Telephone

A

Switching

Telephone switching equipment interprets the number dialed and then completes a path through the network to the called subscriber. For long-distance calls with complicated paths through the network, several levels of switching equipment may be needed. The automatic exchange to which the subscriber’s telephone is connected is the lowest level of switching equipment and is called by various names, including local exchange, local office, central-office switch, or, simply, switch. Higher levels of switching equipment include tandem and toll switches, and are not needed when both caller and called subscribers are within the same local exchange.

Before automatic exchanges were invented, all calls were placed through manual exchanges in which a small light on a switchboard alerted an operator that a subscriber wanted service. The operator inserted an insulated electrical cable into a jack corresponding to the subscriber requesting service. This allowed the operator and the subscriber to converse. The caller told the operator the called party’s name, and the operator used another cord adjacent to the first to plug into the called party’s jack and then operated a key (another type of electrical switch) that connected ringing current to the called party’s telephone. The operator listened for the called party to answer, and then disconnected to ensure the privacy of the call.

Today there are no telephones served by manual exchanges in the United States. All telephone subscribers are served by automatic exchanges, which perform the functions of the human operator. The number being dialed is stored and then passed to the exchange’s central computer, which in turn operates the switch to complete the call or routes it to a higher-level switch for further processing.

Today’s automatic exchanges use a pair of computers, one running the program that provides service, and the second monitoring the operation of the first, ready to take over in a few seconds in the event of an equipment failure.

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Early telephone exchanges, a grouping of 10,000 individual subscriber numbers, were originally given names corresponding to their town or location within a city, such as Murray Hill or Market. When the dialing area grew to cover more than one exchange, there was a need for the dial to transmit letters as well as numbers. This problem was solved by equating three letters to each digit on the dial except for the one and the zero. Each number from two to nine represented three letters, so there was room for only 24 letters. Q and Z were left off the dial because these letters rarely appear in place-names. In dialing, the first two letters of each exchange name were used ahead of the rest of the subscriber’s number, and all exchange names were standardized as two letters and a digit. Eventually the place-names were replaced with their equivalent digits, giving us our current U.S. and Canadian seven- or ten-digit telephone numbers. In other parts of the world, a number may consist of more or less than seven digits.

The greatly expanded information-processing capability of modern computers permits Direct Distance Dialing, with which a subscriber can automatically place a call to a distant city without needing the services of a human operator to determine the appropriate routing path through the network. Computers in the switching machines used for long-distance calls store the routing information in their electronic memory. A toll-switching machine may store several different possible routes for a call. As telephone traffic becomes heavier during the day, some routes may become unavailable. The toll switch will then select a less direct alternate route to permit the completion of the call.

B

Transmission

Calling from New York City to Hong Kong involves using a path that transmits electrical energy halfway around the world. During the conversation, it is the task of the transmission system to deliver that energy so that the speech or data is transmitted clearly and free from noise. Since the telephone in New York City does not know whether it is connected to a telephone next door or to one in Hong Kong, the amount of energy put on the line is not different in either case. However, it requires much more energy to converse with Hong Kong than with next door because energy is lost in the transmission. The transmission path must provide amplification of the signal as well as transport.

Analog transmission, in which speech or data is converted directly into a varying electrical current, is suitable for local calls. But once the call involves any significant distance, the necessary amplification of the analog signal can add so much noise that the received signal becomes unintelligible. For long-distance calls, the signal is digitized, or converted to a series of pulses that encodes the information.

When an analog electrical signal is digitized, samples of the signal’s strength are taken at regular intervals, usually about 8,000 samples per second. Each sample is converted into a binary form, a number made up of a series of 1s and 0s. This number is easily and swiftly passed through the switching system. Digital transmission systems are much less subject to interfering noise than are analog systems. The digitized signal can then be passed through a digital-to-analog converter (DAC) at a point close to the receiving party, and converted to a form that the ear cannot distinguish from the original signal.

There are several ways a digital or analog signal may be transmitted, including coaxial and fiber-optic cables and microwave and longwave radio signals sent along the ground or bounced off satellites in orbit around the earth. A coaxial wire, like the wire between a videocassette recorder, or VCR (see Video Recording), and a television set, is an efficient transmission system. A coaxial wire has a conducting tube surrounding another conductor. A coaxial cable contains several coaxial wires in a common outer covering. The important benefit of a coaxial cable over a cable composed of simple wires is that the coaxial cable is more efficient at carrying very high frequency currents. This is important because in providing transmission over long distances, many telephone conversations are combined using frequency-modulation (FM) techniques similar to the combining of many channels in the television system. The combined signal containing hundreds of individual telephone conversations is sent over one pair of wires in a coaxial cable, so the signal has to be very clear.

Coaxial cable is expensive to install and maintain, especially when it is lying on the ocean floor. Two methods exist for controlling this expense. The first consists of increasing the capacity of the cable and so spreading the expense over more users. The installation of the first transatlantic submarine coaxial telephone cable in 1956 provided only about 30 channels, but the number of submarine cable channels across the ocean has grown to thousands with the addition of only a few more cables because of the greatly expanded capacity of each new coaxial cable.

Another telephone-transmission method uses fiber-optic cable, which is made of bundles of optical fibers (see Fiber Optics), long strands of specially made glass encased in a protective coating. Optical fibers transmit energy in the form of light pulses. The technology is similar to that of the coaxial cable, except that the optical fibers can handle tens of thousands of conversations simultaneously.

Another approach to long-distance transmission is the use of radio. Before coaxial cables were invented, very powerful longwave (low frequency) radio stations were used for intercontinental calls. Only a few calls could be in progress at one time, however, and such calls were very expensive. Microwave radio uses very high frequency radio waves and has the ability to handle a large number of simultaneous conversations over the same microwave link. Because cable does not have to be installed between microwave towers, this system is usually cheaper than coaxial cable. On land, the coaxial-cable systems are often supplemented with microwave-radio systems.

The technology of microwave radio is carried one step further by the use of communications satellites. Most communications satellites are in geosynchronous orbit—that is, they orbit the earth once a day over the equator, so the satellite is always above the same place on the earth’s surface. That way, only a single satellite is needed for continuous service between two points on the surface, provided both points can be seen from the satellite. Even considering the expense of a satellite, this method is cheaper to install and maintain per channel than using coaxial cables on the ocean floor. Consequently, satellite links are used regularly in long-distance calling. Since radio waves, while very fast, take time to travel from one point to another, satellite communication does have one serious shortcoming: Because of the satellite’s distance from the earth, there is a noticeable lag in conversational responses. As a result, many calls use a satellite for only one direction of transmission, such as from the caller to the receiver, and use a ground microwave or coaxial link for receiver-to-caller transmission.

A combination of microwave, coaxial-cable, optical-fiber, and satellite paths now link the major cities of the world. The capacity of each type of system depends on its age and the territory covered, but capacities generally fall into the following ranges: Frequency modulation over a simple pair of wires like the earliest telephone lines yields tens of circuits (a circuit can transmit one telephone conversation) per pair; coaxial cable yields hundreds of circuits per pair of conductors, and thousands per cable; microwave and satellite transmissions yield thousands of circuits per link; and optical fiber has the potential for tens of thousands of circuits per fiber.

IV

Telephone Services

In the United States and Canada, universal service was a stated goal of the telephone industry during the first half of the 20th century—every household was to have its own telephone. This goal has now been essentially reached, but before it became a reality, the only access many people had to the telephone was through pay (or public) telephones, usually placed in a neighborhood store. A pay telephone is a telephone that may have special hardware to count and safeguard coins or, more recently, to read the information off credit cards or calling cards. Additional equipment at the exchange responds to signals from the pay phone to indicate to the operator or automatic exchange how much money has been deposited or to which account the call will be charged. Today the pay phone still exists, but it usually serves as a convenience rather than as primary access to the telephone network. The explosive growth of cell phones has greatly reduced the use of pay phones. The FCC estimates that more than 1 million pay phones have been removed from service since the mid-1990s.

Computer-controlled exchange switches make it possible to offer a variety of extra services to both the residential and the business customer. Some services to which users may subscribe at extra cost are call waiting, in which a second incoming call, instead of receiving a busy signal, hears normal ringing while the subscriber hears a beep superimposed on the conversation in progress; and three-way calling, in which a second outgoing call may be placed while one is already in progress so that three subscribers can then talk to each other. Some services available to users within exchanges with the most-modern transmission systems are: caller ID, in which the calling party’s number is displayed to the receiver (with the calling party’s permission—subscribers can elect to make their telephone number hidden from caller-ID services) on special equipment before the call is answered; and repeat dialing, in which a called number, if busy, will be automatically redialed for a certain amount of time.

For residential service, voice mail can either be purchased from the telephone company or can be obtained by purchasing an answering machine. An answering machine usually contains a regular telephone set along with the ability to detect incoming calls and to record and play back messages, with either an audiotape or a digital system. After a preset number of rings, the answering machine plays a prerecorded message inviting the caller to leave a message to be recorded.

Toll-free 800 numbers are a very popular service. Calls made to a telephone number that has an 800 area code are billed to the called party rather than to the caller. This is very useful to any business that uses mail-order sales, because it encourages potential customers to call to place orders. A less expensive form of 800-number service is now available for residential subscribers.

In calling telephone numbers with area codes of 900, the caller is billed an extra charge, often on a per-minute basis. The use of these numbers has ranged from collecting contributions for charitable organizations, to businesses that provide information for which the caller must pay.

While the United States and Canada are the most advanced countries in the world in telephone-service technologies, most other industrialized nations are not far behind. An organization based in Geneva, Switzerland, called the International Telecommunication Union (ITU), works to standardize telephone service throughout the world. Without its coordinating activities, International Direct Distance Dialing (a service that provides the ability to place international calls without the assistance of an operator) would have been extremely difficult to implement. Among its other services, the ITU creates an environment in which a special service introduced in one country can be quickly duplicated elsewhere.

V

The History of the Telephone

The history of the invention of the telephone is a stormy one. A number of inventors contributed to carrying a voice signal over wires. In 1854 the French inventor Charles Bourseul suggested that vibrations caused by speaking into a flexible disc or diaphragm might be used to connect and disconnect an electric circuit, thereby producing similar vibrations in a diaphragm at another location, where the original sound would be reproduced. A few years later, the German physicist Johann Philip Reis invented an instrument that transmitted musical tones, but it could not reproduce speech. An acoustic communication device that could transmit speech was developed around 1860 by an Italian American inventor, Antonio Meucci. The first to achieve commercial success and inaugurate widespread use of the telephone, however, was a Scottish-born American inventor, Alexander Graham Bell, a speech teacher in Boston, Massachusetts.

Bell had built an experimental telegraph, which began to function strangely one day because a part had come loose. The accident gave Bell insight into how voices could be reproduced at a distance, and he constructed a transmitter and a receiver, for which he received a patent on March 7, 1876. On March 10, 1876, as he and his assistant, Thomas A. Watson, were preparing to test the mechanism, Bell spilled some acid on himself. In another room, Watson, next to the receiver, heard clearly the first telephone message: “Mr. Watson, come here; I want you.”

A few hours after Bell had patented his invention, another American inventor, Elisha Gray, filed a document called a caveat with the U.S. Patent Office, announcing that he was well on his way to inventing a telephone. Other inventors, including Meucci and Amos E. Dolbear, also made claim to having invented the telephone. Lawsuits were filed by various individuals, and Bell’s claim to being the inventor of the first telephone had to be defended in court some 600 times. Gray’s case was decided in Bell’s favor. Meucci’s case was never resolved because Meucci died before it reached the Supreme Court of the United States.

A

Advances in Technology

After the invention of the telephone instrument itself, the second greatest technological advance in the industry may have been the invention of automatic switching. The first automatic exchanges were called Strowger switches, after Almon Brown Strowger, an undertaker in Kansas City, Missouri, who invented the system because he thought his town’s human operators were steering prospective business to his competitors. Strowger received a patent for the switches in 1891.

Long-distance telephony was established in small steps. The first step was the introduction of the long-distance telephone, originally a special highly efficient instrument permanently installed in a telephone company building and used for calling between cities. The invention at the end of the 19th century of the loading coil (a coil of copper wire wound on an iron core and connected to the cable every mile or so) increased the speaking range to approximately 1,000 miles. Until the 1910s the long-distance service used repeaters, electromechanical devices spaced along the route of the call that amplified and repeated conversations into another long-distance instrument. The obvious shortcomings of this arrangement were overcome with the invention of the triode vacuum tube, which amplified electrical signals. In 1915 vacuum-tube repeaters were used to initiate service from New York City to San Francisco, California.

The vacuum tube also made possible the development of longwave radio circuits that could span oceans. Sound quality on early radio circuits was poor, and transmission subject to unpredictable interruption. In the 1950s the technology of the coaxial-cable system was combined with high-reliability vacuum-tube circuits in an undersea cable linking North America and Europe, greatly improving transmission quality. Unlike the first transatlantic telegraph cable placed in service in 1857, which failed after two months, the first telephone cable (laid in 1956) served many years before becoming obsolete. The application of digital techniques to transmission, along with undersea cable and satellites, finally made it possible to link points halfway around the earth with a circuit that had speech quality almost as good as that between next-door neighbors.

Improved automatic-switching systems followed the gradual improvement in transmission technology. Until Direct Distance Dialing became available, all long-distance calls still required the assistance of an operator to complete. By adding a three-digit area code in front of the subscriber’s old number and developing more sophisticated common-control-switching machines, it became possible for subscribers to complete their own long-distance calls. Today customer-controlled international dialing is available between many countries.

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