lion, with annual revenues of nearly $11 billion. It's interesting to note that in 1982, AT&T proudly predicted that there would be over 100,000 cellular subscribers by 1990. Cellular Telephony As modern as cellular telephony is considered to be, it was originally con- ceived in the 1940s at Bell Labs, as part of a plan to overcome the con- gestion and poor service issues associated with mobile telephone systems. There are four key design principles of cellular systems (see Figure 6-10), which are the same today as they were in the 1940s. They are the use of many of low-power, small coverage-area transmitters instead of a single, powerful, monolithic transmitter to cover a wide area; frequency reuse; the concept of cell splitting; and central control and cell-to-cell handoff of calls. These concepts are fairly straight forward. The first relies on the philosophy that the whole is greater than the sum of its parts. By using a large number of low-power transmitters scattered across a broad coverage zone, each capable of handling multiple simul- taneous calls, more users can be supported than with a single, monolithic transmitter. The second, frequency reuse, takes into account the fact that cellular telephony, like all radio-based services, has been allocated a limited num- ber of frequencies by the FCC. By using geographically small, low-power cells, frequencies can be reused by nonadjacent cells. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Access Technologies Chapter 6 Figure 6-10 An array of low-power cells in a cellular network: Notice how the coverage areas overlap, allowing handoff of a call to occur from one cell to another. Also, because of low power and distance, frequencies can be reused. When usage areas become saturated by excessive usage, cells can be split. When traffic engineers observe that the number of callers refused ser