ed to the protection time slots. STS-1 time slot assignments for working traffic at intermediate nodes are unaffected by the fault. Conditions that trigger RPS include total failure modes such as LoS and also degradation in terms of excessive line-layer BIP errors. Because pro- tection switching is revertive in a two-fiber BLSR, traffic automatically returns to its normal routing-without human intervention-after a fault- free state exists for a user-defined wait-to-restore interval. Rerouting of Pass-Through Traffic During Node Failures The two- fiber BLSR architecture also fully protects all restorable traffic in the event of a node failure anywhere along the ring. Although tributaries terminat- ing at the failed node cannot be protected, traffic passing through that node is automatically redirected away from the fault via a time slot loopback sim- ilar to the previous span failure example of Figure 3-50. In an action referred to as squelching, nodes adjacent to the failure replace nonrestor- able traffic with a path-layer AIS to notify the far end of the interruption in service. The squelching feature employs automatically generated squelch maps that do not require manual record keeping. Capacity Advantages of Two-Fiber BLSRs Due to the BLSR's capa- bility to reuse STS-1 channels, a BLSR may offer significant capacity advantages over UPSRs, depending on the traffic pattern (see Figure 3-51). Where traffic is entirely hubbed (as in most access networks), capacity equals that of a UPSR operating at the same line rate. As traffic becomes more distributed (or meshlike), however, a BLSR provides a substantial capacity benefit that can typically range up to 300 percent of the capacity of a UPSR with an identical optical rate. The greatest capacity advantage occurs with the node-to-adjacent-node pattern because this traffic model permits a very high level of channel reuse. See Figure 3-52 for a capacity comparison of all three traffic patterns relative to a UPSR. In a given ring application, the capacity required on a