S-1s Required 0 STS-1s Required 3 STS-1s Required 2 STS-1s Required Solution Without VT-Managed VT-Managed OC-12 BLSR OC-12 BLSR 6 STS-1s Required 8 STS 1s Required 4 STS-1s Required 6 STS-1s Required Hub Site Hub Site Solution requires 14 Solution requires 10 (approximately STS-1 ports on DCS STS-1 ports on DCS 30% savings) Traditional SONET Architectures OC-12 BLSR since a total of STS-14 working bandwidth would be required at the hub site. Besides crossconnect port and core resource savings, a VT-managed OC- 12 BLSR reduces transport shelf tributary hardware requirements due to the efficient concentration of traffic to a minimum number of DS1 mapper plug-ins. Fiber bandwidth savings increase the effective capacity of the net- work and thus defer or avoid the need for additional fiber pairs and more network elements. The net result is a substantial reduction in capital costs in the typical small-city network. Also, operational costs and labor are reduced because fewer back-hauled services mean less management com- plexity. 3.5 Packet over SONET (PoS) Figure 3-68 depicts the migrations of the broadband data communications protocol stack that have occurred over the past few years or are expected to occur in the near future. In particular, there has been a migration from IP over ATM, IP directly over SONET, or even directly over optics. The goal is to improve efficiency and reduce the cell tax that was associated with the use of ATM. 15 This technology saw deployment in the late 1990s and is cur- rently fairly popular with ISPs. PoS can be used to connect users' routers to the WAN or LAN-to-LAN connections using the SONET/SDH carrier struc- ture. PoS was originally defined in the IETF RFC 1619, "PPP over SONET and SDH Circuits" (see Table 3-5 for related RFCs) as well as in RFC 1661 and RFC 1662. PoS is a point-to-point solution that eliminates ATM's need for multiple switched virtual circuits. PoS is a Layer 2 transport technology that encapsulates IP packet data directly into SONET. Figure 3-69 is the PoS stack.