Editing Signaling (telecommunications)

{{Short description|Exchange of information required to set up a telecommunications connection}}
{{refimprove|date=January 2008}}
{{Use American English|date=September 2020}}

In [[telecommunications]], '''signaling''' is the use of [[signal]]s for controlling [[communication]]s. This may constitute an information exchange concerning the establishment and control of a [[telecommunication circuit]] and the management of the network.

==Classification==
Signaling systems may be classified based on several principal characteristics.

===In-band and out-of-band signaling===
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 message (the callers' voice) is using. An example is [[dual-tone multi-frequency signaling]] (DTMF), which is used on most [[telephone line]]s to customer premises.

'''Out-of-band signaling''' is telecommunication signaling on a dedicated [[Communication channel|channel]] separate from that used for the message. 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 internationally.<ref>{{cite web|url=https://www.pcmag.com/encyclopedia_term/0,2542,t=signaling+inout-of-band&i=51333,00.asp|title=Signaling in/out-of-band Definition|publisher=PC Magazine Encyclopedia|access-date=11 April 2012}}</ref><ref>{{cite book|author=Annabel Z. Dodd|title=The Essential Guide to Telecommunications|edition=3rd|url=https://books.google.com/books?id=H4POZ66FaS0C&q=SS6+SS7&pg=PA219|publisher=[[Prentice Hall PTR]]|isbn=0-13-064907-4|year=2002|page=219}}</ref>

In the mid-20th century, supervision signals on [[Long-distance calling|long-distance]] trunks in North America were primarily in-band, 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 [[E-carrier|digital trunks]], supervision signals are carried by [[Robbed-bit signaling|robbed bit]]s or other bits in the [[E1-carrier]] dedicated to signaling.

===Line versus register 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 as ''[[Supervision (telephony)|supervision]]''), [[Ringing (telephony)|ringing]], and [[hook flash]].  
[[Register signaling]] is concerned with conveying addressing information, such as the calling and/or called [[telephone number]]. In the early days of [[telephony]], with [[Switchboard operator|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 [[Pulse dialing|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===
[[Channel-associated signaling]] (CAS) employs a signaling channel that is dedicated to a specific [[B channel|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===
[[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.<ref>{{cite web|url=http://www.its.bldrdoc.gov/fs-1037/dir-008/_1159.htm|title=compelled signaling|publisher=National Telecommunications and Information Administration|access-date=11 April 2012}}</ref>

Most forms of [[R2 signaling|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 (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 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.
*[[Signalling System No. 7|SS7]] (e.g., TUP or ISUP) is an out-of-band, common-channel signaling system that incorporates both line and register signaling.
*[[Metering pulse]]s (depending on the country, these are 50&nbsp;Hz, 12&nbsp;kHz or 16&nbsp;kHz 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 3400&nbsp;Hz) and channel-associated. They are generally regarded as line signaling, although this is open to debate.
*[[E and M signaling]] (E&M) 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 (which typically employs a 2280&nbsp;Hz tone of various durations) 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.

==Protocols==
A '''signaling protocol''' is a type of communications protocol for [[encapsulation (networking)|encapsulating]] 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:
* [[ALOHAnet|ALOHA]]
* [[Digital Subscriber System No. 1]] (EDSS1)
* [[Dual-tone multi-frequency signaling]]
* [[H.248]]
* [[H.323]]
* [[H.225.0]]
* [[Jingle (protocol)|Jingle]]
* [[Media Gateway Control Protocol]] (MGCP)
* [[Megaco]]
* [[Multi-frequency|Regional System R1]]
* [[NBAP|NBAP (Node B Application Part)]]
* [[Signalling System R2]]
* [[Session Initiation Protocol]]
* [[Signaling System No. 5]]
* [[Signaling System No. 6]]
* [[Signaling System No. 7]]
* [[Skinny Client Control Protocol]] (SCCP, ''Skinny'')
* [[Q.931]]
* [[QSIG]]

==See also==
*[[Control character]]
*[[In-band control]]
*[[Metadata]]
*[[Out-of-band control]]

==References==
{{reflist}}
*{{FS1037C MS188}}

{{telecommunications}}

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