| 摘要 |
After receipt of the answerback tone in a conventional handshaking procedure, the originate modem is connected to the communication line, sends a short echo control training (ECT) sequence to the answer modem, and starts a timer. Upon receipt of the ECTo sequence, the answer modem sends its own ECTa sequence to the originate modem and starts its own timer. Upon receiving the ECTa sequence, the originate modem measures the round trip propagation delay (TRTPD) and transmits a second ECTo sequence. The time of receipt of this sequence at the answer modem is used by the answer modem to determine TRTPD. Either modem may now start data transmission. Because of the measurements made, the receiver at the transmitting modem is aware that the echo from its transmission will be delayed by TRTPD. Accordingly, it will open an "echo monitor window" in a time interval around TRTPD during which time frame any received energy is assumed to be an echo. If an echo is detected in such time interval, the receiver will be squelched until energy is no longer detected. Moreover, knowledge of TRTPD also permits the modem to select an optimum length training sequence because the quality of the communication line depends on its length.
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