Method and apparatus for remote signalling by substituting a message for the data conveyed by a digital transmission link

The method consists in momentarily replacing the data in a transmitted digital data stream by a remote signalling message which is built up from consecutive sequences, each of which is obtained by repeating a specific binary pattern chosen from a plurality of possible binary patterns which are generated from the second and fourth sub-harmonics of a clock signal at the bit rate of the digital stream, and from logical combinations thereof. FIG. 2 is a circuit diagram of a transmitter circuit for transmitting a remote signalling message. Said transmitter circuit is located in equipments on a very high data rate digital link, which equipments include a circuit (6') for detecting and handling alarms, and a circuit (3) for recovering the bit rate from the digital stream. Divide-by-two circuits (10, 14) and a logic gate (16) serve to generate three kinds of pattern from the recovered clock rate, namely: 0101, 0011, 1000. A sequencer (21) builds up sequences of patterns within a remote signalling message by means of a multiplexer (20) whose inputs are encoded with the message to be transmitted, a pattern generator selector circuit (11, 15, 16), and switching means (18) for inserting the remote signalling message instead of the digital stream.