In telecommunication, signaling (signalling in British spelling) has the following meanings:
Signaling systems can be classified according to their principal properties, some of which are described below:
In-band versus out-of-band signaling
In the public switched telephone network (PSTN), in-band signaling is the exchange of call control information within the same channel that the telephone call itself is using. An example is dual-tone multi-frequency signaling (DTMF), which is used on most telephone lines to customer premises.
Out-of-band signaling is telecommunication signaling on a channel that is dedicated for the purpose and separate from the channels used for the telephone call. Out-of-band signaling is used in Signaling System 7 (SS7), the standard for signaling among exchanges that has controlled most of the world's phone calls for some twenty years.
Line versus register
Line signaling is concerned with conveying information on the state of the line or channel, such as on-hook, off-hook (Answer supervision and Disconnect supervision, together referred to as supervision), ringing current (alerting), and recall. In the middle 20th Century, supervision signals on long distance trunks in North America were usually inband, for example at 2600 Hz, necessitating a notch filter to prevent interference. Late in the century, all supervisory signals were out of band. With the advent of digital trunks, supervision signals are carried by robbed bits or other bits in the E1-carrier dedicated to signaling.
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