriate availability of capacity). However, using such a ring approach on a national or an international scale is likely to be cost prohibitive. This may lead to an approach which allows a combination of ring and digital cross-connect systems for transmission layer restoration, although it should be kept in mind that the spare capacity provided for such restoration is unused during normal network
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