Optical Networks: A Practical Perspective - Part 62. This book describes a revolution within a revolution, the opening up of the capacity of the now-familiar optical fiber to carry more messages, handle a wider variety of transmission types, and provide improved reliabilities and ease of use. In many places where fiber has been installed simply as a better form of copper, even the gigabit capacities that result have not proved adequate to keep up with the demand. The inborn human voracity for more and more bandwidth, plus the growing realization that there are other flexibilities to be had by imaginative use of the fiber, have led people. | 580 Network Survivability These protection routing tables are similar to the routing tables maintained in IP networks which work well even in very large IP networks with thousands of nodes. However we need to realize that routing tables in IP networks are not always consistent. If the tables are inconsistent routing pathologies such as looping can be present in the network with fairly high probabilities. For example at the end of 1995 the likelihood of encountering a major routing pathology in the Internet was Pax97j. These pathologies can cause packets to be forwarded incorrectly in the network but these packets eventually find their way to their destination or are dropped by the network. In the latter event the packets are retransmitted by a higher-layer protocol TCP . While this approach works well in IP networks we cannot afford to have routing pathologies in transport networks because they could prevent restoration of service after a failure. Therefore fast and reliable topology update mechanisms need to be in place to maintain the protection routing tables. We now look at the different variations of mesh protection. One aspect of this is whether the entire network is protected as a single domain or whether it is broken down into multiple domains with each domain protected independently and the different domains then tied together. In a degenerate scenario each domain could be a single ring in which case we get back to the usual mode of ring-based protection. Another important aspect that differentiates protection schemes is whether the protection routes are precomputed ahead of time offline or whether they are computed after a failure has occurred online . In both cases another dimension to consider is the degree of distributed implementation. This affects the complexity of the signaling protocols required and has a direct impact on the speed of restoration. Let us first consider the case where the protection routes are precomputed. In this case the .
