et carriers have used their facilities-based CLEC models, along with optical Ethernet technology, to carve a path into the RBOC's stranglehold on metro access " Equipment manufacturers are quoting an internal rate of return (IRR) of 87.6 percent and a payback in less than 18 months. This kind of advocacy is intended to get ILECs to purchase the new next- generation SONET or other packet-mode network elements. However, the cited financial metrics are unachievable in our opinion when the total cost of rendering service is appropriately taken into account, such as fiber riser costs, building-owner costs, building fiber spur costs, metro backbone fiber costs, building and CO equipment costs, CO rent/power costs, network man- agement costs, engineering and provisioning costs, other operations costs, and SGA costs. 4.4 Approaches In the past, carriers typically built overlay networks to accommodate dif- ferent characteristics of various media (voice, data, video, and the Internet) because there was no single technology or product that could meet the requirements for each service. New SONET systems are aimed at address- ing these requirements. In addition, they can be used in conjunction with embedded systems, as Figure 4-9 demonstrates. As Figure 4-10 indicates, there are a number of variants of next- generation SONET products. Traditional SONET provides a robust physi- cal-layer protection, but it employs a rigid multiplexing hierarchy that is not well suited to data-centric line rates (although it has definitions for VTs for DS1s and DS2s, as briefly discussed in Chapter 3). For example, a 10Base-T Ethernet interface (10 Mbps) delivered in a SONET network is traditionally mapped to an STS-1 (51 Mbps), effectively wasting 40 Mbps of Chapter 4 Figure 4-9 Use of next- generation SONET Access Ring, Campus A with an embedded SONET metro core OPTera Metro OPTera Metro 3300/3400/3500 3300/3400/3500 ring. with OPTera Packet Edge Module Interoffice ISP 2 Collector Ring High-Speed OPTera Metro IP Router or Access to ISP, 3300/