Editing Signaling (telecommunications) (section)

==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.