Narrowband narrowband or subvoice grade channels

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Unformatted text preview: rcuits are seldom used because a return path is generally needed to send acknowledgement, control, or error signals. Half-Duplex A half-duplex system can transmit data in both directions, but only in one direction at a time. Thus, a half-duplex line can alternately send and receive data. It requires two wires. This is the most common type of transmission for voice communication because only one person is supposed to speak at a time. It is also used to connect a terminal with a computer. The terminal might transmit data and then the computer responds with an acknowledgement. Full-Duplex In a half-duplex system, the line must be "turned-around" each time the direction is reversed. This involves a special switching circuit and requires a small amount of time (approximately 150 milliseconds). With high-speed capabilities of the computer, this turn-around time is unacceptable in many instances. Also, some applications require simultaneous transmission in both directions. In such cases, a full-duplex system is used that allows information to flow simultaneously in both directions on the transmission path. Use of a full-duplex line improves efficiency as the line turn-around time required in a half-duplex arrangement is eliminated. It requires four wires. DATA TRANSMISSION SPEED A term used to describe the data-handling capacity of a communication system is bandwidth. Bandwidth is the range of frequencies that is available for the transmission of data. A narrow range of frequencies in a communication system is analogous to a narrow road. The flow of information in such a system - its data transfer rate - is limited, just as is the flow of traffic in a narrow road. Wider bandwidths permit more rapid information flow. Thus, the wider the bandwidth of a communication system, the more data it can transmit in a given period of time. When dealing with computer input/output devices, we think in terms of characters per second. However, in case of data transmission, we talk in terms of bits per second. The ASCII code uses 7 data bits per character plus a parity bit. For data communication, additional bits are added to control the process. Althou...
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