GSM: NSS (Network Subsystem) Interfaces

To enable GSM customers to use speech-, fax- and data services wherever they are within the network, information and data must be exchanged within as well as between different networks. This process is called signalling. To set up a call, signalling takes place between subscriber interface points, that is between the user and the network, as well as between different network elements. To make sure that the individual elements through which the information travels can understand each other, they must, as it were, agree on a common official language. This language is specified by protocols. The protocol used in the Network Subsystem NSS is called Signalling System No 7 - or SS7. SS7 is based on the Open System Interconnection model, also called the OSI reference model.