es to accountability, usage-based chargeback is being replaced by mul- tiple flat-rate service levels. From the standpoint of network ownership, today extensive wholesaling and reselling of other carriers' facilities replace the view that complete network ownership is required for success. Finally, the business model is also changing: New-era service providers don't want to be just transport businesses any longer. In the new service environment, there is a growing commonality between the network infrastructures of both service providers and enterprises. Fewer and fewer networks will be 100% facilities based. The service provider hosting sites are look- ing a great deal like enterprise data centers. As discussed in Chapter 11, service providers and large enterprises alike are taking advantage of many developments in the optical realm, including dark fiber, wavelength services, and WDM. There is a common emphasis on user service-level management, accounting, and rapid deployment. Finally, IP and Ethernet are becoming more pervasive in both worlds. The Next-Generation Network Infrastructure The vision of next-generation networks originated in the Internet community over 20 years ago. The development and growth of packet-switching networks provided the basis for the concept of next-generation networks. The key principles defining this vision included connectionless datagram transport, best-effort packet delivery, and the separation of service creation from IP transport. Today, virtually all service providers recognize the need to evolve their infra- structures to support multimedia and content delivery services. Telecommunica- tions service providers, both wireline carriers and mobile operators, face the challenge of providing seamless migration of the circuit-switched voice services onto an IP-based backbone while retaining all the important traditional features, particularly those required by law. The key PSTN capabilities that must be pre- served include public safety, law enforcement, fraud prevention, precedence