 Spur Routes Optical Hub in Optical Hub in Point-to-Point Linear ADM and Applications Ring Applications Chapter 3 Service-Terminating Service-Terminating Figure 3-42 Network Element Cascaded Regenerators Network Element Extended reach application supported by cascaded Mixed Tributaries Mixed Tributaries regenerators. reconstitute the optical signal on both the working and protection fibers of a 1+1/1:1 protected point-to-point or linear system without route diversity. A diverse route regenerator supports the single bidirectional optical channel associated with two-fiber BLSRs and other route diversity arrangements (such as the example depicted in Figure 3-33). 3.4.2 Understanding OC-3 UPSRs A UPSR is a survivable, closed-loop transport architecture that protects against cable cuts and node failures by providing duplicate, geographically diverse paths for each service. Adjacent nodes on the ring are intercon- nected using a single pair of optical fibers. UPSRs feature a unique unidi- rectional traffic flow, as illustrated in Figure 3-43. Working traffic travels in only one direction (clockwise) on the ring, whereas a second unidirectional protection path is provided in the counterclockwise direction. OC-3 UPSRs are typically deployed in access networks where traffic from multiple carrier serving areas (CSAs) terminates at a CO hub site. (BLSRs are often the preferred choice for other types of traffic.) Because the UPSR supports virtual ring configurations, isolated CSAs/nodes can be easily placed on the same ring with several other physically interconnected nodes. Some advantages are exclusive to the UPSR architecture. One such advantage is that services may originate and terminate on the same UPSR or may be passed to an adjacent access or interoffice ring for survivable transport to the service-terminating location. OC-3 UPSRs support matched-node inter-ring gateways that ensure survivable interconnections between rings irrespective of the type (UPSR or BLSR), line rate, or equip- ment supplier. UPSR Operation Al