De Forest's tube used a small, varying voltage on a gridlike element to impose matching variations, even at high frequencies, on a much larger flow of electrons between a heated filament and a plate. The inventor's understanding of his device was imperfect, however. He thought that ionized gas in the tube was somehow involved. In 1913 a Bell physicist named H. D. Arnold showed that, on the contrary, the completeness of the vacuum dictated the performance. Arnold and his colleagues designed superior tubes and related circuitry to amplify long-distance telephone transmissions, and service was opened between New York and San Francisco in 1915. Alexander Graham Bell made the first call, speaking to Thomas Watson, who had helped him develop a working telephone four decades earlier. The transcontinental path had 130,000 telephone poles, 2,500 tons of copper wire, and three vacuum-tube devices to strengthen the signals. A 3-minute conversation that year cost $20.70. By the mid-1920s long distance lines connected every part of the United States. Their capacity was expanded by a technique called frequency multiplexing, which involves electronically shifting the frequencies of speech (about 200 to 3,400 cycles per second) to other frequency bands so that several calls could be sent along a wire simultaneously. After World War II, the Bell system began to use coaxial cable for this kind of multiplexing. Its design—basically a tube of electrically conducting material surrounding an insulated central wire—enabled it to carry a wide range of frequencies. Stretching coaxial cable beneath oceans posed difficulties so daunting that the first transatlantic link, capable of carrying 36 calls at a time, wasn't established until 1956. But radio had been filling the oceanic gaps for several decades by then while also connecting ships, planes, and cars to the main telephone system or to each other. After mid-century, a previously unexploited form of radio—the microwave frequencies above a billion cycles per second—took over much of the landbased long-distance traffic. Microwaves travel in a straight line rather than following the curvature of the earth like ordinary radio waves, which means that the beam has to be relayed along a chain of towers positioned 26 miles apart on average. But their high frequency permits small antenna size and high volume. Thousands of two-way voice circuits can be crammed into a single microwave channel.
The never-ending need for more capacity brought steady strides in switching technology as well. A simple architecture had been developed early on. Some switching stations handled local circuits, others connected clusters of these local centers, and still others dealt with long-distance traffic. Whenever congestion occurred, the routing was changed according to strict rules. By the 1970s Bell engineers had devised electromechanical switches that could serve more than 30,000 circuits at a time, but an emerging breed of computer-like electronic switches promised speed and flexibility that no electromechanical device could match. The never-ending need for more capacity brought steady strides in switching technology as well. A simple architecture had been developed early on. Some switching stations handled local circuits, others connected clusters of these local centers, and still others dealt with long-distance traffic. Whenever congestion occurred, the routing was changed according to strict rules. By the 1970s Bell engineers had devised electromechanical switches that could serve more than 30,000 circuits at a time, but an emerging breed of computer-like electronic switches promised speed and flexibility that no electromechanical device could match. The move to electronic switching began in the 1960s and led to all-digital systems a decade later. Such systems work by converting voice signals into on-off binary pulses and assigning each call to a time slot in a data stream; switching is achieved by simply changing time slot assignments. This so-called time division approach also boosts capacity by packing many signals into the same flow, an efficient vehicle for transmission to and from communications satellites. Today's big digital switches can handle 100,000 or more circuits at a time, maintaining a remarkably clear signal. And like any computer, the digital circuits are versatile. In addition to making connections and generating billing information, their software enables them to provide customers with a whole menu of special services—automatically forwarding calls, identifying a caller before the phone is answered, interrupting one call with an alert of another, providing voice mail, and more. In recent decades, long-distance transmission has undergone a revolution, with such calls migrating from microwave and coaxial cable to threadlike optical fibers that channel laser light. Because light waves have extremely high frequencies, they can be encoded with huge amounts of digital information, a job done by tiny semiconductor lasers that are able to turn on and off billions of times a second. The first fiber-optic telephone links were created in the late 1970s. The latest versions, transmitting several independently encoded streams of light on separate frequencies, are theoretically capable of carrying millions of calls at a time or vast volumes of Internet or video traffic. Today, the world is wrapped in these amazing light pipes, and worries about long-distance capacity are a thing of the past (see Lasers and Fiber Optics).
Another technological triumph is the cell phone, a radio-linked device that is taking the world by storm. Old-style mobile telephones received their signals from a single powerful transmitter that covered an area about 50 miles in diameter, an interference-prone method that provided enough channels to connect only a couple of dozen customers at a time. Cellular technology, by contrast, uses low-powered base stations that serve "cells" just a few square miles in area. As a customer moves from one cell to another, the phone switches from a weakening signal to a stronger one on a different frequency, thus maintaining a clear connection. Because transmissions are low powered, frequencies can be reused in nonadjacent cells, accommodating thousands of callers in the same general area.
Do'stlaringiz bilan baham:
|
