The term compelled signalling refers to a class of
telecommunications
Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...
signalling protocols where receipt of each discrete signal needs to be explicitly acknowledged before the next signal is able to be sent.
ITU-T Recommendation Q.9 - Vocabulary of Switching and Signalling Terms
For example, in
R2 signalling, R2 register signalling, the transmission of each signal involves the following five events:
*The outgoing trunk begins sending the tones that correspond to the signal it wishes to convey in the forward direction;
*The incoming trunk, once it is sure to have correctly detected the above signal, begins sending a set of confirmation tones in the backward direction;
*The outgoing trunk, once it is sure to have correctly detected the confirmation tones, stops sending the signal tones in the forward direction;
*The incoming trunk, once it is sure to have detected the end of the signal tones (that is: that the forward channel is now silent) stops sending its confirmation tones in the backward direction;
*Before being able to begin sending the next signal, if any, the outgoing trunk needs to ensure that it is receiving silence on the backward channel.
The term is only relevant in the case of signalling systems that use discrete signals (e.g. a combination of tones to denote one digit), as opposed to signalling systems which are message-oriented, such as
Signaling System 7 (SS7) and ISDN Q.931, where each message is able to convey multiple items of information (e.g. multiple digits of the called telephone number).
Compelled signalling is not suitable for
satellite communication
A communications satellite is an artificial satellite that relays and amplifies radio telecommunication signals via a transponder; it creates a communication channel between a source transmitter and a receiver at different locations on Earth. C ...
due to the long
propagation delay
Propagation delay is the time duration taken for a signal to reach its destination. It can relate to networking, electronics or physics. ''Hold time'' is the minimum interval required for the logic level to remain on the input after triggering ed ...
.
The contrary of compelled signalling is referred to as ''non-compelled'' signalling. An example includes
DTMF
Dual-tone multi-frequency signaling (DTMF) is a telecommunication signaling system using the voice-frequency band over telephone lines between telephone equipment and other communications devices and switching centers. DTMF was first developed ...
, where the originating side sends tones and silence in the forward direction, without being able to ascertain whether each tone has been correctly received by the terminating side. In order to minimise the risk of signalling errors, minimum durations are imposed, both on the tones and on the intervening periods of silence.
References
{{DEFAULTSORT:Compelled Signalling
Telephony signals