Signaling System 7

Signaling System Number 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 as Signaling System No. 7, Signaling System #7, or just SS7. 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.^[1] There are however, many national variants of the SS7 protocols. Most national variants are based on two widely deployed national variants as standardized by ANSIETSI, 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. and

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.

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