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.
Signaling systems may be classified based on several principal characteristics.
In the public switched telephone network (PSTN), in-band signaling is the exchange of call control information within the same physical channel, or within the same frequency band, 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 dedicated channel separate from that used for the telephone call. Out-of-band signaling has been used since Signaling System No. 6 (SS6) was introduced in the 1970s, and also in Signalling System No. 7 (SS7) in 1980 which became the standard for signaling among exchanges ever since.
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, and hook flash. In the middle 20th 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.
Common-channel signaling (CCS) employs a signaling channel which conveys signaling information relating to multiple bearer channels. These bearer channels therefore have their signaling channel in common.
The term is only relevant in the case of signaling systems that use discrete signals (e.g. a combination of tones to denote one digit), as opposed to signaling systems which are message-oriented (such as SS7 and ISDN Q.931) where each message is able to convey multiple items of formation (e.g. multiple digits of the called telephone number).
Subscriber signaling refers to the signaling between the telephone and the telephone exchange. Trunk signaling is the signaling between exchanges.
Every signaling system can be characterized along each of the above axes of classification. A few examples:
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.