Telephony

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 features—such as call forwarding, 800 numbers, calling party identification, and mobility—which 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 stronger—in fact, the two industries may merge—if "voice-over-IP" becomes practical and universally used.

Richard A. Thompson

Bibliography

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

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