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The data to be transmitted must first be encoded into a form suitable for specific transmission handling, as the transmission over a communication line produces attenuation and phase delay; it is impractical to transmit the digital pulses in their raw form over telephone lines. It is necessary to modulate the data to be transmitted over analogue telephone lines and to demodulate the signal at the receiver end. Similarly as to the speech, data transmission is generally possible to both directions, although not simultaneously. The data transmitter receiver which perform the dual process of modulation and demodulation is consequently referred to by the abbreviated form of modem. The modem performs the operation of trans­lation between the binary data pulses and the voice frequency wave­form; hence it could also be regarded as analogue to digital and digi­tal to analogue converters.

Without regard to the communication media, there are three types of transmission, (a) Simplex transmission, where a line carries data in one direction only. Half-duplex transmis­sion, where a line can carry data in either direction but only in one direction at a time. Here identical modems are inserted at both end of the transmission path. This is a most popular data transmission configuration, as it can be used in any public tele­phone network, (c) Duplex (or full duplex) transmission, where a line can carry data in both directions simultaneously. This double direction can be achieved by either transmission over two different frequency bandwidths or by a 4-wire line circuit.

Although data can flow in both directions simultaneously in a dup­lex configuration, it is common practice to operate it in a half-duplex mode. In these applications the digital information flows in one direc­tion while conversely the control data flows in the other direction to indicate either an acknowledgement or a request for retransmission of the message. This mode of operation saves time when two terminals or two computers communicate. It must be appreciated that for most communication procedures an answer-back is essential.



Date: 2015-04-20; view: 172

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