node on a four-fiber BLSR may terminate traffic fed from either side of the ring. With a fiber pair in each direction entirely dedicated to working traffic, an individual node on a four-fiber BLSR has twice the traffic- terminating capacity of a two-fiber BLSR node operating at the same line rate. For example, at the OC-192 rate, a four-fiber BLSR node terminates Figure 3-54 Simplified view of four-fiber BLSR architecture. Bidirectional Fiber Pair for Working Traffic Bidirectional Fiber Pair for Protection Four-Fiber BLSR Node Chapter 3 the equivalent of up to STS-384 fully protected tributary traffic. In a two- fiber configuration, fully protected tributary traffic is limited to a maximum equivalent bandwidth of STS-192. (This comparison assumes a sufficient number of tributary ports as in Nortel's S/DMS TransportNode OC-192 sys- items.) Similarly, the maximum levels of pass-through traffic and unprotected extra traffic terminations are doubled in a four-fiber BLSR. Refer to Table 3-4 for a concise listing of the maximum capacities offered by OC-192 two- fiber and four-fiber BLSRs. Span Protection Switching Four-fiber BLSR 1/1:1 span protection switching bypasses unidirectional or bidirectional faults that affect only the working fiber pair. Examples include a defective splice, faulty connector, and optical transmitter/receiver problems. A cable cut also affects only the working fiber pair in applications where geographically diverse routes link adjacent nodes. Because span protection switching operates independently on each link, service is maintained in the presence of several concurrent working path faults. Thus, in a representative dual-fault scenario (see Fig- ure 3-55), span protection switching can be activated between Nodes A and B to address an optics module failure, while at the same time the protec- tion fibers between Nodes C and D are in use to bypass a defective splice. Four-Fiber BLSR Ring Protection Switching (RPS) If a fault affects both the working and protection fibers (a node failure or a cab