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What is Signalling System No. 7 (SS7) ?

Module by: Lekulana Kolobe. E-mail the author

Summary: This module breifly introduces signalling system no 7 and its usefulness in telecommunications networks.

Signalling System 7(SS7) is the dedicated, robust, and independent data network that allows exchanges (switches) in a PSTN network to echange messages about call setup, management, and tear down amogst themselves. SS7, together with Internet Call Processing model are the two commonly used call processing models used in signalling. The SS7 network is independent from the PSTN network and it is overlayed over the PSTN network. Its composed of high speed data links, and is very reliable.

The SS7 network comprises of Signalling Points, which are the actual gateways to the PSTN network. SS7 provides a universal structure for telephony network signalling, messaging, interfacing, and network maintenance. It deals with establishment of a call, exchanging user information, call routing, different billing structures, and supports Intelligent network (IN) services.

SS7 clearly splits the signalling planes and voice circuits. An SS7 network has to be made up of SS7 capable equipment from end to end in order to provide its full functionality. The network is made up of several link types (A, B, C, E, and F) and three signalling nodes - Service switching points (SSPs), signal transfer point (STPs), and service control point (SCPs). Each node is identifed on the network by a number, a point code. Extended services are provided by a database interface at the SCP level using X.25. The links between nodes are full-duplex 56 kbps and/or 64 kbps. In Europe they are usually timeslots (DS0s) within an E1 trunk. In contrast to the US, trunks with signalling links usually also carry bearer channels, called associated signalling. In the US, SS7 links are usually carried over a network that is separated from the bearer channels, called nonassociated signaling. Quasi-associated signalling is similar to nonassociated signaling with a logical separation, but uses a smaller number of dedicated STPs to handle the signalling path. In terms of ayering it only has four layers like TCP/IP as opposed to seven layers of the OSI model.


Hanrahan H. Integrated Digital Communications. School of Electrical and Information Engineering. University of Witwatersrand, Johannesburg. 2006.

Wikipedia 2006, Last Accessed on 13 March 2006

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