e. Working Fiber Pair Node A Fiber Cut Bidirectional Loopback to Protection Fibers Redirected Traffic on Protection Fibers Bidirectional Loopback Node D Node B to Protection Fibers Locally Locally Terminating Redirected Traffic on Terminating Traffic Protection Fibers Traffic Redirected Traffic on Protection Fibers Node C Working Fiber Pair Working Fiber Pair Locally Four-Fiber Terminating BLSR Node Traffic gateways can provide survivable interconnections between an OC-3 UPSR from one equipment supplier and an OC-192 four-fiber BLSR from a differ- ent supplier, for example. Survivable Service Interconnections Between Rings To ensure sur- vivability for services traversing multiple interconnected rings, the matched-node configuration provides redundant routing across inter-ring boundaries as in the BLSR example of Figure 5-58. In the outbound direc- tion, drop-and-continue routing is employed within a primary gateway net- work element to simultaneously pass an exiting signal to both the neighboring ring and a remote node configured as a secondary inter-ring gateway. For the opposite direction of transmission, the primary inter-ring gateway incorporates a service selector that chooses either the primary or secondary input from the adjacent ring. Traditional SONET Architectures Figure 3-57 Survivable Service Interconnection Between Rings Inter-ring service transport using Primary Inter-Ring Gateways survivable matched- node gateways. To/From SONET Tributaries To/From Adjacent Adjacent Vendor/Rate Independent Matched Vendor/Rate Independent UPSR or BLSR Nodes UPSR or BLSR To/From SONET Tributaries To/From Adjacent Adjacent Vendor-Independent Secondary Inter-Ring Gateways Vendor-Independent UPSR or BLSR node UPSR or BLSR node Inter-Ring Service "A" Inter-Ring Service "A" Figure 3-58 Duplicate inter-ring Primary Inter-Ring Gateways service paths provided by Drop & Continue matched-node Routing for gateways-a BLSR Outbound Signal Failed example is shown. Vendor-Independent BLSR Nodes Vendor-Independent BLSR Service Sele