Signalling System 7
| SS7 protocols by OSI layer | |
| Application | TCAP, CAP, ISUP, ... |
|---|---|
| Network | MTP Level 3 + SCCP |
| Data link | MTP Level 2 |
| Physical | MTP Level 1 |
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 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 (SCTP) transport mechanism.
SS5 and earlier systems used in-band signaling, in which the call-setup information was sent by playing special multi-frequency tones into the telephone lines, known as bearer channels. As the bearer channel was directly accessible by users, it was exploited with devices such as the blue box, which played the tones required for call control and routing. As a remedy, SS6 and SS7 implemented out-of-band signaling, carried in a separate signaling channel, thus keeping the speech path separate. SS6 and SS7 are referred to as common-channel signaling (CCS) protocols, or Common Channel Interoffice Signalling (CCIS) systems.
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