Line signaling is category of telecommunications signaling protocols. Line signaling is responsible for off-hook, ring, answer, and on-hook unidirectional messaging in each direction from calling party to called party and vice versa. After an off-hook, line signaling initiates register signaling to accomplish the exchange of telephone-numbers of called party and in more modern line-signaling protocols, the calling party as well. While register signaling occurs, line signaling remains quiescent unless the calling party goes on-hook or an abnormal cessation of the call occurs, such as due to equipment malfunction or shutdown or due to network outage upstream in that call-attempt's series of spanned trunks.

Line signaling can be conveyed in a single DS0 channel of a trunk. In modern PCM telecommunications, line signaling is represented by the ABCD bits in DS0 #16 of an E1 or a selected DS0 within a T1. Line signaling can also be conveyed in-band, e.g. as in L1 signaling where 2280 Hz tone pulses are used.

Telecommunications