Telephone

V

Circuits and Exchanges

A telephone call starts with the person making a call lifting the handset off its base and listening for a dialling tone. This closes an electrical switch called the switchhook (originally “hook switch”, named after its shape). Closing this switch starts the flow of an electric current over the caller’s line, also called the loop, between the caller’s location and the building containing the automatic exchange, a part of the switching system. This is a DC or direct current which does not change direction of flow although its intensity or amplitude may vary. The exchange detects the current and returns dialling tone, a precise combination of two notes to permit reliable detection by machines as well as by people.

Once the dialling tone is heard, the caller enters a sequence of digits on pushbuttons mounted either on the handset or base. This sequence is unique to one other telephone subscriber, the party being called. The switching equipment in the exchange removes 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. Ringing current is 20 Hz alternating current. This alternating or AC current flows in each direction 20 times a second. Each subscriber’s telephone contains a ringer which responds to a ringing current, usually by making a sound which can be heard throughout the room containing the telephone. If the called party answers the telephone by picking up her or his handset, DC current starts to flow in the called party’s line and is detected by the exchange. The exchange then stops applying the ringing and sets up a connection between the calling and called parties that can be used for talking.

If the called party is in a different exchange from the calling party, the calling exchange sets up a connection over the network to the called party’s exchange. As part of this process, the calling exchange must tell the called exchange who the called party is. The called exchange then handles the process of ringing, detecting answering, and notifying the calling exchange and billing machinery whether 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. This opens the switchhook and stops the flow of DC current. The exchange then initiates the process of taking down the connection including again notifying the billing equipment if appropriate. Billing equipment may or may not be involved as calls within the local calling area may be either flat rate or message rate. The local calling area includes several nearby exchanges. In flat rate service, the subscriber is allowed an unlimited number of calls for a fixed fee each month. Message rate subscribers pay a charge for each call which 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.

In early telephones the current was generated by a battery. The local circuit included, in addition to a battery and a transmitter, one winding of a transformer called an induction coil; the other winding, connected to the line, stepped up the sound wave voltage. Connections between telephones were made manually, by operators working at switchboards located in central switching offices.

As telephone systems grew, manual switching proved too slow and labour intensive. This provided the impetus for developing a series of mechanical and electronic devices that allowed switching to be done automatically. In the modern telephone, an electronic device transmits either a number of successive impulses of current or a series of audible tones corresponding to the number being called. Electronic equipment at a central switching station automatically translates the signal and routes the call to the receiving party.

Solid-state technology enables these central exchanges to process calls at speeds of one-millionth of a second, so that large numbers of calls can be handled simultaneously. First the input circuit converts the caller’s voice into digital signal pulses. These pulses are then transmitted through the network by high-capacity systems that exchange individual calls by means of computerized mathematical switching operations. Instructions for operating the system are stored in computer memory. Equipment maintenance is facilitated by duplication of components. When a defect becomes manifest, a standby unit automatically begins handling calls. Using computer techniques to handle telephone calls, data messages, and even visual signals, the system can make speed calls, both local and long distance, by swiftly determining the most efficient route.

Today there are no telephones served by manual exchanges in the United States and Britain. All telephone subscribers are served by automatic exchanges. In an automatic exchange switching equipment performs the functions of the human operator. A line current relay in a line circuit replaces the switchboard light and a crosspoint switch replaces the cords. Other relays replace the key. Since computers only now are beginning to be able to understand spoken commands, about a century too late for the earliest automatic exchanges, the dial is used to indicate what number is being called. Incoming registers store the number being dialled and then pass it to the exchange’s central computer which in turn operates the crosspoint switch array to complete the call or route it to a higher level switch for further processing.

VI

Transoceanic Telephony

Overseas radio-telephone service was introduced commercially in 1927, but the problem of amplification prevented the laying of telephone cables until 1956, when the world’s first transoceanic submarine telephone cable, extending between Newfoundland Island and Scotland, was placed in service.

VII

Carrier-Current Telephony

Through the use of frequencies above the voice range, extending from about 4,000 to several million cycles per second, or hertz, as many as 13,200 telephone messages can be carried simultaneously over a single conducting medium. Carrier-current telephony techniques are also being used to send telephone messages over the normal distribution lines without interfering with regular service. With the growth in size and complexity of systems, solid-state amplifiers, called repeaters, are used to amplify the messages at regular intervals.

VIII

Coaxial Cable

Developed in 1936, the coaxial cable uses cable conductors to carry a large number of circuits. The modern coaxial cable consists of copper tubes 0.95 cm (0.375 in) in diameter. Each has a thin copper wire held exactly in the centre of the tube by plastic disc insulators about 2.5 cm (1 in) apart. The tube and the wire have the same centre; that is, they are coaxial. The copper tubes shield the transmitted signal from electrical interference and prevent energy losses by radiation. A cable, consisting of up to 22 coaxial tubes arranged in tight rings sheathed in polyethylene and lead, can carry 132,000 messages simultaneously.

IX

Optical Fibres

Coaxial cables are increasingly being replaced by optical glass fibres. Messages are digitally coded into pulses of light and transmitted over great distances by these slender fibres. A fibre cable may contain up to 50 fibre pairs, each pair carrying up to 4,000 voice circuits. The basis of the new fibre optics technology, the laser, exploits the visible region of the electromagnetic spectrum, where frequencies are thousands of times higher than in radio and thus able to carry much larger volumes of information. The light-emitting diode (LED), a simpler device, is adequate for most transmission purposes.

One fibre-optic cable, TAT 8, carries more than twice the number of transatlantic circuits that were available in the 1980s. Used in a system that stretches from New Jersey to Britain and France, it can transmit up to 50,000 conversations at once. Such cables also provide channels for high-speed transmission of computer data that are more secure than those offered by communications satellites. Another major advance in telecommunications, TAT 9, which is an even higher capacity fibre cable, came into operation in 1992 and can carry 75,000 calls simultaneously.