Traffic on BLSRs If desired, the protection band- width on a two-fiber (or four-fiber) BLSR may be used to transport unpro- tected extra traffic. The additional capacity permits the introduction of new revenue-enhancing services without further investment in fiber plant. As an example, Nortel's S/DMS TransportNode OC-48 BLSRs offer extra traffic features with an equivalent OC-192 offering now coming to market. Extra traffic can be terminated along with regular protected traffic using any available tributary ports on the network element. If all tributary capac- ity is exhausted, an additional collocated BLSR node may be deployed to provide additional tributary interfaces as required. Extra traffic is automatically removed from the protection channels when protection switching occurs on the BLSR. 3.4.4 Understanding Four-Fiber BLSRs The four-fiber BLSR architecture ensures service survivability through duplicate, geographically diverse paths similar to a two-fiber BLSR. It dif- Figure 3-53 ATM Network ATM network Element interconnected by a STS-3c two-fiber BLSR. Payload A STS-3c STS-3c Payload A Payload A OC-12/48/192 Two-Fiber BLSR ATM Network ATM Network Element Element Two-Fiber BLSR Node STS-3c Payload A ATM Network Element Traditional SONET Architectures fers from the two-fiber configuration in that four fibers (or two bidirectional fiber pairs) link adjacent nodes on the ring, as shown in Figure 3-54. The additional fiber enhances the architecture in two important ways: A doubling in the traffic-handling capacity of the ring since twice the number of fibers are available Two protection-switching modes-automatic RPS and traditional 1+ 1/1:1 span switching for self-healing operations during multiple fault conditions The allocation of bandwidth in a four-fiber BLSR is identical to point-to- point and linear add/drop topologies. One fiber pair between nodes carries working traffic exclusively, while the other pair serves as a protection facility. Capacity Advantages of Four-Fiber BLSRs As in a two-fiber BLSR, a 