Making A Telephone Call
A telephone call starts when the caller lifts a handset off the base. This closes an electrical switch that initiates the flow of a steady electric current over the line between the user’s location and the exchange. The exchange detects the current and returns a dial tone, a precise combination of two notes that lets a caller know the line is ready.
Once the dial tone is heard, the caller uses a rotary or push-button dial mounted either on the handset or base to enter a sequence of digits, the telephone number of the called party. The switching equipment in the exchange removes the dial tone from the line after the first digit is received and, after receiving the last digit, determines whether the called party is in the same exchange or a different exchange. If the called party is in the same exchange, bursts of ringing current are applied to the called party’s line. Each telephone contains a ringer that responds to a specific electric frequency. When the called party answers the telephone by picking up the handset, steady current starts to flow in the called party’s line and is detected by the exchange. The exchange then stops applying ringing and sets up a connection between the caller and the called party.
If the called party is in a different exchange from the caller, the caller’s exchange sets up a connection over the telephone network to the called party’s exchange. The called exchange then handles the process of ringing, detecting an answer, and notifying the calling exchange and billing machinery when the call is completed (in telephone terminology, a call is completed when the called party answers, not when the conversation is over).
When the conversation is over, one or both parties hang up by replacing their handset on the base, stopping the flow of current. The exchange then initiates the process of taking down the connection, including notifying billing equipment of the duration of the call if appropriate. Billing equipment may or may not be involved because calls within the local calling area, which includes several nearby exchanges, may be either flat rate or message rate. In flat-rate service, the subscriber is allowed an unlimited number of calls for a fixed fee each month. For message-rate subscribers, each call involves a charge that depends on the distance between the calling and called parties and the duration of the call. A long-distance call is a call out of the local calling area and is always billed as a message-rate call.
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.
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-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.
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