Node B Traffic Traffic Service B-C OC-12/48/192 Bidirectional OC-12/48/192 Fiber Pair Bidirectional "Reused" Bandwidth Fiber Pair (STS-1 Time Slot 1) Node C Service A-D-C Service A-B STS-1 Time Slot 1 Terminating Traffic STS-1 Time Slot 1 Traditional SONET Architectures transport traffic having STS-3 total bandwidth while using only a single STS-1 channel. Automatic Healing of Failed or Degraded Optical Spans In the event of failure or degradation in an optical span, automatic RPS reroutes affected traffic away from the fault within 50 ms, preventing a service out- age. Traffic is redirected by looping back STS-1 time slots (see Figure 3-50). Logically, the normally unused protection bandwidth bridges the defective span thereby maintaining service for all terminating and pass-through traffic. When setting up the bidirectional loopback path, OC-12 systems map working STS-1 time slots 1 through 6 to protection time slots 7 through 12 (respectively), OC-48 network elements map working STS-1 time slots 1 through 24 to protection time slots 25 through 48, and OC-192 Rerouted Service A-B Figure 3-50 Two-fiber BLSR Unaffected Traffic OC-48 BLSR response to an Node A optical span failure- an OC-48 application is shown. OC-48 Bidirectional STS-1 Fiber Cut Bidirectional Loopback to Protection Fiber Pair Rerouted Time Slot Service A-B Bidirectional STS-1 Loopback to Protection Time Slot Pass-through Routing of Protection Bandwidth Rerouted Service A-B Node D Node B Unaffected Rerouted OC-48 Traffic Service A-B Bidirectional Fiber Pair OC-48 Bidirectional Fiber Pair Node C Pass-through Routing of Denotes STS-1 time slots for Protection Bandwidth Denotes STS-1 time slots for working traffic (1 through 24) working traffic (25 through 48) Chapter 3 systems map working STS-1 time slots 1 through 96 to protection time slots 97 through 192. STS-1 time slot bridging occurs only at the nodes adjacent to the fault, with intermediate nodes (for example, Nodes C and D in Figure 3-50) sim- ply passing through the redirected traffic mapp