telephony Telephony ( ) is the field of technology involving the development, application, and deployment of telecommunication services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is i ...

, a service switching point (SSP) is the

telephone exchange telephone exchange, telephone switch, or central office is a telecommunications system used in the public switched telephone network (PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital syste ...

that initially responds, when a telephone caller dials a number, by sending a query to a central database called a service control point (SCP) so that the call can be handled. The service switching point uses the

Signalling System No. 7 Signalling System No. 7 (SS7) is a set of telephony signaling protocols developed in 1975, which is used to set up and tear down telephone calls in most parts of the world-wide public switched telephone network (PSTN). The protocol also perform ...

(SS7) protocols which are responsible for the call setup, management, and termination with other service switching points.

Relationship between SSP and SCP

With the introduction of the

Intelligent Network The Intelligent Network (IN) is the standard network architecture specified in the ITU-T Q.1200 series recommendations. It is intended for fixed as well as Global System for Mobile Communications, mobile telecommunication, telecom networks. It al ...

architecture, service functionality (e.g. UK translation of 0800 non-geographic telephone numbers) is being removed from the actual telephone exchange and devolved out into other computer nodes. In this new architecture, the telephone exchange is known as an SSP and the node that contains the services (and hence ''controls'' the progression of a call) is known as service control point (SCP).

Example of a 0800 number translation service

In the UK 08XXX numbers are non-geographic numbers – that is, the number does not refer to a telephone number in any particular region of the UK. To route a call to such an 08XXX number, the number must be translated into a geographic number (e.g. 0121 XXX XXXX for

Birmingham Birmingham ( ) is a city and metropolitan borough in the metropolitan county of West Midlands in England. It is the second-largest city in the United Kingdom with a population of 1.145 million in the city proper, 2.92 million in the We ...

numbers). An SSP telephone exchange receives a call to an 0800 number. This causes a trigger within the SSP that causes an SCP (Service Control Point) to be queried using SS7 protocols (

INAP INAP stands for Intelligent Network Application Protocol or Intelligent Network Application Part. It is the signalling protocol used in Intelligent Networking (IN). It is part of the Signalling System No. 7 (SS7) protocol suite, typically layer ...

, TCAP). The SCP responds with a geographic number, e.g. 0121 XXX XXXX, and the call is actually routed to a phone. By this architecture: *08XXX (non-geographic numbers) can be set up in a few SCP nodes rather than having to be set up in every telephone exchange in the country. *geographic numbers can be hidden *revenue can be generated by non-telecoms companies from people making telephone calls to services – e.g. telephone voting

External links

SS7 Tutorial
– Signalling architecture, including SSP description. ink Dead Signaling System 7 {{telephony-stub