Editing Signaling (telecommunications) (section)

===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.