Telephone Transmission

Telephone signals between subscriber sets and the urban exchange, and between urban and long-distance exchanges, are carried by aerial lines, underground or aerial cables, or radio links. Operating these transmission media presents serious challenges, especially regarding communication security and economic efficiency. At first glance, it might seem that each long-distance conversation requires its own pair of wires, either as an aerial line or as one pair within a cable bundle. In certain cases, the pair of wires may be replaced by a radiotelephone link when distance, terrain, or other factors make wire installation impractical. Because a long-distance telephone pair is extremely costly, it becomes essential to maximize existing resources and design new installations capable of carrying not just one conversation, but many simultaneously.

The most important solution to this problem is the carrier-wave transmission system. Its basic principle lies in the ability to shift voice frequencies (300 to 3,500 cycles per second) to another region of the frequency spectrum using electronic components grouped in what is known as a modulator. For example, a modulator can add a fixed frequency—called the carrier—such as 10,000 cycles, to all the frequencies of the conversation. Thus, the original 300–3,500-cycle band becomes 10,300–13,500. Another conversation can be shifted to 14,300–17,500 cycles, and so on. In this way, many conversations can occupy distinct positions—or channels—in the spectrum while remaining independent. These mixed frequencies can be transmitted simultaneously over a single pair of wires. At the receiving end, special filter circuits separate the channels and send them to individual modulators, where the added carrier frequency is removed, restoring the original signal. Ordinary aerial lines can transmit between 2 and 24 simultaneous conversations, depending on their construction. Special coaxial cables and microwave radio systems can handle thousands of conversations at once.

A schematic representation of the system appears in Figure 12.

Principles of Automatic Telephony

It is often said—perhaps with some exaggeration—that if the entire U.S. telephone system still operated under manual switchboard principles, half of the country’s adult female population would have to work as operators. New York City alone has more than three million telephones. Given the density of normal telephone traffic in a major commercial center, it is unlikely that one operator could manage more than 30 or 40 lines. This would require about 100,000 operators per shift. Since no operator can work more than six hours a day, four shifts would be necessary (the night shift being smaller). Adding weekly rest, absences, illness, maternity leave, and auxiliary services, nearly half a million women would be needed to operate one city’s service. Fortunately, automation has prevented such an absurd scenario.

Today, nearly all large cities have automatic service, giving each subscriber access to an impressive system capable of establishing the desired connection instantly. Automatic service now extends not only to urban calls but, in many countries, across entire national territories and even into neighboring countries. In the United States, a New York subscriber can connect with one in San Francisco within seconds thanks to automatic interconnection.

The principle of the automatic exchange is simple, although its practical realization requires enormous quantities of delicate equipment. When a subscriber wishes to place a call, the first step is to lift the handset. This action releases the hook switch and connects the set to the exchange, closing the central-battery circuit. The subscriber then dials the desired number. For example, placing a finger in the hole corresponding to number 8, rotating the dial to the stop, and releasing it causes the dial to return to its resting position while actuating a contact that interrupts the circuit eight times. These impulses travel through the line to the exchange, where they activate an electromagnet controlling a selector.