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Intelecommunications,signaling is the use ofsignals for controllingcommunications. This may constitute an information exchange concerning the establishment and control of atelecommunication circuit and the management of the network.
Signaling systems may be classified based on several principal characteristics.
In thepublic 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 message (the callers' voice) is using. An example isdual-tone multi-frequency signaling (DTMF), which is used on mosttelephone lines to customer premises.
Out-of-band signaling is telecommunication signaling on a dedicatedchannel separate from that used for the message. Out-of-band signaling has been used sinceSignaling System No. 6 (SS6) was introduced in the 1970s, and also inSignalling System No. 7 (SS7) in 1980 which became the standard for signaling among exchanges internationally.[1][2]
In the mid-20th century, supervision signals onlong-distance trunks in North America were primarily in-band, for example at2600 Hz, necessitating anotch filter to prevent interference. Late in the century, all supervisory signals had been moved out of band. With the advent ofdigital trunks, supervision signals are carried byrobbed bits or other bits in theE1-carrier dedicated to signaling.
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 assupervision),ringing, andhook flash.Register signaling is concerned with conveying addressing information, such as the calling and/or calledtelephone number. In the early days oftelephony, withoperator 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 arotary dial, which rapidly breaks the line current intopulses, with the number of pulses conveying the address. Finally, starting in the second half of the century, address signaling is byDTMF.
Channel-associated signaling (CAS) employs a signaling channel that is dedicated to a specificbearer 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 refers to signaling where the receipt of each signal from an originating register needs to be explicitly acknowledged before the next signal can be sent.[3]
Most forms ofR2 register signaling are compelled, while R1multi-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 formation (e.g. multiple digits of the called telephone number).
Subscriber signaling refers to the signaling between the telephone and thetelephone 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.
Asignaling protocol is a type of communications protocol forencapsulating the signaling between communication endpoints and switching systems to establish or terminate a connection and to identify the state of connection.
The following is a list of signaling protocols:
This article incorporatespublic domain material fromFederal Standard 1037C.General Services Administration. Archived fromthe original on 2022-01-22. (in support ofMIL-STD-188).
