capacity interoffice networks. Source: Recent Nortel study. Cost-effective DCS deployment Number of STS-1/DS3 Terminations per Office Traditional SONET Architectures Service DS1 Figure 3-65 Termination Point Back-hauling traffic to a crossconnect hub. Service DS1 Termination Point Traffic Back-Haul Channels Transport Network Elements Digital Crossconnect System at Hub Site Bandwidth Management Solutions Based on OC-12 BLSRs Solu- tions based on VT-managed OC-12 BLSRs offer a bandwidth management alternative for small- to medium-capacity metropolitan networks. A VT- managed TBM ring provides a distributed crossconnect capability that can either enhance an existing crossconnect hub architecture or eliminate the need for an external DCS entirely (see Figure 3-66). The TBM's VT1.5 time slot assignment features groom, consolidate, and route traffic without cum- bersome back hauling to a shared DCS. This conserves fiber span capac- ity as well as DCS ports and core resources. In addition, end-to-end survivability is improved because services don't have to exit the ring for grooming. Fiber capacity and DCS resource savings stem from two important advantages of the VT-managed BLSR: Efficient packing of DS1 traffic into STS-1 channels Fewer services back-hauled to a hub site for grooming These savings can be substantial, depending on the desired network management strategy of the individual service provider. In the small-city traffic demand of Figure 3-67, a VT-managed TBM ring reduces the number of DCS ports required from 14 to 10 for a savings of nearly 30 percent. Note also that this application cannot be handled with a single STS-managed Chapter 3 Service DS1 Figure 3-66 Termination Point Traffic routing in a VT- managed BLSR. Direct Routing of DS1 Traffic Service DS1 OC-12 BLSR Termination Point S/DMS TransportNode Digital Cross-Connect System Optional Figure 3-67 1 DS1 Crossconnect savings 6 DS1s 5 DS1s in a typical small-city 30 DS1s network. 6 DS1s 44 DS1s 195 DS1s Hub Site 2 STS-1s 2 STS-1s Required Required 3 ST