Abstract: A transmission system for transmission between end devices (1) and (2) is provided for transmitting information signals at a certain common bit rate. This bit rate depends on a symbol rate and a carrier frequency. Via a processing by a processor (9) which cooperates with an instruction memory (10), the largest possible bit rate is determined for exchanging information signals between end devices.

Abstract: A system for testing a data transmission link between first and second terminal devices such as modems. The first terminal device transmits a test data sequence which it is received at an output access (20) of the second terminal device. That device also has an input access (21), first and second clocks which set the timing of data at the input and output accesses, and a loop-back circuit for connecting the output access to the input access to retransmit a received test sequence back to the first terminal device. Since different modems often operate at different data rates, the input and output clock rates may differ. However, during loop-back operation the clocks must be synchronized in order to avoid loss of data.

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.

Abstract: A transmission system wherein the data transmission rate is optimized for communication between modems which are each capable of operating at various transmission rates and wherein the transmission medium is of variable quality. Prior to synchronization each modem determines an optimum transmission rate based on line quality, and during a synchronization setup phase the modems exchange information identifying their minimum and maximum acceptable transmission rates. An initial transmission rate is adopted and then adjusted up or down, depending on line quality, so that the rate is as close to the optimum rate as is consistent with line quality and the rates acceptable to both modems. Thus, the optimum rate calculated by each modem is taken into account and is imposed on the remote modem only if the latter is capable of operation at that rate. The system is therefore highly flexible and also reliable.