5. Signage

5.1 General principles

A telephone exchange acts on requests made to it remotely, either by subscriber stations or terminals, or by remote exchanges. Establishing communications therefore requires exchanges of information between the PBX and its external environment. These exchanges of information are of different types, depending on whether the relationship is between the exchange and subscribers directly, or via a link between two exchanges. The set of procedures governing these exchanges is called telephone signalling. There are two types of signalling, commonly referred to as :

• subscriber signage ;

• network signaling.

The way these signals are transmitted has evolved...