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