Variable data rate channels for digital networks

A method of varying the bit-rate available for digital signals to be transmitted by a communication system which employs a plurality of first fixed bit-rate channels, comprising the steps of

• opening for operation between two stations a variable bit-rate second channel comprising at least one of the first channels, and
• changing, as required in the course of operation, the number of first channels comprising the second channel while continuing to transmit data over the second channel,
• wherein the number of first channels forming the second channel is reduced, when required, by
• transmitting, in the second channel, a predetermined distinctive sequence of digital signals from one of the two stations, the predetermined distinctive sequence being one which is not likely to occur in data to be transmitted by the communication system,
• ceasing to transmit data in one of the first channels which is to be removed from the second channel at a time having a first predetermined relationship to the transmission of the predetermined distinctive sequence, and
• at the said other station, omitting, at a time having a second predetermined relationship to the reception of the predetermined distinctive sequence, any data received in the said one first channel from the output of the second channel,
• wherein opening a variable bit-rate second channel or increasing the number of first channels in the second channel comprises the steps of
• transmitting a first further predetermined sequence of digital signals from both stations in one of the first channels which is carrying a second predetermined sequence of digital signals indicating that the channel is idle,
• detecting separately at each station the reception of the first further predetermined sequence of digital signals and then transmitting a third further predetermined sequence of digital signals separately at each station followed by digital signals representing data, and
• combining the digital signals received after the third further sequence with digital signals received in all other first channels already forming the second channel to provide an output for the second channel.

Apparatus for use in transmitting and receiving variable bit-rate digital signals, comprising

• means for operating a plurality of fixed bit-rate first duplex channels, each of which first channels have the same bit-rate, at a first location for communication to a second location,
• means for combining a variable number of the first channels to provide a variable bit-rate second channel, and
• means for changing the number of first channels forming the second channel as required in the course of operation while continuing to transmit data over the second channel, comprising means for transmitting a predetermined distinctive sequence of digital signals from the first location when the number of first channels forming a second channel is to be reduced, the predetermined distinctive sequence being one which is not likely to occur in data to be transmitted by the second channel, and
• means for detecting the predetermined distinctive sequence when transmitted from the second location and received at the first location,
• the means for combining the first channels being arranged to cease to transmit data in one of the first channels which is to be removed from the second channel at a time having a first predetermined relationship to the transmission of the predetermined distinctive sequence and to omit, at a time having a second predetermined relationship to the reception of the predetermined distinctive sequence, any data received in the said one first channel from the output of the second channel.

Apparatus for use in transmitting and receiving variable bit-rate digital signals, comprising

• means for operating a plurality of fixed bit-rate first duplex channels at a first location for communication to a second location,
• means for combining a variable number of the first channels to provide a variable bit-rate second channel, and
• means for changing the number of first channels forming the second channel as required in the course of operation while continuing to transmit data over the second channel, comprising means for transmitting a predetermined distinctive sequence of digital signals from the first location when the number of first channels forming a second channel is to be reduced, the predetermined distinctive sequence being one which is not likely to occur in data to be transmitted by the second channel, and
• means for detecting the predetermined distinctive sequence when transmitted from the second location and received at the first location,
• the means for combining the first channels being arranged to cease to transmit data in one of the first channels which is to be removed from the second channel at a time having a first predetermined relationship to the transmission of the predetermined distinctive sequence and to omit, at a time having a second predetermined relationship to the reception of the predetermined distinctive sequence, any data received in the said one first channel from the output of the second channel, wherein
• the means for operating the first channels comprises multiplex means for time division multiplexing a high bit-rate channel to provide the first channels, the first channels then being transmitted in a series of frames, and further comprising
• means for defining a series of synchronisation times which are separated from one another by the same number of frames, and wherein
• the means for changing the number of first channels is arranged to cease combining data from the said one channel at the frame following the next synchronisation time after reception of the predetermined distinctive sequence.