The earliest computers communicated only with their expert handlers, who programmed them by connecting circuits by hand and read their outputs on paper. Today one of a computer's essential tasks is to communicate with other computers. This would be impossible without a vast network for transmitting signals reliably and at high speeds over long distances, and such a network has evolved during the last century. A few of its aspects are described below.
Communication networks bundle slow signals together to make fast ones. In the phone system, the slowest or most basic signal is the voice signal. Early in the history of telephony it was realized that it would be more cost-effective to combine many voice signals into a few high-speed signals for long-distance transmission than to try to transmit voice signals everywhere. It was also realized that these high-speed signals might, for very long hauls, be profitably combined into even higher-speed signals. ("Speed" here refers not to message velocity--even a voice line is as "fast" as could be desired, in this sense--but to how quickly messages can be fed through the line.) Computers send and receive data through a similar hierarchy of slow, fast, and faster signals. Sometimes they use the telephone system, sometimes dedicated communication systems.
In any given system a baseband signal is the basic message stream. In the telephone network, voice signals or dial-up modem signals are baseband signals. A baseband signal is the slowest in any given system and generally has a physical channel (pair of wires or some other form) all to itself. In the digital communications hierarchy the stream of bits that a single computer or other digital device sends or receives is a baseband signal. The rate at which such a signal transmits symbols is known as its baud rate. If each binary digit (bit) is counted as a symbol, then a signal's baud rate equals its bit rate; if each 8-bit binary word is counted as a symbol, then the baud rate is one-eighth the bit rate.
Baseband signals do not travel very far on their separate channels before being bundled into faster signals for efficient transmission. The usual method for bundling digital baseband signals is time-division multiplexing, the basic principle of which is familiar from everyday life: if one must tend many pots, stir one at a time. Similarly, if one must combine, say, 24 similar baseband signals into one fast signal, proceed as follows: (1) Simultaneously save one word from each of the 24 baseband channels. (2) Transmit the 8-bit word from the first baseband channel in 1/24th the time it took that channel send it. (3) Next transmit the word saved from the second baseband channel; then the next, then the next, and so on, until all 24 saved words have been sent. (4) Meanwhile, collect the next round of 24 words from the baseband channels. (5) When finished sending the first round of words at high speed, send the next round.
The resulting combined or "multiplexed" signal will just keep up with the 24 baseband signals. At the receiving end, the process can be undone by distributing the 24 baseband signals to as many outputs like a deck of cards being dealt rapidly to 24 players.
What has just been described is the T1 line, originally designed by the American Telephone and Telegraph Company to combine 24 digitized voice signals into one 1.544 Mbps (megabits per second) signal. Several T1 signals can be combined into higher-speed signals just as 24 baseband signals are combined to make a single T1; four T1's make a T2, seven T2's make a T3, and so on. Today, dedicated T1 service can be purchased for non-voice data communications. A typical fiber-optic cable carries 2.5 Gbps (gigabits per second), the equivalent of just over 1,600 T1 signals; individual optical fibers will soon carry 1,000 Gbps or more (over 640,000 T1's, or about 15 million phone conversations).
Any signal formed by combining two or more baseband signals is often called a "broadband" signal. "Broadband" is also used to mean all relatively high-speed signals or, alternatively, as a technical label restricted to signals faster than a T1. Whatever they are called, fast signals are in increasing demand, as users increasingly wish to receive images and high-quality audio at baud rates faster than old-fashioned baseband signals can handle. Large organizations, for example, have for some years been investing in dedicated networks (local-area networks) to interlink their in-house computers with high-speed signals. Such systems have usually relied on coaxial cable, but in the last ten years or so a new radio technique--spread spectrum-communications--has been increasingly enabling digital devices to go mobile and wireless. Spread spectrum is "new" only in the commercial sense, having been conceived during World War II and developed by the military for some 40 years; it has only recently been declassified. Since it will probably become nearly ubiquitous in the near future, replacing many or most of the telephone lines, coaxial cables, and other hardwire connection technologies that have become so familiar, it is worthwhile to look briefly at how it works.
In spread-spectrum radio communications, a baseband signal—the data stream from a single device, such as a telephone or a computer--is smeared out over a much wider interval of the radio spectrum than it normally occupies. The result sounds rather like noise, an effect that the military values because it makes such signals hard to jam or intercept. For consumers the value is that spread spectrum signals have high immunity to interference from other signals, including each other. Thus a great many digital devices in a given area can enjoy high-speed wireless data links without getting in each other's way. One method for generating a spread-spectrum signal is to generate a rapid series of pseudo-random numbers at the transmitter. Each pseudo-random number is used to specify a different broadcast frequency; the message signal, whatever it may be, is then broadcast on that frequency very briefly, until a new pseudo-random number specifies a new frequency. The receiver generates the same pseudo-random number series and listens on all the right frequencies at the right times. The pseudo-random number generators at the transmitter and receiver must, of course, be synchronized in some way. This is the frequency hop method for spread-spectrum communications.
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