ally diverse path to an interexchange car- rier (IXC) point of presence (POP). Path protection switching in a virtual ring operates in exactly the same manner as a conventional UPSR since any intermediate BLSR nodes (or other foreign nodes) are completely transparent to the virtual ring. Simi- larly, an intermediate BLSR protects its portion of the path using standard line-layer BLSR ring protection switching (RPS). Note that survivability is completely ensured without use of protected optics on the OC-3 links between the virtual ring and an intermediate BLSR. This advantage can yield substantial circuit pack inventory savings and frees up shelf space for other revenue-generating services. 3.4.3 Understanding Two-Fiber BLSRs As with the OC-3 UPSR discussed previously, the two-fiber BLSR is a sur- vivable SONET transport architecture that protects against cable cuts and node failures by providing duplicate, geographically diverse paths for each service. With the capability to reuse bandwidth as traffic is added/dropped at various locations around the ring, two-fiber BLSRs are ideal for distrib- uted mesh and node-to-adjacent-node traffic patterns typical of interoffice networks and are sometimes found in access networks as well. Traditional SONET Architectures Isolated UPSR Node Figure 3-45 Virtual OC-3 UPSR Isolated Business architecture. Campus Site OC-12/48/192 Virtual UPSR Interoffice BLSR BLSR Node OC12/48/192 Virtual UPSR UPSR Node Nortel's S/DMS TransportNode offers two-fiber BLSRs operating at a choice of line rates: OC-12, OC-48, or OC-192. All S/DMS TransportNode two-fiber BLSRs comply with GR-1230 industry standards and support matched-node inter-ring gateways to ensure survivability for services tra- versing multiple interconnected rings. These gateways are fully interoper- able with two-fiber BLSRs, four-fiber BLSRs, and UPSRs irrespective of line rate or equipment vendor. Nortel's S/DMS TransportNode also offers BLSR configurations supporting extra traffic-a feature that enhances the revenue potentia