What is SS7

SS7 is a packet based protocol suite that provides for the exchange of control and signaling information. Responsible for the routing and transport of communications between a mix of IP based and circuit switched central office exchanges, SS7 is the key enabler of the modern public switched telephone network (PSTN) around the World. Without SS7, there would be no integrated services digital network (ISDN), intelligent networks (INs), and public land mobile networks (PLMNs).

AT&T developed SS7 in 1975 and the CCITT adopted it in 1980. In North America, SS7 tends to refer to standards produced by Telcordia (formerly Bellcore) and the American National Standards Institute (ANSI) for North American networks. These standards are based on the ITU-T recommendations. The abbreviations CCS7, CS7, or C7 tend to refer to ITU-T recommendations, or non-North American implementations, of SS7.

Every telephone call placed or released that extends beyond the local exchange, requires SS7 signaling to set up and reserve dedicated network resources for the call. At the end of the call, SS7 takes action to return the resources to the network for future allocation.

Each time a cellular phone is powered up, SS7 based transactions identify, authenticate, and register the subscriber. Before a cellular call can be made, further transactions check that the cellular phone is not stolen (network dependent option) and qualify permission to place the call (for example, the subscriber may be barred from International usage). In addition, the SS7 network tracks the cellular subscriber to allow call delivery, as well as to allow a call that is already in progress to remain connected, even when the subscriber is mobile.