Signalling System No. 7 - History

History

Common Channel Signaling protocols have been developed by major telephone companies and the ITU-T since 1975; the first international Common Channel Signaling protocol was defined by the ITU-T as Signalling System No. 6 (SS6) in 1977. Signalling System No. 7 was defined as an international standard by ITU-T in its 1980 (Yellow Book) Q.7XX-series recommendations. SS7 was designed to replace SS6, which had a restricted 28-bit signal unit that was both limited in function and not amenable to digital systems. SS7 has substantially replaced SS6, Signalling System No. 5 (SS5), R1 and R2, with the exception that R1 and R2 variants are still used in numerous nations.

SS5 and earlier systems used in-band signaling, in which the call-setup information was sent by playing special multi-frequency tones into the telephone lines, known as bearer channels in the parlance of the telecom industry. This led to security problems with blue boxes. SS6 and SS7 implement out-of-band signaling protocols, carried in a separate signaling channel, explicitly keep the end-user's audio path—the so-called speech path—separate from the signaling phase to eliminate the possibility that end users may introduce tones that would be mistaken for those used for signaling. See falsing. SS6 and SS7 are referred to as so-called Common Channel Interoffice Signalling Systems (CCIS) or Common Channel Signaling (CCS) due to their hard separation of signaling and bearer channels. This required a separate channel dedicated solely to signaling, but the greater speed of signaling decreased the holding time of the bearer channels, and the number of available channels was rapidly increasing anyway at the time SS7 was implemented.

The common channel signaling paradigm was translated to IP via the SIGTRAN protocols as defined by the IETF. 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.