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In telecommunication, 'signalling' (UK spelling) or 'signaling' (US spelling) 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.
Signalling systems can be classified according to their principal properties, some of which are described below:

Contents

In-Band Signalling versus Out-Of-Band Signalling

Line Signalling versus Register Signalling

Channel-Associated Signalling versus Common-Channel Signalling

Compelled Signalling

Subscriber vs trunk signaling

Classification examples

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 standard for the signalling that has controlled the world's phone calls for some twenty years.

Line Signalling versus Register Signalling

'Line signalling' is concerned with conveying information on the state of the line or channel, such as on-hook, off-hook (together referred to as supervision), ringing current (alerting), and recall. In the middle 20th Century, supervision signals on long 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 bits in the digital stream dedicated to signalling.
'Register signalling' is concerned with conveying addressing information, such as the calling and/or called telephone number. In the early days of telephony, with operator handling calls, the addressing information is by voice as "Operator, connect me to Mr. Smith please". In the first half of the 20th century, addressing information is by using a rotary dial, which rapidly breaks the line current into pulses, with the number of pulses conveying the address. Finally, starting in the second half of the century, address signalling is by DTMF.

Channel-Associated Signalling versus Common-Channel Signalling

'Channel-Associated signalling' employs a signalling channel which is dedicated to a specific bearer channel.
'Common-Channel signalling' is so-called, because it employs a signalling channel which conveys signalling information relating to multiple bearer channels. These bearer channels therefore have their signalling channel in common.

Compelled Signalling

The term 'Compelled signalling' refers to the case where receipt of each signal needs to be explicitly acknowledged before the next signal is able to be sent.
Most forms of R2 register signalling are compelled (see R2 signalling), while R1 multi-frequency is not.
The term is only relevant in the case of signalling systems that use discrete signals (e.g. a combination of tones to denote one digit), as opposed to signalling systems which are message-oriented (such as SS7 and ISDN Q.931) where each message is able to convey multiple items of information (e.g. multiple digits of the called telephone number).

Subscriber vs trunk signaling

Subscriber signaling is between the telephone and the telephone exchange. Trunk signaling is between exchanges.

Classification examples

Note that every signalling system can be characterized along each of the above axes of classification. A few examples:

★ DTMF is an in-band, channel-associated register signalling system. It is not compelled.

★ SS7 (e.g. TUP or ISUP) is an out-of-band, common-channel signalling system that incorporates both line and register signalling.

★ Metering pulses (depending on the country, these are 50Hz, 12kHz or 16kHz pulses sent by the exchange to payphones or metering boxes) are out-of-band (because they do not fall within the frequency range used by the telephony signal, which is 300 through 3400Hz) and channel-associated. They are generally regarded as line signaling, although this is open to debate.

★ E&M signalling is an out-of-band channel-associated signalling system. The base system is intended for line signalling, but if decadic pulses are used it can also convey register information. E&M line signalling is however usually paired with DTMF register signalling.

★ By contrast, the L1 signalling system (which typically employs a 2280Hz tone of various durations) is an in-band channel-associated signalling system as was the SF 2600 Hz system formerly used in the Bell System.

★ Loop start, Ground start, Reverse Battery and Revertive Pulse systems are all DC, thus out of band, and all are channel-associated, since the DC currents are on the talking wires.
Whereas common-channel signaling systems are out-of-band by definition, and in-band signaling systems are also necessarily channel-associated, the above metering pulse example demonstrates that there exist channel-associated signaling systems which are out-of-band.
=Sources=

★ Federal Standard 1037C

★ MIL-STD-188