Multiplexing provider's backbone, thus intermixing everyone's presumed-to-be-private data into one seething, heaving mass. Because of the problems with leased lines, the astute network manager at Bob's thinks about ways to make the network less expensive. One solution, shown in Figure 1.13, is to use multiplexers. Multiplexers always come in pairs, SO if you have one at one end, you must have one at the other end. They are also symmetri- cal, so if there are five outputs available in San Francisco, there must also be five inputs in the New York location. The key savings in this scenario comes from using only one leased line between New York and California. In San Francisco, short leased lines, referred to as tail circuits, run from the centrally placed multiplexer to each of the individual locations. Thus, five locations are sharing one high-cost leased line, rather than each having its own leased line. Intelligence embedded in the multiplexers allows the network manager to manage access to that bandwidth and to allocate network services to the endpoints. Various techniques-including Frequency Division Multiplexing (FDM), Time Division Multiplexing (TDM), Statistical Time Division Multiplexing (STDM), Code Division Multiple Access (CDMA), intelligent multiplexing, inverse multi- plexing, Wavelength Division Multiplexing (WDM), Dense Wavelength Division Multiplexing (DWDM), and Coarse Wavelength Division Multiplexing (CWDM)- enable multiple channels to coexist on one link. The following sections examine Location A Front-end Location B Processor Leased Line Tail Circuits Figure 1.13 A network with multiplexers Chapter 1 Telecommunications Technology Fundamentals these techniques (except CDMA, which is covered in Chapter 13, "Wireless Com- munications Basics"). With FDM, the entire frequency band available on the communications link is divided into smaller individual bands or channels (see Figure 1.14). Each user is assigned to a different frequency. The signals all travel in parallel over the same communications li