Abstract: The present invention is a quantization-level-sampling (QLS) modem which includes means for separately equalizing each loop of an end-to-end digital telephone system network connection by employing a plurality of transmitter filters and a plurality of receiver filters in such a way that, in the direction of transmission from modem to central office, the voltage samples seen by the coder/decoder stations are equivalent to the network quantization levels transmitted by the modem, and in the direction of transmission from central office to modem, the voltage samples seen by the modem are equivalent to the network quantization levels encoded by the coder/decoder stations. The invention also includes a QLS modem communications system which includes the aforesaid digital telephone system network and a plurality of QLS modems. In preferred embodiments, an adaptation service unit within the telephone system network assists in the determination of the settings of the transmitter and receiver filters.

Abstract: In many data communications systems, crosstalk is a significant channel impairment. If all transmitters in such a system must utilize the same shaping function, and if the transmitters are furthermore nominally Nyquist bandlimited, then the crosstalk generally cannot be mitigated by equalization. However, by extending the bandwidth of the transmitted signal in accordance with the principles of the invention, the resultant crosstalk can be efficiently mitigated by equalization. This extended bandwidth signal is produced by adding significant additional energy outside the traditional Nyquist band.

Abstract: A new receiver structure is disclosed for a full-duplex modem in which a fractionally spaced equalizer, which is normally present to mitigate the effects of linear impairments present on a communication channel, also performs echo cancellation. Specifically, an additional echo signal is formed in the full-duplex modem and added to a received signal, which includes an echo signal, thereby forming a composite signal. This composite signal is then applied to the fractionally spaced equalizer for equalization, which mitigates the echo signal.

Abstract: Coefficient drift, in fractionally-spaced equalizers, is constrained by combining a received data signal with a cyclostationary signal of the same symbol rate as the received data signal.

Abstract: In a voiceband data multipoint network, each tributary modem transmits, in a trailer which follows the user data, a tributary modem identification, a sequence number which advances for each start-up and a checksum. The observation at the receiver of a gap in the series of sequence numbers in messages determined to have originated from a particular tributary modem is used to accurately compute a missed message rate for that modem.

Abstract: In a voiceband data multipoint network, each tributary modem transmits, in a trailer which follows the user data, a tributary modem identification, a sequence number which advances for each start-up and a checksum. The observation at the receiver of a gap in the series of sequence numbers in messages determined to have originated from a particular tributary modem is used to accurately compute a missed message rate for that modem.