ake a direct connection through MANs and long-haul networks, with the associated protocols. And, of course, they want guaranteed QoS. IP over WDM Today, bandwidth reservation and intelligent IP switches can prioritize voice and video traffic to ensure that high-priority traffic gets the first shot at the underlying bandwidth. New generations of IP-based switches provide the capability to meet QoS commitments. Layer 3/Layer 4 switching services allow the switch to priori- tize and guarantee packets, based on predetermined criteria within a switch. Higher-level protocols (such as RSVP) can reserve bandwidth across an entire net- work. This creates a value proposition for the service provider: The ISP can deliver high bandwidth in a format that users want, for less cost, while approximating the QoS guarantees that the end user expects for high-priority traffic. As discussed in Chapter 4, the PSTN was not built to be dynamic. It was based on a system of 64Kbps channels, or DS-0s/CEPT-Os, aggregated by time division multiplexers into DS-1/CEPT-1 or DS-3/CEPT-3 facilities that would deliver traffic into cross-connects and switches at the network core. More time division multi- plexers were required at the other end to reverse the process and to distribute the DS-0s/CEPT-Os. Time division multiplexers are expensive, and they often require manual configuration, which slows provisioning and further increases costs. Whereas TDM is reaching its limits in terms of network elements and switch- ing technologies, DWDM is just beginning. But with any new technology, obsta- cles arise, and in the case of DWDM, the obstacles include management and performance impedance mismatches between networks. The International Tele- communication Union (ITU; www.itu.int) has formed a study group that will look into the interoperability standards to ensure that traffic can move between vendor networks despite the underlying differences in the many different ven- dors' equipment. End-to-End Optical Networking Emerging WDM Applications Emerging 