and OC-3 (155Mbps) in order to build an enterprise backbone. Service providers, like ISPs, often use the OC-12 (622Mbps) and OC-48 (2.5Gbps) levels, while the largest of carriers rely on OC-192 (10Gbps) backbones. In today's environment, given the high cost of such leased lines, the customers that use the SDH/SONET levels include airports, aerospace companies, universities that have medical campuses or significant art schools, entertainment companies, large government agencies such as the U.S. Internal Revenue Service, and the military. These organizations typically have a greater use of multimedia and visualization applications, requiring this type of bandwidth. To build a private SDH/SONET network, a customer would need two types of multiplexers: terminal muxes and add/drop muxes (ADMs). A customer would also need two types of cross-connects to build such a network: wideband cross- connects and broadband cross-connects. (Chapter 4 describes these muxes and cross-connects.) Dark Fiber With dark fiber, the customer leases the fiber itself and buys the necessary equip- ment to actually activate the fiber. The customer pays for the physical media, not for bandwidth, and as the customer adds equipment that can either pulse more bits per second or extract more wavelengths out of the underlying fiber, the bandwidth essentially becomes cheaper and cheaper. Networks Based on ISDN Another circuit-switched WAN option is ISDN. The International Telecommunica- tion Union Telecommunications Standardization sector (ITU-T) formalized the ISDN standard in 1983. According to the ITU-T (www.itu.org), ISDN is "a network evolved from the telephony integrated digital network that provides end-to-end connectivity to support a wide range of services, including voice and nonvoice ser- vices, to which users have access by a limited set or standard multipurpose cus- tomer interfaces." One of the ideas behind Narrowband ISDN (N-ISDN), as the first generation of ISDN was called, was to give customers one access into the network, from which