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Unformatted text preview: 9.4 Why Optical Layer Protection 541 goes completely around the ring forming a closed loop. Label swapping is allowed for working LSPs and their protection tunnels. When a node detects a failure, it transmits a request to protection switch to the other node adjacent to the failure. In the case of wrapping, when the node detects a failure or a request to protection switch, it switches traffic to the protection tunnel. In the case of steering, the source and destination nodes of the LSP switch to the protection tunnel. Note that MPLS fast reroute can also be implemented on a ring network topology. A comparison of the MPLS fast reroute and T-MPLS ring protection can be found in [YS08]. T-MPLS ring protection has higher complexity due to its APS and complexity in configuration, but it keeps packets in order. In MPLS fast reroute, there is a possibility of packet misordering. 9.4 Why Optical Layer Protection The optical layer provides lightpaths for use by its client layers, such as the SONET, IP, and 10-Gigabit Ethernet. We have seen that extensive protection mechanisms are available in the SONET layer and other client layers. These layers were all designed to work independently of each other and not rely on protection mechanisms available in other layers. We will see below that there is a strong need for protection in the optical layer, despite the existence of protection mechanisms in the client layers. SONET/SDH networks incorporate extensive protection functions. However, other networks such as IP networks do not provide the same level of protection. As we saw in Section 9.3.3, IP traffic for the most part is best-effort traffic. One way to protect data networks is to rely on optical layer protection, which can be quite cost-effective and efficient. Significant cost savings can be realized by making use of optical layer protection instead of client layer protection. We illustrate this with two examples. Consider an example of a WDM ring network with lightpaths carrying higher- layer traffic. Figure 9.18 illustrates an example where there is no optical layer pro- tection. Two SONET line terminals (LTEs) are connected to each other through lightpaths provided by the optical layer, as are two IP routers. For simplicity we look at a undirectional lightpath from LTE A to LTE B and another lightpath from router C to router D. These two lightpaths are protected by the SONET and IP layers, respectively, using 1 + 1 protection. The working connection from LTE A to LTE B is established on wavelength 1 along the shortest path in the ring, and the other protection connection is established, say, on the same wavelength 1 around the ring. Likewise, the working connection from router C to router D 542 Network Survivability Working fiber OADM OADM OADM OADM SONET LTE SONET LTE l 1 l 1 l 1 l 2 Protection fiber A C D B IP router IP router Figure 9.18 A WDM ring built using optical add/drop multiplexers (OADMs), sup- porting two interconnected SONET line terminals (LTEs) and two interconnected IP...
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- Spring '09