Communication Devices

The versatility of a computer is enhanced considerably if it can communicate with other computers or a number of users. The very first computers were large machines that occupied an entire room with a control console at the center. The input and output devices for these machines were located at that console and only one user could operate the machine at a time. However, not long after the first computers were made, engineers realized that it would be more efficient if users could access the machine from their desks via a "terminal." This was the beginning of the "mainframe" computer.

The key to a mainframe computer was a communications network that would allow users to link to the machine. The terminals had no computing power themselves, but were connected to the mainframe computer via copper wire, sometimes called twisted pair. As computers became larger and faster, developers realized that computing time could be sold to othercompanies or offered to other offices of the same company. This necessitated gaining access to the computer via long distance communication. In the beginning, during the late 1950s and 1960s, the only available long distance communications device was the telephone line.

However, telephonic communications were designed for voice transmission, while computers require data transmission. Computer data can not be applied to a telephone line directly but must be transformed into a signal that is compatible with the telephone system. To enable this compatibility, a signal, usually a sine or cosine function, called a carrier, is modulated, or modified in some way. This is done via a modulator—a device that produces a signal that can be handled by the telephone system without undue distortion, and without the modulated signal interfering with the operation of the telephone system.

On the receiving end, the modulated signal must be "demodulated" in order to retrieve the digital data. For two-way communications, the computer must be equipped with a modulator for outgoing data and a demodulator for incoming data. This device is now known as a modem, a term that was derived from the combination of MOdulator and DEModulator.

The first computer modems modulated the carrier signal by changing the frequency of the sine or cosine function. The frequency of a carrier is the rate at which it repeats. For example, if the function repeats 1,000 times each second, its frequency is 1,000 Hertz or 1,000 Hz. A modem can have two distinct frequencies, called tones, at 1,200 and 2,200 Hz, where 1,200 Hz represents a "mark" and the 2,200 Hz represents a "space." In the early days, the modem would shift between these two frequencies, a method of modulation called frequency shift keying or FSK. This technology was already in place before the invention of computers and was used for teleprinters, better known by their brand name, Teletype machines. It was therefore easy to attach a logic one or zero to the mark and space respectively and use the modem to transmit digital data.

The speed of the modem is expressed in baud, named after Jean-Maurice-Émile Baudot, who invented the fixed-length teleprinter code in 1874. This code was the model on which many computer codes were configured. In modern terminology, baud represents bits per second.

The first computer modems were very slow, at 300 baud. However, the computer input/output devices were Teletype machines, which could not print any faster than 100 baud. When electronic terminals appeared, the 300 baud modem became a major bottleneck, and the speed of the FSK modem was increased to 1200, 2400, 4800 and later 9600 baud. Increasing the speed of modems beyond 9600 baud required a modulation scheme more sophisticated than the simple FSK.

Later modulation schemes use much more sophisticated techniques that vary the amplitude, angle, or combinations of both. In addition, improved encoding and error detection techniques are used. With the creation of the Internet, the need for even faster modems increased. The quality of the telephone line, which is the communications "channel," now becomes the limiting factor for increased data rates. The highest data rate available from a commercial product for telephone line use is 56,000 baud, 56 kilobaud, or 56kB. When a communications channel is used at speeds beyond its limits,errors occur. These high-speed telephone modems automatically adjust the data rate downward when the errors increase.

Jean-Maurice-Émile Baudot was famous for his telegraph code, created in 1874, that became more popular than Morse Code by the mid-twentieth century.

Modems faster than 56kB require a higher capacity channel, which is available on cable television systems. The bandwidth of a telephone channel is about 3.5 kHz. In comparison, the bandwidth of one television channel is 6 MHz. The cable television system is an inherently broadband system, making it possible to add high-speed data signals to the cable television system without interfering with existing television signals. In these cases as well, modems are used to perform exactly the same function they do when connected to a telephone line. Data rates vary but a typical cable modem data rate is more than 1 million baud or 1 MB.

Another high speed modem is the digital subscriber loop (DSL). This system uses telephone wires, but not the telephone channel. Just as the cable modem connects to the cable television system while the normal television service is uninterrupted, DSL connects to the telephone system with no effect on normal telephone service.

Special equipment is required by the subscriber and the telephone company to "multiplex" computer data on the telephone line with normal telephone service. The only difference is that telephone wires being used for the DSL are incapable of being handled by any telephone equipment. Therefore, the digital signals must be separated at the point where the telephone wires enter the telephone network, which is usually a central office.

DSL uses telephone wires that were never intended to handle high speed data. Therefore, the digital signals are quickly reduced in strength, limiting the distance these signals can travel to the central office. Because of this limitation, some telephone customers, who are too distant from the central office, cannot obtain DSL. Other variations on DSL, which allow users to take advantage of the maximum distance and data rate, such as the asymmetric DSL or ADSL, are also available now. This system has a higher speed down-link than up-link.

Wireless modems are also available with data rates up to several MB. These systems are usually privately owned by large companies or other organizations. There have been some installations of public access wireless systems but the most common is the cellular telephone system. New cellular telephone systems, called "third generation" or 3G, will offer extensive data transmission capability.

The highest performing data links are those based on fiber optics. These links offer data rates measured in the hundreds of MB. These systems are used within a company or an industrial complex. Until there is a fiber optic infrastructure capable of delivering fiber to the home, or FTH, this option is not available to many individual subscribers. Fortunately, these FTH systems are being installed at various locations around the world.

Albert D. Helfrick

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Glossbrenner, Alfred, and Emily Glossbrenner. The Complete Modem Handbook. New York: MIS Press, 1995.

Hakala, David. Modems Made Easy: The Basics and Beyond. Berkeley, CA: Osborne McGraw-Hill, 1995.