In Chapter 6, the process of transmitting voice sounds in virtual media channels, via codecs, was presented. In order for those media channels to be set up, monitored , and destroyed when needed, a PBX calls on signaling. Different methods are used for different kinds of endpoints and trunks. On the PSTN, the SS7 network handles signaling. On a POTS voice channel, the signaling is accomplished using analog FXS signaling.
SS7, FXS, and the dozens of other signaling technologies in use on the PSTN, though all signaling protocols, are outside the realm of VoIP. They could all be considered legacy technology, since just about all of their signaling functions have been replicated using several new, modern, open TCP/IP-centric standards. Even though SS7 is a packet-based protocol and there are attempts underway to make it compatible with VoIP softPBX systems (Asterisk included), its roots are in the PSTN, not the Internet.
This chapter describes the standards for call signaling in a softPBX-based VoIP network; it also describes the ways these standards compete with and complement one another.