The term compelled signalling refers to a class of telecommunications signalling protocols where receipt of each discrete signal needs to be explicitly acknowledged before the next signal is able to be sent.
For example, in R2 register signalling, the transmission of each signal involves the following five events:
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).
The contrary of compelled signalling is referred to as non-compelled signalling. An example includes DTMF, 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.