le cut in a span without route diversity), automatic RPS redirects traffic in a manner similar to a two-fiber BLSR. However, instead of looping back time slots within the same fiber pair as in a two-fiber BLSR, the four-fiber architec- ture loops back traffic from the working fiber pair to the protection fiber pair (see Figure 3-56). Other aspects of four-fiber BLSR RPS are similar to the operation of a two-fiber BLSR. Table 3-4 Max. Tributary Max. Pass- OC-192 Two- Capacity for Fully Through Traffic on Max. Extra Traffic Fiber/Four-Fiber BLSR Type Protected Traffic Working Time Slots Terminations BLSR Node Capac- ity Comparison Two-Fiber STS-192 STS-96 STS-192 Four-Fiber STS-384 STS-192 STS-384 Traditional SONET Architectures Locally Figure 3-55 Terminating Optics Module Four-fiber BLSR span Traffic Fault switching example. Working Fiber Pair Node A Working Fiber Pair Bypassed Protection Fibers Activated Locally Protection Fibers Terminating Unused Traffic Node D Node B Locally Protection Fibers Protection Fibers Terminating Activated Unused Traffic Splice Failure Four-Fiber BLSR Node Node C Working Fiber Pair Working Fiber Pair Bypassed Locally Terminating Traffic 3.4.5 Understanding Matched Nodes Whereas the route diversity provided by ring topologies fully protects all working traffic passing from node to node along a ring, service paths must be protected on an end-to-end basis. This means survivable ring intercon- nections are required as traffic passes from an access ring through one or more interoffice rings and then to another access ring serving the service termination point. The SONET matched-node configuration (see Figure 3-57) has been expressly designed to provide these survivable connections between adjacent rings. Matched-node gateways are completely vendor/technology independent, enabling seamless interworking between rings of different types without regard to line rate or equipment supplier. Thus, matched-node inter-ring Chapter 3 Locally Figure 3-56 Terminating Four-fiber BLSR RPS Traffic exampl