Mạng và viễn thông P24

Mobile and Radio Data NetworksJust as computers and datacommunication are revolutionizing office life, so mobile and radio data networks are enabling corporate computer networks be extendedto every part of the comto pany’s business, including the mobile sales force, the haulage fleet and the travelling executive. Data network techniques can now also be usedto trace and pinpoint trucks on the roador ships at sea.
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Mạng và viễn thông P24 Networks and Telecommunications: Design and Operation, Second Edition. Martin P. Clark Copyright © 1991, 1997 John Wiley & Sons Ltd ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic) 24 Mobile and Radio Data Networks Just as computers and datacommunication are revolutionizing office life, so mobile and radio data networks are enabling corporate computer networks be extendedto every part of the com- to pany’s business, including the mobile sales force, the haulage fleet and the travelling executive. Data network techniques can now also be usedto trace and pinpoint trucks on the roador ships at sea. This chapter discusses someof the most recent radio data network technologies, covering the principles of ‘radiopaging’, mobile data networking, ‘wireless LANS’, as well as describing radiodetermination services.24.1 RADIOPAGING Radiopaging was the first major type of network that enables transfer of short data messages to mobile recipients. Initially it was a method of alerting an individual in a remote or unknown location (typically by ‘bleeping’him) to the fact that someone wishes to converse with him by phone. Subsequently, the possibility to send a short text message to the mobile recipient became commonplace. To be paged an individual needs to carry a special radioreceiver, called a radiopager. The unit is about thesize of a cigarette box, and designed to be worn on abelt or clip- is ped inside a pocket. The person carrying the pager may roam freely and can be paged provided they are within the radiopaging service area. The service may provide a full nationwide coverage. Figure 24.1 illustrates a typical radiopaging receiver. The initial radiopagers were allocated a normal telephone number as if they were standard telephones.Pagingwasachieved by diallingthisnumber, as if makinga normal telephone call. Instead of being connected through to the radiopager the caller either speaks to a radiopaging service operator, or hears a recorded message confirming that the radiopager has been paged. Paging is done by sending a radio signal to the radiopager, causing it to emit an audible ‘bleeping signal’ to alert its wearer. The simplest types of radiopager, even today, provide no further information to the wearer than thebleep. Having been alerted, the wearer must find a nearby telephone and 425426 NETWORKS DATA MOBILE AND RADIO Figure 24.1 Messagedisplayradiopager. A relativelysophisticatedradiopager,allowingnot only bleeping facilities, but also the conveyance of a short textual message. (Courtesy of British Telecom)RADIOPAGING 421 ring a pre-arranged telephone number (say the radiopaging operator, or the wearer’s own secretary) to be given the message or the telephone numberof the caller whowishes to speak with him. Thus for a caller to page an individual they must first inform the intermediary office (the radiopaging operator or the ‘roaming individual’s’ secretary). The caller leaves either a message for the paged person, or a telephone number to be called. Figure 24.2 gives a general schematic view of a radiopaging network. The key elements of a radiopaging system are the paging access control equipment ( P A C E ) , thepaging transmitter andthe paging receiver. ThePACE,containsthe electronics necessary for the overall controlof the radiopaging network. It is the PACE which codes up the necessary signal to alert only the appropriate receiver. This signal is distributed to alltheradiotransmittersservingthewhole of thegeographicarea covered by theradio-pagingservice.On receiving itsindividualalertingsignalthe receiver bleeps. A special code is used between the PACE andall the paging receivers.It enables each receiver to be distinguished and alerted. The earliest codes used a discret ...