Signaling System #7 (SS7)

Signaling System #7 (SS7 or C7) is the signaling protocol used in the public switched telephone network (PSTN) around the world. It is specified in a series of ITU-T Recommendations published in the 1980s and 1990s.

SS7 is a message-oriented protocol, wherein the messages travel independently of the voice and data paths. The messages are referred to as packets. The packets travel through various nodes in an SS7 network, including the Service Switching Point (SSP), Signal Transfer Point (STP), and Service Control Point (SCP). The bearer path is (generally) separate from the SS7 signaling path. The bearer is comprised of 56 or 64Kbps digital circuit or link.

SS7 opened a new door for providing a range of new services in the PSTN, including number portability and "800" (or "freephone") services.

The various ITU-T standards related to SS7 include:

Q.700-Q.709 - Message Transfer Part (MTP)
Q.710 - PBX Application
Q.711 - Q.716 - Signaling Connection Control Part (SCCP)
Q.721 - Q.725 - Telephone User Part (TUP)
Q.730 - ISDN Supplementary Services
Q.741 - Data User Part (DUP)
Q.761 - Q.766 - ISDN User Part (ISUP)
Q.771 - Q.775 - Transaction Capabilities Application Part (TCAP)
Q.791 - Q.795 - Monitoring, Operations, and Maintenance
Q.780 - Q.783 - Test Specifications

Resources: Performance Technologies' SS7 Tutorial, IEC's SS7 Tutorial