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Telephony is the electronic point-to-point communication of audio signals. At the new millennium, the world has 750 million telephone lines, half of which are in the United States. Half the people in the world have never talked on a telephone! Every telephone "homes on" a switching office, either by copper wire or wireless link. Transmission lines, typically fiber or microwave links carrying thousands of simultaneous conversations, connect these switching offices to each other. Starting with Alexander Graham Bell's invention of the telephone in 1876, telephony has become a trillion-dollar-a-year worldwide industry. This industry consists of: (1) the carriers (in some countries, a government branch) that own the networks and provide telephone service and (2) the manufacturers of the switching and transmission equipment that the carriers use. Telephony and computing have had a synergistic relationship for about forty years as of 2002.

Computing Supports Telephony

In the United States, Bell's company grew into the Bell System, a telephone monopoly that was broken up in 1984. When the Bell System introduced area codes and automated long-distance calling in the 1950s, the cost of modifying the nation's 40,000 switching offices motivated a change from the wired control used in electromechanical technology to program control. The world's first computer-controlled switching office, now called the "1E," was developed by the Bell System and was installed in 1965. The 1E processor had a 1-MHz clock and no operating system. Now, almost every switching office in the United States, and many of the offices in the world, are program-controlled by huge programs that run on modern processors.

Complicated switching software recognizes off-hooks and on-hooks, processes calls, controls special features and services, implements the numbering plan, routes calls through the carriers' networks, maintains the switching office, manages the network, and communicates with software in other switching offices. Program control not only manages telephony's complexity, but it also enables new featuressuch as call forwarding, 800 numbers, calling party identification, and mobilitywhich makes the programs even more complicated. Switching software has become so complicated, and changes so frequently, that only large companies, such as global giants Alcatel, Lucent, or Siemens, can afford the thousands of programmers required to maintain and update the programs.

Computer-controlled switching equipment was common by the 1980s when the world's carriers started using Signaling System 7, a global data network by which switch processors send packets to each other for call control and routing. Then, in a 1990s evolution called the "Intelligent Network," the SS7 network added special processors and databases for special service control, translation, and network management. The operation of switching software, the SS7 network, and these IN processors is illustrated by the following three examples.

Long-distance Busy Tone.

Suppose Dave in Denver calls Alice in Atlanta. Dave dials Alice's number into the USWest switching office on which his telephone homes. This office in Denver sends a packet to the BellSouth switching office on which Alice's telephone homes and requests Alice's busy/idle status. If Alice's telephone is idle, the Denver office determines Dave's default long-distance carrier (MCI, for example), routes the call to MCI's gateway office in Denver, and sends a packet containing the calling and called telephone numbers to this gateway. Then, MCI routes the call through its national network to the BellSouth office in Atlanta, which rings Alice's telephone. If Alice's telephone had been busy, rather than route the call to Atlanta, the USWest office in Denver would connect Dave to a local busy tone.

800-call. Suppose Dave dials 1-800-555-4567 into the USWest switching office in Denver on which his telephone homes. This office sends a packet to a special database on the SS7 network, which returns a packet identifying AT&T, for example, as the carrier that leases this 800 number. Then, the USWest office routes Dave's call to AT&T's gateway office in Denver and sends a packet containing the calling and called telephone numbers to this gateway. This office sends a packet to AT&T's 800-number translation database, which returns the actual telephone number that AT&T assigned to this 800 number. The call is then routed through AT&T's national network to this real telephone.

Cell Phone Roaming.

Suppose Dave is not home in Denver today; he is in Atlanta and he brought his cell phone. Every cellular carrier's switching office maintains a pair of databases, the home location register (HLR) and the visitor location register (VLR). Dave's cell phone is registered in his cellular carrier's HLR in Denver. When Dave turns on his cell phone in Atlanta, the telephone communicates with a cellular switching office in Atlanta. This office records Dave's phone number (and home office) in its VLR and sends a packet to Dave's home office in Denver, which records the Atlanta office's ID in Dave's HLR entry. If anyone tries to call Dave's cell phone, the Denver HLR enables the call to be routed to Atlanta. If Dave tries to place a call, the VLR in Atlanta sends billing information to Denver.

It should be apparent that these services, and hundreds of others, would be practically impossible without computers, software, and the SS7 network.

Telephony Supports Computing

The companies that manufacture switching equipment have a long history of innovation in computing. Most notable was Bell Labs, when it was the R&D (research and development) branch of the former Bell System. Bell Labs' scientists built a prototype computer in the late 1930s using electromechanical technology, and they developed the UNIX operating system and the C programming language in the 1970s.

More recently, the Internet would be practically impossible without telephone industry resources. Besides using their networks of transmission lines for telephone calls, the carriers also lease channels, typically for use as enterprise private networks for voice and data. Such channels, leased from the various telephone carriers, interconnect the Internet's IP routers, providing its global backbone network. In addition, most residential personal computers get access to Internet service providers by using a modem (the word is an abbreviation of modulator-demodulator ). The PC's modem makes a telephone call to a modem at the ISP. The modems convert computer data (in the PC and ISP) to audio signals, which transmit like voice over the telephone channel.

The relationship between telephony and computing may get even strongerin fact, the two industries may mergeif "voice-over-IP" becomes practical and universally used.

see also Bell, Alexander Graham; Bell Labs; Internet; Satellite Technology; Telecommunications; Wireless Technology.

Richard A. Thompson


McDonald, J. C. Fundamentals of Digital Switching, 2nd ed. New York: Plenum, 1990.

Thompson, Richard A. Telephone Switching Systems. Boston: Artech House, 2000.

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Telephony is the technology that allows telephones to work. When a telephone call is placed, sounds are converted into electrical signals, transmitted over telephone lines, and converted back into sounds. Early telephony was analog; a single pair of copper wires carried signals over short distances. As the number of telephone calls increased, telephone companies attempted to overcome the limitations of early analog telephony by devising a process known as frequency division multiplexing; this allowed a single pair of copper wires to handle more than one call by routing calls through different bits of the frequency spectrum in much the same way radio stations are assigned a specific frequency. Eventually, analog telephony was replaced by digital telephony, which uses a binary format. By digitizing speech into a binary code, in a manner similar to the digitization of music on CDs, telephone companies were able to reduce noise, increase capacity, and reduce costs.

Internet telephony, the use of Internet technology to make and receive phone calls, emerged in the mid 1990s. In February of 1995, Vocaltec Communications Ltd., which was founded six years prior, created its Internet Phone software application. Internet Phone allowed Internet surfers around the world, most of whom used a local phone connection to access the Internet, the ability to speak with other Internet Phone users at a much cheaper rate than offered by traditional long-distance providers. Soon thereafter, the Information Systems Group of Motorola Inc. began including a copy of Internet Phone with its 28.8 desktop modems. Eventually, personal computer (PC) owners were able to gain access to Internet telephony technology simply by purchasing a telephony board, which integrated modem, sound board, speaker phone, and voice-mail hardware and software. Many Internet telephony packages also included microphones and headphones.

The leading Internet telephony carrier, Net2Phone, Inc. was founded in 1996. The firm released the world's first PC-to-phone application that November. Within four months, more than 2 million Net2Phone calls had been completed. However, by most accounts, the industry was still in its infancy; in fact, Internet telephony accounted for less than half a percent of all telephone usage in 1997. By the year's end, Net2Phone had unveiled its phone-to-phone over IP (Internet Protocol) technology; it launched its Net2Fax service, a fax-to-fax over IP service, and Click2Talk, an e-commerce customer service solution that allowed online shopper to speak to customer service representatives, early in 1998. That year, Frost & Sullivan named Net2Phone the Internet Telephony Services leader. IDC dubbed the firm the IP Telephony Services leader in 1999.

Panasonic announced its intent to incorporate Net2Phone technology into its new cordless phones in 2000. As part of a similar alliance, software behemoth Microsoft Corp. added the technology to its MSN Messenger. In March of that year, Internet portal Yahoo! purchased a 5 percent stake of Net2Phone for $150 million. Telecommunications giant AT&T Corp. paid $1.4 billion for a 32 percent stake in Net2Phone in August. Minutes per day of use reached 8 million in December, compared to 1 million in July. Although sales grew to $72.4 million, the firm posted a loss of $118 million. Customers totaled 1 million.

Despite the many advances made in Internet telephonyparticularly in transmission quality and reliabilityin the late 1990s and early 2000s, voice over IP (VOIP) calls accounted for less than one percent of all voice traffic in 2001. According to a February 2001 issue of Business Communications Review, this is due, at least in part, to reluctance on the part of traditional telecommunications firms to enter the fledgling market. "While start-ups have forged into the market, the established carriers have moved much more slowly." MCI WorldCom's decision to begin testing VOIP services in mid-2000 may well prove to be a turning point if other telecommunications giants follow its lead. "If the major carriers were to join the fray, the analysts believe customers will accept the concept."


"Analogue Telephony." London: Wilco Telephony Ltd., 2001. Available from

"Equipment and Services: A Fast-Growing Market for a Fast-Moving Industry." IEEE Computing Online. 2001. Available from

Korzeniowski, Paul. "VOIPStill Only a Drop in the Bucket." Business Communications Review. February 2001.

Net2Phone, Inc. "Net2Phone Timeline." Newark, NJ: Net2Phone, Inc., 2001. Available from

"Telephony." Webopedia, Darien, CT:, 2001. Available from

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te·leph·o·ny / təˈlefənē; ˈteləˌfōnē/ • n. the working or use of telephones.

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