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.S.households subscribed towireless phone service, and a majority of them had two or moremobile phones.Over 15 percent of cellular subscribers use theservice for more than half of their long-distance calls, whileabout 10 percent use it for more than half their local calls.Developing countries that do not have an advanced com-munications infrastructure are increasingly turning to cellu-lar technology so that they can take part in the globaleconomy without having to go through the resource-intensivestep of installing copper wire or optical fiber.Explosivegrowth is occurring in India and China.Even among indus-trialized countries, there is continued high growth in cellularusage.In Japan, the number of cell phones now exceeds thenumber of analog fixed-line phones.Technology ComponentsCellular networks rely on relatively short-range transmit-ter/receiver (transceiver) base stations that serve small sec-tions (or cells) of a larger service area.Mobile telephoneusers communicate by acquiring a frequency or time slot inthe cell in which they are located.Amaster switching centercalled the  mobile transport serving office (MTSO) linkscalls between users in different cells and acts as a gatewayto the PSTN.Figure C-5 illustrates the link from the MTSOto the base stations in each cell.The MTSO also has links tolocal telephone central offices so that cellular users can com-municate with users of conventional phones.Cell Sites Cell boundaries are neither uniform nor constant.The usage density in the area, as well as the landscape, thepresence of major sources of interference (e.g., power lines,buildings), and the location of competing carrier cells, con-tributes to the definition of cell size.Cellular boundarieschange continuously, with no limit to the number of fre-quencies available for transmission of cellular calls in anarea.As the density of cellular usage increases, individualCELLULAR VOICE COMMUNICATIONS 51Mobile Transport Serving OfficeCentral OfficeBaseStationBaseStationBaseStationFigure C-5 Atypical cellular network configuration.cells are split to expand capacity.By dividing a service areainto small cells with limited-range transceivers, each cellu-lar system can reuse the same frequencies many times.Technologies such as Code Division Multiple Access (CDMA)and Expanded Time Division Multiple Access (E-TDMA)promise further capacity gains in the future.Master Switching Center In a typical cellular network, themaster-switching center operates similar to a telephone cen-tral office and provides links to other offices.The switchingcenter supports trunk lines to the base stations that estab-lish the cells in the service area.Each base station supportsa specific number of simultaneous calls from 3 to 15,depending on the underlying technology (i.e., CDMA,TDMA, or some derivative).Transmission Channels Most cellular systems provide twotypes of channels: a control channel and a traffic channel.52 CELLULAR VOICE COMMUNICATIONSThe base station and mobile station use the control channelto support incoming and outgoing calls, monitor signal qual-ity, and register when a user moves into a new zone.Thetraffic channel is used only when the station is off-hook andactually involved in a call.The control and traffic channels are divided into time slots.When the user initiates access to the control channel to placea call, the mobile station randomly selects a subslot in a gen-eral-use time slot to reach the system; the system thenassigns a time slot to the traffic channel.For an incoming callto a mobile station, the base station initiates conversations onthe control channel by addressing the mobile station in a timeslot, which at the same time reserves that time slot for the sta-tion s reply.If a user s call attempt collides with another user scall attempt, both instruments automatically reselect a sub-slot and try again.After repeated collisions, if no time slotsare available within a predetermined time, the system rejectsservice requests for incoming and outgoing calls.When a mobile telephone user places a call, the cell inwhich the user is traveling allocates a slot for the call [ Pobierz całość w formacie PDF ]