l traffic is managed at the path (service) layer on an OC-3 UPSR. As each individual DS1, DS3, or STS-1 service travels in one direction around the ring, a duplicate signal passes in the opposite direction for protection (see Figure 3-44). A path selector continuously Traditional SONET Architectures Typical Service Path Figure 3-43 Simplified view of OC-3 UPSR architecture. Single Fiber for Working Traffic (Clockwise Flow) Typical Service Single Fiber for a Protection Facility (Counterclockwise Flow) UPSR Node Signal Entry Point Signal Entry Point Figure 3-44 OC-3 UPSR protection switching-one Protection direction of Protection Working Path transmission is shown. Path Selector Path Selector Signal UPSR Nodes UPSR Nodes Point Signal Exit Path Failure Point Detected UPSR in Normal, Fault-Free State UPSR Response to a Cable Cut monitors both the working and protection signals at each end of the path and automatically switches to the protection signal in the event of optical span or node failure or degradation. The path selector detects path failure Chapter 3 and signal degradation based on industry-standard parameters such as the following: Path AIS Path LoP Signal degrade (SD) Excessive path-layer BIP errors Once protection switching has been activated, traffic flows in a tradi- tional bidirectional pattern that is similar to a linear add/drop topology. The S/DMS TransportNode OC-3 Express product line supports UPSR protec- tion switching in accordance with the GR-1400 industry standard. Support for Virtual Rings Because intermediate nodes are essentially transparent in the UPSR protection-switching scheme, other survivability architectures such as a two-fiber or four-fiber BLSR may be incorporated within a UPSR in a hybrid arrangement known as a virtual ring or path- in-line ring. This alternative is an excellent choice for interconnecting an isolated business campus or CSA with several remote sites that share a common access network, as shown in Figure 3-45. Virtual rings can also provide a survivable, geographic