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=Signalling In Telecommunications=

In telecommunication, signalling (or signaling) has the following meanings:


 * The use of signals for controlling communications.
 * In a telecommunications network, the information exchange concerning the establishment and control of a connection and the management of the network, in contrast to user information transfer.
 * The sending of a signal from the transmitting end of a circuit to inform a user at the receiving end that a message is to be sent.

Classification
Signalling systems can be classified according to their principal properties, some of which are described below:

In-Band Signalling versus Out-Of-Band Signalling
In the public switched telephone network, (PSTN), in-band signalling is the exchange of signalling (call control) information within the same channel that the telephone call itself is using. An example is DTMF signalling.

Out-of-band signalling is telecommunication signalling (exchange of information in order to control a telephone call) that is done on a channel that is dedicated for the purpose and separate from the channels used for the telephone call. Out-of-band signalling is used in Signalling System #7 (SS7), the latest standard for the signalling that controls the world's phone calls.

Line Signalling versus Register Signalling
Line signalling, also called supervision, is concerned with conveying information on the state of the line or channel, such as on-hook, off-hook, ringing current (alerting), and recall. In the middle 20th Century, supervision signals on long trunks 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.

Register signalling is concerned with conveying addressing information, such as the calling and/or called telephone number.