Brief Description :
Signaling System #7 (SS7) is a set of telephony signaling protocols which are used to set up most of the world's public switched telephone network telephone calls. The main purpose is to set up and tear down telephone calls.
Other uses include number translation, prepaid billing mechanisms, short message service (SMS), and a variety of other mass market services.It is usually abbreviated to SS7 though in North America it is often referred to as CCSS7, an acronym for "Common Channel Signaling System 7".
In some European countries, specifically the United Kingdom, it is sometimes called C7 (CCITT number 7) and is also known as number 7 and CCIS7. (ITU-T was formerly known as CCITT.)There is only one international SS7 protocol defined by ITU-T in its Q.700-series recommendations.
There are however, many national variants of the SS7 protocols. Most national variants are based on two widely deployed national variants as standardized by ANSI and ETSI, which are in turn based on the international protocol defined by ITU-T. Each national variant has its own unique characteristics. Some national variants with rather striking characteristics are the China (PRC) and Japan (TTC) national variants.
The Internet Engineering Task Force (IETF) has also defined level 2, 3, and 4 protocols that are compatible with SS7 MTP2 (M2UA and M2PA) MTP3 (M3UA) and SCCP (SUA), but use an SCTP transport mechanism. This suite of protocols is called SIGTRAN.
History :
Common Channel Signaling protocols have been developed by AT&T, BT and the ITU-T since 1975 and the first international Common Channel Signaling protocol was defined by the ITU-T as Signalling System No. 6 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 Signalling System No. 6, 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, SS5, R1 and R2, with the exception that R1 and R2 variants are still used in numerous nations.
SS5 and earlier used in-band signaling, where 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. Modern designs of telephone equipment that implement out-of-band signaling protocols 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.
SS6 and SS7 moved to a system in which the signaling information was out-of-band, carried in a separate signaling channel. This avoided the security problems earlier systems had, as the end user had no connection to these channels. 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.
