Signalling System No. 7 (SS7) is a set of telephony signaling protocols developed in the 1970s that is used to setup and teardown 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 signaling between No. 4ESS switch and No. 4A crossbar toll offices. The SS7 protocol is defined for international use by the Q.700-series recommendations of 1988 by the ITU-T. 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. 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).
History Signaling System No. 5 and earlier systems use in-band signaling, in which the call-setup information is sent by generating special multi-frequency tones transmitted on the telephone line audio channels, also known as bearer channels. Since the bearer channels are directly accessible by users, they can be exploited with devices such as the blue box, which can replicate the tones used by the network for call control and routing. As a remedy, SS6 and SS7 implements out-of-band signaling, carried in a separate signaling channel, thus keeping the call control and speech paths separate. SS6 and SS7 are referred to as common-channel signaling (CCS) protocols, or Common Channel Interoffice Signaling (CCIS) systems. Another element of in-band signaling addressed by SS7 is network efficiency. With in-band signaling, the voice channel is used during call setup which makes it unavailable for actual traffic. For long-distance calls, the talk path may traverse several nodes which reduces usable node capacity. With SS7, the connection is not established between the end points until all nodes on the path confirm availability. If the far end is busy, the caller gets a busy signal without consuming a voice channel. Since 1975, CCS protocols have been developed by major telephone companies and the International Telecommunication Union Telecommunication Standardization Sector (ITU-T); in 1977 the ITU-T defined the first international CCS protocol as Signaling System No. 6 (SS6). In its 1980 Yellow Book Q.7XX-series recommendations ITU-T defined the Signaling System No. 7 as an international standard. SS7 replaced SS6 with its restricted 28-bit signal unit that was both limited in function and not amenable to digital systems. SS7 also replaced Signaling System No. 5 (SS5), while R1 and R2 variants are still used in numerous countries. The Internet Engineering Task Force (IETF) defined SIGTRAN protocols which translate the common channel signaling paradigm to the IP Message Transfer Part (MTP) level 2 (M2UA and M2PA), Message Transfer Part (MTP) level 3 (M3UA) and Signaling Connection Control Part (SCCP) (SUA). 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.
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