l of existing fiber plant. The Two-Fiber BLSR Architecture In the two-fiber BLSR architec- ture, SONET OC-12/48/192 network elements are interconnected in a closed loop using bidirectional spans consisting of two fibers (see Figure 3-46). Each fiber handles one direction of transmission similar to traditional point-to- point and linear add/drop topologies. The perimeter of the ring can measure up to 1,200 km (746 miles) and up to 16 traffic-terminating nodes can be Chapter 3 Figure 3-46 Simplified view of a two-fiber BLSR architecture. Bidirectional Span Consisting of Two Fibers Two-Fiber BLSR Node Up to 16 Traffic-Terminating Network Elements placed on the ring. Regenerators or optical amplifiers may be deployed between nodes as needed to maintain the level of the optical signal. Allocation of Bandwidth Exactly one-half of the bandwidth available between adjacent nodes can be used for working traffic, and the remaining bandwidth is reserved for protection. As Figure 3-47 suggests, this band- width partitioning is accomplished by mapping traffic into STS-1 channels (time slots) 1 through 6 in OC-12 applications, into channels 1 through 24 in OC-48 applications, or into channels 1 through 96 in OC-192 applica- tions. Note that STS-1 time slot usage within a two-fiber BLSR does not impose a bandwidth penalty since any service can be routed from either side of the ring (see Figure 3-48). Thus, a two-fiber BLSR node has a maxi- mum fully protected traffic termination capacity equivalent to STS-12 (for OC-12 applications), STS-48 (for OC-48 applications), or STS-192 (for OC- 192 applications) bandwidth. Traditional SONET Architectures Working STS-1 Time Slots Node E Node D Figure 3-47 Bandwidth Pipe Model Span bandwidth allocation in a two- fiber BLSR. Shared Protection Facility (Automatically Selected During Protection STS-1 Time Slots Span or Node Failure) Node A Node B Node C STS-1 Time Slot Assignment in a Two-Fiber BLSR Working Traffic Working Traffic Protection STS-1 Equivalent STS-1 Equivalent OC-12 1 through 6