| 摘要 |
Ethernet frames are communicated via a telephone line between master and sla ve modems using half duplex communication of data and control information to av oid collisions, data to be transmitted being buffered in each modem. Different communication modes, involving different transmitted symbol frequencies, mod ulation methods, and bits per symbol to provide different transmission rates, can be used depending on errors on the line. To reduce signal attenuation and interferen ce both of which increase with increasing frequency, a lowest frequency communication m ode is used for transmitting control information when there is no data in the buffe r for transmission, whereas a higher transmission rate communication mode is used for transmission of data and control information when the buffer is not empty. T he communication mode can also be adjusted in dependence upon signal to noise r atio, signal and noise energies being monitored by the modem receiver respectively during and in a quiet period immediately following each received signal burst. Chan ges especially to a higher transmission rate communication mode can be made sele ctively depending upon a monitoring in the quiet period by the modem receiver of noi se in the higher communication mode, the relevant signal to noise ratio being determin ed from this and a storedsignal level for this mode.
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