| 摘要 |
An echo cancellation modem for full-duplex communication over a channel with a remote device in accordance with one of several possible modulation schemes suitable for different channel qualities, respectively. The modem includes an echo canceller for reducing echo components appearing in the channel signal while leaving a residual echo component in the echo reduced channel signal, a monitor for determining the quality of the channel based on an analysis of the residual echo component, and a controller for selecting an acceptable modulation scheme, based on the analysis of the monitor. The modem selects the acceptable modulation scheme prior to data communication and it also requests changes to the appropriate level of modulation during data communication when the monitor indicates that the channel quality has changed significantly. The modem requests the remote device to operate at the selected rate and begins operating at the selected rate only if the remote device concurs. In another aspect, the modem includes a fall forward flag which is used to prevent contention between the modem and the remote device which would occur if the modem repeatedly requested an increase to a higher level modulation scheme which the remote device cannot support. In another aspect, the modem includes re-initialization logic which selects the appropraite thresholds used to monitor the quality of the channel depending upon whether the modulation option is coded or uncoded.
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