vice because of congestion reaches a critical level, they can split the original cell into two cells, by installing a second site within the same geographical area that uses a different set of nonadjacent frequencies. This has an extended impact; as the cells become smaller, the total num- ber of calls that the aggregate can support climbs dramatically. Because of cellular geometry, if the radius of the cell is halved, the number of sup- ported calls is quadrupled. The smaller the cell, therefore, the larger the total number of simultaneous callers that can be accommodated. Of course, this also causes the cost of deployment to climb dramatically, and while the architectural goal of most cellular providers is to create a mosaic of thousands of very small cells-called microcells or picocells- that's an expensive proposition and will not happen immediately. Finally, cellular systems rely on a technique called cell-to-cell handoff, which simply means that the cellular network has the ability to track a call (using relative signal strength as an indicator) as the user moves across a cell. When the signal strength detected by the current cell is per- ceived by the system to be weaker than that detected by the cell the user is approaching, the call is handed off to the second cell. Ideally, the user hears nothing. Cell handoff and other cellular system capabilities are under the central control of a mobile telephone switching office, or MTSO (sometimes pronounced "Mitso"). 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 Access Technologies OK, But How Does It Work? When a user turns a cellular phone on, several things happen. First, the phone identifies itself to anyone willing to listen (hopefully, a local cell) by transmitting a unique identification code on a frequency that is designated as a control channel. The control chan