In telecommunication, signaling is the use of signals for controlling communications. This may constitute an information exchange concerning the establishment and control of a telecommunication circuit and the management of the network—in contrast to manual setup of circuits by users or administrators, for example the sending of a signal from the transmitting end of a telecommunication circuit to inform a user at the receiving end that a message is to be sent.
Classification
Signaling systems may be classified based on several principal characteristics.
is concerned with conveying information on the state of the line or channel, such as on-hook, off-hook, ringing, and hook flash. In the middle20th century, supervision signals on long-distance trunks in North America were primarily inband, for example at 2600 Hz, necessitating a notch filter to prevent interference. Late in the century, all supervisory signals had been moved 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. Register signaling 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 formation is by voice as "Operator, connect me to Mr. Smith please". In the first half of the 20th century, addressing formation is done 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 signaling is by DTMF.
Channel-associated versus common-channel signaling
employs a signaling channel which is dedicated to a specific bearer channel. Common-channel signaling employs a signaling channel which conveys signaling information relating to multiple bearer channels. These bearer channels therefore have their signaling channel in common.
Compelled signaling
refers to signaling where receipt of each signal from an originating register needs to be explicitly acknowledged before the next signal is able to be sent. Most forms of R2 register signaling are compelled, while R1 multi-frequency signaling is not. The term is only relevant in the case of signaling systems that use discrete signals, as opposed to signaling systems which are message-oriented where each message is able to convey multiple items of formation.
Subscriber versus trunk signaling
Subscriber signaling refers to the signaling between the telephone and the telephone exchange. Trunk signaling is the signaling between exchanges.
Examples
Every signaling system can be characterized along each of the above axes of classification. A few examples:
DTMF is an in-band, channel-associated register signaling system. It is not compelled.
SS7 is an out-of-band, common-channel signaling system that incorporates both line and register signaling.
Metering pulses are out-of-band and channel-associated. They are generally regarded as line signaling, although this is open to debate.
E and M signaling is an out-of-band channel-associated signaling system. The base system is intended for line signaling, but if decadic pulses are used it can also convey register information. E&M line signaling is however usually paired with DTMF register signaling.
By contrast, the L1 signaling system is an in-band channel-associated signaling system as was the SF 2600 hertz 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.
