TELEPHONE AND TELEPHONE SYSTEM

instrument designed for simultaneous two-way voice communication and the technological system through which it is employed. It is a central part of modern telecommunication. The functional components of the modern telephone are described in the article telephone. In this article the development of the telephone instrument is traced, as is the development of what is known as the public switched telephone network (PSTN). In order to understand the many concepts represented in the PSTN, it is helpful to review the processes that take place in the making of a single telephone call. To make a call, a telephone subscriber begins by taking the telephone off-hookin the process, signaling the local central office that service is requested. The central office, which has been monitoring the telephone line continuously (a process known as attending), responds with a dial tone. Upon receiving the dial tone, the customer enters the called party's telephone number, using either a rotary dial or a push-button pad. The central office stores the entered number, translates the number into an equipment location and a path to that location, and tests whether the called party line is already in use (or busy). The called party number may lie in the same central office (in which case the call is designated intraoffice), or it may lie in another central office (requiring an interoffice call). If the call is intraoffice, the central office switch will handle the entire call process. If the call is interoffice, it will be directed either to a nearby central office or to a distant central office via a long-distance network. In the case of interoffice calls, a separate signaling network is employed to coordinate the call progression through a multitude of switches and telephone trunks. Assuming, however, that the call is an intraoffice call, if the called party's line is busy, the telephone switch will return a busy signal until the calling party returns to the on-hook condition. If the called party's line is not busy, it will be alerted, or rung. At the same time that the line is rung, an audible signal will be returned to the calling party to indicate that ringing is taking place. If the called party answers by going off-hook, ringing will be discontinued and a voice path will be established through the switching system to both the calling and called parties. The voice path is maintained until either party goes back on-hook. At that moment the voice path is disconnected, and call charging is recorded. From the example described above, it is evident that telephone systems consist of four major components: (1) transmission, between the central switching office and subscribers' telephone sets and also between central offices, (2) switching, between telephone sets and between trunks, as required, (3) signaling, between the telephone sets and the central offices as well as between central offices when needed, and (4) the telephone instruments (or station apparatuses) employed in the call. Each of these major components of a telephone system is discussed in turn in this section. Following these descriptions of wire telephony are discussions of radiotelephones, videotelephones, facsimile transmission, and modems. Additional reading M.D. Fagen et al. (eds.), A History of Engineering and Science in the Bell System (1975 ), looks at the developments of telephone technology during the period 18751925. James E. Brittain (ed.), Turning Points in American Electrical History (1977), collects some of the original papers on telephone systems by inventors including Alexander Graham Bell. A. Michael Noll, Introduction to Telephones & Telephone Systems, 2nd ed. (1991), provides an overview of telephone systems and of recent developments.Bruce R. Briley, Introduction to Telephone Switching (1983), is a readable engineering text. Robert J. Chapuis, 100 Years of Telephone Switching (18781978), 2 vol. (198290), despite the title, chronicles worldwide telephone switching developments during the period 18781985. John C. McDonald (ed.), Fundamentals of Digital Switching, 2nd ed. (1990), presents a modern view of electronic switching systems, including ISDN and software-related issues.Bell Telephone Laboratories, Inc., Transmission Systems for Communications, 5th ed. (1982), is the standard engineering reference on the design of telephone transmission systems, including wire, cable, and microwave systems.William C. Jakes, Jr. (ed.), Microwave Mobile Communications (1974, reissued 1994), is the classic engineering text on mobile communications systems. D.M. Balston and R.C.V. Macario (eds.), Cellular Radio Systems (1993), provides comprehensive overviews of the existing cellular radio systems throughout the world.Arun N. Netravali and Barry G. Haskell, Digital Pictures: Representation, Compression, and Standards, 2nd ed. (1995), describes video and still-image compression, videotex, and teletext. Ralph E. Taggart, Video Formats and Display Systems, chapter 4 in his Weather Satellite Handbook, 5th ed. (1994), pp. 110, is an amateur radio overview of weather satellite systems, including descriptions of equipment needs.Dennis Bodson, Kenneth R. McConnell, and Richard Schaphorst, Fax: Digital Facsimile Technology and Applications, 2nd ed. (1992), is an engineering reference on fax systems and includes fax markets, history, and recommendations for purchasing. William R. Bennett and James R. Davey, Data Transmission (1965), looks at early modem technology. Richard D. Gitlin, Jeremiah F. Hayes, and Stephen B. Weinstein, Data Communications Principles (1992), addresses many aspects of today's telephone systems with a special emphasis on modem technology. David E. Borth Radiotelephone In addition to the wireline telephones described in Development of the telephone instrument, there exist a number of wireless instruments that are connected to the public switched telephone network (PSTN). At the present time, these wireless telephones generally fall into one of three categories: cordless telephones, cellular radio systems, or personal communication systems. Eventually these systems will be expanded to include global satellite-based telephony. Cordless telephones Cordless telephones are devices that take the place of a telephone instrument within a home or office and permit very limited mobility (up to a hundred metres). Because they are plugged directly into an existing telephone jack, they essentially serve as a wireless extension to the existing home or office wiring. Cordless transceivers communicate with the plugged-in base unit over a pair of frequencies in the 46- and 48-megahertz bands or over a single frequency in the 902928-megahertz band.