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COMMUNICATIONS HARDWARE

Simple communications between a computer and an attached device such as a printer can be ac­complished without special communications hardware. However when data is transmitted between incompatible devices or over longer distances, special devices are required.

Modems. While computers manage data using digital methods that are expressed in binary form (either on or off), most communications devices use analog methods that deal with continuous patterns of sound frequencies. When computers transmit data using standard communications systems (such as the phone system), binary data must be converted to the analog signals used by that system.

This is known as modulation. When such data is received by computers, it must be converted back again to its digital form. This is known as demodulation.

A communications device that carries out both tasks is referred to as a modem (a contraction of the terms modulation and demodulation). A variety of modems are available. The types of modems that are used with microcomputers directly to the phone line are the most common.

The types of modems that are used with microcomputers transmit and receive data at speeds from 300 bps to 9,600 bps or more. Faster modems are available, but are usually more expensive.

Multiplexers. A multiplexer is a communications device that allows a number of computing devices to share the same communications line. Such devices are often used to make signals from several computing devices and combine them for transmission via a modem over high-speed channels. Another multiplexing device at the other hand of the connection is used to split the signals back into their original components.

Today, multiplexers are also playing another role in communications systems. Because data is often transmitted in a variety of different formats (known as protocols), many organizations use multiplexers that include a protocol converter. They provide a way for different types of computers using different types of transmission methods to communicate with each other. Such systems are often used to allow communications between microcomputers and mainframes.

Concentrators. Multiplexing can also be done by a concentrator. This device, which may be a computer with special multiplexing capabilities, divides the data channel into separate channels. It allocates channel space as the need arises by providing internal storage of the transmitted data when traffic on the channel is high and then forwarding the data later when the channel is available. A concentrator can also have additional data-management capabilities making it more flexible than other multiplexers.

10. Ответьте на вопросы:

1. When are special devices required?

2. What is a modem?

3. What allows a number of computing devices to share the same communication line?

4. What is a protocol converter used for?

5. What device divides the data channel into separate channels?