1.4 The Signaling System Number 7
For establishing, maintaining and clearing a connection, signaling information needs to be exchanged between the end user and network devices. In the fixed-line network, analog phones signal their connection request when the receiver is lifted off the hook and by dialing a phone number that is sent to the network either via pulses (pulse dialing) or via tone dialing, which is called dual tone multifrequency (DTMF) dialing. With fixed-line ISDN phones and GSM mobile phones, the signaling is done via a separate dedicated signaling channel, and information such as the destination phone number is sent as messages.
If several components in the network are involved in the call establishment, for example, if originating and terminating parties are not connected to the same switching center, it is also necessary that the different nodes in the network exchange information with each other. This signaling is transparent for the user, and a protocol called the signaling system number 7 (SS-7) is used for this purpose. SS-7 is also used in GSM networks and the standard has been enhanced by ETSI to fulfill the special requirements of mobile networks, for example, subscriber mobility management.
The SS-7 standard defines three basic types of network nodes: