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Signaling System No.7 (SS7) is a telecommunications signaling architecture traditionally used for the set up and tear down of telephone calls. It has a robust protocol stack that uses out-of-band signaling to communicate between elements of the public switched telephone network (PSTN). In recent years it has been superseded by the Diameter signaling protocol on all-IP networks.
Signaling System No.7 is also known as Common Channel Signaling System 7 (CCSS7), Common Channel Interoffice Signaling 7 (CCIS7), CCITT Number 7 (C7) or simply Number 7.
In the past all communication between elements of the PSTN took place on the same channel as telephone conversations. This was called “in-band signaling.” It became a problem when curious pranksters (including Steve Wozniak and Steve Jobs) discovered that they could emulate telecom signals and explore secret telecom codes using something called a “blue box.”
The solution was to move internal phone company communication over to a channel that was not available to the average phone user. SS7 was the result of this effort to develop “out-of-band signaling.” The use of common-channel signaling (CCS) allowed providers to set up and tear down calls without interference from the hacking public.
Over time, SS7 evolved into a powerful set of protocols that could do more than make calls. Functions such as call waiting, conference calling, call forwarding and voice mail were added as either standard or premium features. Eventually SS7 enabled telecom companies to offer a rich array of call-related services. Billing, number translation, short message service and prepaid functions are among the capabilities available.
The SS7 protocol stack can be compared to the OSI model. The physical, data link and network layers of the OSI model match Message Transfer Part (MTP) levels one to three in the SS7 stack. Signaling Connection Control Part (SCCP) is on layer four, like OSI’s transport layer. Transaction Capabilities Application Part (TCAP) can be compared to OSI layers five and six, and Mobile Application Part (MAP) and Intelligent Network Application Part (INAP) sit at the uppermost layer of the SS7 architecture.
In recent years, hackers have found ways to exploit SS7 vulnerabilities. Experts have been warning about potential weaknesses in the protocol architecture for years. In 2017, a mobile phone provider in Germany confirmed that hackers were able to siphon money from bank customers through an SS7 exploit.