Optical Networks - _9_3 Protection in the Client Layer_108

Optical Networks - _9_3 Protection in the Client Layer_108...

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532 Network Survivability back, as shown in Figure 9.11. The interconnection is done using signals typically at lower bit rates than the line bit rate. For instance, two OC-12 UPSRs may be interconnected by DS3 signals. In many cases, a digital crossconnect is interspersed between the two rings to provide additional grooming and multiplexing capabilities. The problem with this approach is that if one of the ADMs fails, or there is a problem with the cabling between the two ADMs, the interconnection is broken. A way to deal with this problem is to use dual homing . Dual homing makes use of two hub nodes to perform the interconnection, as shown in Figure 9.12. For traffic going between the rings, connections are set up between the originating node on one ring and both of the hub nodes. Thus if one of the hub nodes fails, the other node can take over, and the end user does not see any disruption to traffic. Similarly, if there is a cable cut between the two hub nodes, alternate protection paths are now available to restore the traffic. Rather than set up two separate connections between the originating node and the two hub nodes, the architecture uses a multicasting or drop-and-continue feature present in the ADMs. Consider the connection shown between an end node and the two hub nodes (hub 1 and hub 2) in Figure 9.12. In the clockwise direction of the ring, the ADM at hub 1 drops the traffic associated with the connection but also simultaneously allows this traffic to continue along the ring, where it is again dropped at hub 2. Likewise, along the counterclockwise direction, the ADM at hub 2 uses its drop-and-continue feature to drop traffic from this connection as well as pass it through to hub 1. Note that additional bandwidth is used up between the two hub nodes on each ring to support this capability. Dual homing is being deployed in business access networks to interconnect access UPSRs with interoffice BLSRs as well as to interconnect multiple BLSRs. It can also be applied to interconnections between two subnetworks, not necessarily two rings (although rings are the major application). In general, for dual homing to work, the dual node interconnect itself must be a protected subnetwork, so that alternate paths are available if any of the hub nodes or the links interconnecting them fails. 9.3 Protection in the Client Layer We will describe survivability mechanisms in other client layer networks and in particular IP, MPLS, Ethernet, and Resilient Packet Ring (RPR) networks. MPLS, Ethernet, and RPR protocols have carrier grade protection switching of 60 ms to re- store a failed connection. MPLS has a fast reroute mechanism that protects segments of a connection. MPLS and Ethernet has path switching for path connections (see Figure 9.2(b)) which is often referred to as linear protection .
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9.3 Protection in the Client Layer 533 RPR has been designed for a ring topology and is protected by ring switching (see Figure 9.2(d)). Ethernet and MPLS also have protection switching schemes when they operate on ring topologies. In SONET/SDH BLSR, RPR, and MPLS ring networks,
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Optical Networks - _9_3 Protection in the Client Layer_108...

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