Abstract: A modem for transmission and reception of data samples over a transmission line includes a transmission line break detection device having a correlation meter for providing a measure of the degree of correlation between transmitted and received data samples. Such device further includes a threshold meter for triggering a line break alarm when the correlation exceeds a threshold value. The correlation relates to a limited number of data samples occurring during a portion of an echo period for reception of an echo of transmitted data samples.

Abstract: The transmission system and the modem according to the invention use trellis-coded modulation based on a constellation split up into N sub-sets. For each received symbol in this constellation a decision arrangement makes it possible to determine the combination of the N points closest to the received symbol in each of the N sub-sets. Therefore, the constellation is split up into zones corresponding each to a given combination, and the arrangement comprises a zone searching module (2) in which any received point is situated. This module then controls the reading of a Table (3) which gives a combination of points which combination corresponds to this zone.