| 摘要 |
The present invention is generally directed to an xDSL modem that is capable of transmitting at multiple data rates. Specifically, the xDSL modem is configured to generally transmit at a high data rate (typically the highest data rate supported by the line), but will reduce its data transmission rate in response to one or more sensed conditions. In this regard, it has been found that transmitting at a data rate that is low enough to avoid significant corruption, and thus retransmission, achieves a higher overall data transmission rate than that which is achieved if re-transmissions are required. Further, various conditions may be sensed that will prompt the xDSL modem to alter its transmission rate, including detecting an unfiltered POTS device going Off-Hook. In accordance with one aspect of the present invention, at modem startup, the Off-Hook condition and other line problems are detected by direct measurement of the line impedance. Given a first impedance measurement in which the impedance is approximately equal to the impedance of the typical twisted-pair loop, the digital signal processor (DSP) of the present invention is able to detect that either all phones are on hook or that any Off-Hook equipment is protected with micro data filters. In response to this detected line condition, the xDSL communication is transmitted at the highest data rate supported by the transmission line.
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