Signalling System No. 7 (SS7) is a set of telephony signaling protocols developed in 1975, which is used to set up and tear down most of the world's public switched telephone network (PSTN) telephone calls. It also performs number translation, local number portability, prepaid billing, Short Message Service (SMS), and other mass market services.
In North America it is often referred to as CCSS7, abbreviated for Common Channel Signalling System 7. In the United Kingdom, it is called C7 (CCITT number 7), number 7 and CCIS7 (Common Channel Interoffice Signaling 7). In Germany, it is often called N7 (Signalisierungssystem Nummer 7).
The only international SS7 protocol is defined by ITU-T's Q.700-series recommendations in 1988. Of the many national variants of the SS7 protocols, most are based on variants of the international protocol as standardized by ANSI and ETSI. National variants with striking characteristics are the Chinese and Japanese (TTC) national variants.
The Internet Engineering Task Force (IETF) has defined level 2, 3, and 4 protocols compatible with SS7 which use the Stream Control Transmission Protocol (SCTP) transport mechanism. This suite of protocols is called SIGTRAN.