for protection as in traditional SONET rings that transport voice and other circuit-based services. This yields an effective doubling of the bandwidth available for working traffic relative to circuit-based service transport. Mixed packet data and TDM circuit-based services may be placed on the same ring as desired. In these applications, circuit-based services are protected at Layer 1 using standard SONET unidirectional path-switched rings (UPSR) or bidirectional line-switched rings (BLSR) protection switching. Thus, both Layer 1 and Layer 2 protection switching can coexist on the same ring as needed to concurrently support a broad mix of service types. A more inclusive treatment is available in the book Ethernet-Based Metro Area Networks by Minoli and Johnson. 13 Figure 4-13 plots savings as claimed by the vendor. 14 Naturally, any savings need to be passed onto the customer if he or she is to become interested-a 20 to 25 percent decrease in price (based on a 20 to 25 percent decrease in cost shown in the figure) is just about the savings Next-Generation SONET and Optical Architectures Figure 4-13 RPR savings claimed by vendors. Hubbed Network Meshed Network ATM over SONET 25% + 40% + Optical Typical Savings 40 to 60% Savings Ethernet RPR 25 to 60% Optical Ethernet RPR over PoS Demand Size per Link (Mbps) Demand Size per Link (Mbps) Packet over SONET (PoS), where routers interface to the transport system via TDM interfaces (DS1 to OC-12 and beyond) ATM over SONET, where ATM switches aggregate traffic from routers and interfaces with the transport system via TDM interfaces (DS1 to OC-12 and beyond) Optical Ethernet with RPR, where routers interface to the transport system via Ethernet, fast Ethernet, or Gigabit Ethernet interfaces on RPR NEs level that could motivate an enterprise customer to consider switching his or her service from an existing provider/service to another. It's unclear under what circumstances, if any, an existing provider would lower its own price and cannibalize an existing PoS-delivered cir