Signalling System No. 7

Signalling System No. 7 (SS7) is a set of telephony signaling protocols developed in the 1970s that is used to set up and tear down telephone calls on most parts of the global public switched telephone network (PSTN). The protocol also performs number translation, local number portability, prepaid billing, Short Message Service (SMS), and other services.

The protocol was introduced in the Bell System in the United States by the name Common Channel Interoffice Signaling in the 1970s for signalling between No. 4E SS switch and No. 4A crossbar toll offices.^[1][2] The SS7 protocol is defined for international use by the Q.700-series recommendations of 1988 by the ITU-T.^[3] Of the many national variants of the SS7 protocols, most are based on variants standardized by the American National Standards Institute (ANSI) and the European Telecommunications Standards Institute (ETSI). National variants with striking characteristics are the Chinese and Japanese Telecommunication Technology Committee (TTC) national variants.

SS7 has been shown to have several security vulnerabilities, allowing location tracking of callers, interception of voice data, intercept two-factor authentication keys, and possibly the delivery of spyware to phones.^[4]

The Internet Engineering Task Force (IETF) has defined the SIGTRAN protocol suite that implements levels 2, 3, and 4 protocols compatible with SS7. Sometimes also called Pseudo SS7, it is layered on the Stream Control Transmission Protocol (SCTP) transport mechanism for use on Internet Protocol networks, such as the Internet.

In North America, SS7 is also often referred to as Common Channel Signaling System 7 (CCSS7) (or CCS7). In the United Kingdom, it is called C7 (CCITT number 7), number 7 and Common Channel Interoffice Signaling 7 (CCIS7). In Germany, it is often called Zentraler Zeichengabekanal Nummer 7 (ZZK-7).