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COMMUNICATIONS OPTIONS

When computer data is transmitted, the information is sent as a coded sequence that is decoded by the receiving device. The way this data is sent can vary. Therefore both of the communicating devices must be capable of encoding and decoding specific types of transmitted information. Most systems include special software that gives users a way to set up their data receiving system to match the data-sending capabilities of another computer. The communications options that can be selected before transmissions can take place include signal type, data rate, data flow, and transmission method.

Signal type. A system that uses digital signals sends information coded as a set of bits that can have one of two values. For example, a high pulse can carry the value of 1, and a low pulse can carry the value of 0. Before transmission, patterns of these 1 bits and 0 bits are grouped into bytes and encoded using standard computer coding methods. Systems that use analog signals are somewhat different. They send data as a wave pattern that varies continuously. Computers manage data in digital form. The telephone system, however, uses an analog signal type.

Data Rate. Since the rate at which information is transmitted over a channel varies, commu­nicating computer devices must be capable of transmitting and receiving data at differing rates. Data rates are measured in bits per second or bps. Each type of channel has a maximum rate at which data can be transmitted, based on the type of media used in the channel and its design. Generally, channels with data rates less than 300 bps are referred to as narrowband. Rates of 300 to 9,600 bps are known as voiceband or voice-grade. The fastest channels are referred to as wideband or broadband. They are considered to be high-speed channels.

Data Flow. Data flow can be managed in one of three modes — simplex, half-duplex, or full-duplex. These three modes refer to the direction of the data flow. If you are using a communications system in the simplex mode, data can only travel through the channel in one direction. Since this mode restricts communication to a one-way transmission, either sending or receiving, it is not used very often for communications between two computers. Using the half-duplex mode, on the other hand, data can be sent in both directions, but not at the same time.

Using the full-duplex mode, data can be transmitted in both directions at the same time. Systems with special types of wiring may be able to transmit data at higher speeds using the full-duplex mode. However, the half-duplex mode is more commonly used when transmitting data between two computers.

Transmission Method. Two methods are used to transmit characters over a channel. The asynchronous transmission method is used to send one character at a time. Since the transmission is synchronized by sending a start bit and a stop bit, data can be sent at any time. The synchronous transmission method is used to send blocks (groups) of characters in a timed sequence. Although this method requires more sophisticated communications equipment, it can be used to send data at higher transmission rates.