Editing Signalling System No. 7 (section)

==History==
[[Signaling System No. 5]] and earlier systems use [[in-band signaling]], in which the call-setup information is sent by generating special [[Multi-frequency signaling|multi-frequency]] tones transmitted on the telephone line audio channels, also known as ''bearer channels''. Since the bearer channels are directly accessible by users, they can be exploited with devices such as the [[blue box]], which can replicate the tones used by the network for call control and routing. As a remedy, SS6 and SS7 implements out-of-band signaling, carried in a separate signaling channel,<ref name=Ronayne/>{{rp|141}} thus keeping the call control and speech paths separate. SS6 and SS7 are referred to as [[common-channel signaling]] (CCS) protocols, or ''Common Channel Interoffice Signaling'' (CCIS) systems.

Another element of in-band signaling addressed by SS7 is network efficiency. With in-band signaling, the voice channel is used during call setup which makes it unavailable for actual traffic. For long-distance calls, the talk path may traverse several nodes which reduces usable node capacity. With SS7, the connection is not established between the end points until all nodes on the path confirm availability. If the far end is busy, the caller gets a busy signal without consuming a voice channel.

Since 1975, CCS protocols have been developed by major telephone companies and the International Telecommunication Union Telecommunication Standardization Sector (ITU-T); in 1977 the ITU-T defined the first international CCS protocol as [[Signaling System No. 6]] (SS6).<ref name=Ronayne>{{cite book|author=Ronayne, John P|date= 1986|title= The Digital Network Introduction to Digital Communications Switching|edition=1|location= Indianapolis|publisher= Howard W. Sams & Co., Inc|isbn= 0-672-22498-4}}</ref>{{rp|145}} In its 1980 Yellow Book Q.7XX-series recommendations ITU-T defined the Signaling System No. 7 as an international standard.<ref name="q700"/> SS7 replaced SS6 with its restricted 28-bit signal unit that was both limited in function and not amenable to digital systems.<ref name=Ronayne/>{{rp|145}} SS7 also replaced [[Signaling System No. 5]] (SS5), while [[Multi-frequency signaling|R1]] and [[R2 signaling|R2]] variants are still used in numerous countries.{{citation needed|date=December 2014}}

The [[Internet Engineering Task Force]] (IETF) defined [[SIGTRAN]] protocols which translate the common channel signaling paradigm to the IP Message Transfer Part (MTP) level 2 (M2UA and M2PA), Message Transfer Part (MTP) level 3 ([[M3UA]]) and Signaling Connection Control Part (SCCP) (SUA).{{citation needed|date=January 2015}} While running on a transport based upon IP, the SIGTRAN protocols are not an SS7 variant, but simply transport existing national and international variants of SS7.<ref name="rfc2716">{{cite IETF |title=Framework Architecture for Signaling Transport|rfc=2719|publisher=[[Internet Engineering Task Force|IETF]]}}</ref>{{clarify|date=January 2015}}