Importance

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Abstract

Ethernet frames are communicated via a telephone line between master and slave modems using half duplex communication of data and control information to avoid collisions, data to be transmitted being buffered in each modem. Different communication modes, involving different transmitted symbol frequencies, modulation methods, and bits per symbol to provide different transmission rates, can be used depending on errors on the line. To reduce signal attenuation and interference both of which increase with increasing frequency, a lowest frequency communication mode is used for transmitting control information when there is no data in the buffer for transmission, whereas a higher transmission rate communication mode is used for transmission of data and control information when the buffer is not empty. The communication mode can also be adjusted in dependence upon signal to noise ratio, signal and noise energies being monitored by the modem receiver respectively during and in a quiet period immediately following each received signal burst. Changes especially to a higher transmission rate communication mode can be made selectively depending upon a monitoring in the quiet period by the modem receiver of noise in the higher communication mode, the relevant signal to noise ratio being determined from this and a stored signal level for this mode.

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First Claim