In telecommunication, signaling (signalling in British spelling) has the following meanings:
Signaling systems can be classified according to their principal properties, some of which are described below:
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In the public switched telephone network (PSTN), in-band signaling is the exchange of call control information within the same channel 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 channel that is dedicated for the purpose and separate from the channels used for the telephone call. Out-of-band signaling is used in Signaling System 7 (SS7), the standard for signaling among exchanges that has controlled most of the world's phone calls for some twenty years.
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 current (alerting), and recall. In the middle 20th Century, supervision signals on long distance 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 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 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 signaling is by DTMF.
Channel Associated Signaling (CAS) employs a signaling channel which is dedicated to a specific bearer channel.
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.
Compelled signaling is 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 signaling are compelled (see R2 signaling), while R1 multi-frequency signaling is not.
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 information (e.g. multiple digits of the called telephone number).
Subscriber signaling is between the telephone and the telephone exchange. Trunk signaling is signaling between exchanges.
Note that 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.