What is SS7?

Common Channel Signalling System No. 7 (i.e., SS7 or C7) is a global standard for telecommunications defined by the International Telecommunication Union (ITU) Telecommunication Standardization Sector (ITU-T). Signalling System 7 was introduced by AT andT in 1975 and approved by worldwide standards bodies in 1980. The standards define the procedures and protocol by which network elements in the public switched telephone network (PSTN) exchange information over a digital signalling network to effect wireless (cellular) and wireline call setup, routing and control as well as network management and maintainence. SS7 uses out-of-band signalling i.e. it uses a separate digital channel for the exchange of signalling information

The SS7 network and protocol can be used for:

>Basic call setup, management, and tear down
>Wireless services such as personal communications services (PCS), wireless roaming, and mobile subscriber authentication
>Local number portability (LNP)
>Toll-free (800/888) and toll (900) wireline services
>Enhanced call features such as call forwarding, calling party name/number display, and three-way calling
>Efficient and secure worldwide telecommunications

SS7 messages are exchanged between network elements over 56 or 64 kilobit per second (kbps) bi-directional channels called signalling links. Signalling occurs out-of-band on dedicated channels rather than in-band on voice channels.

The networks consist of three types of network elements:

SSP: Signal Switching Points are telephone exchanges equipped with SS7 capable software. They are responsible for originating, terminating or switching calls. SSP can be an origination or destination point for signalling messages. Exp. MSC etc.

STP: Signal Transfer Points are packet switches that receive and route signalling messages to their proper destination. Exp. STP, SSCP etc.

SCP: Signalling Control Point is a database that provides information necessary for advanced call-processing capabilities. Exp. IN, HLR