Signalling is the exchange of switching information that is used to control the transmission of useful signals. Signalling is part of signalling. This involves the establishment and termination of connections
. Signaling can take place directly in connection with the communication signal, but it can also take place on channels that are physically independent of the communication signal. If the signaling signal is transmitted on the same channel and the same physical transmission medium as the data, it is referred to as innerband signaling, as in the analogtelephone network with the dual-tonemultifrequency (DTMF) and dual-tone multifrequency (DTMF) methods. The switching standard for innerband signaling is the C5 standard. If the signaling data and the information data are transmitted on separate channels, then it is outerband signaling. Typical examples of out-of-band signalling in the D channel ofISDN
as well as signalling system No. 7
. In the case of very complex signalling where a wide variety of services are supported and signalling channels are required, this is known as meta-signalling. This is used in B-ISDN signalling.