Switching to VoIP

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Chapter 7

CHAPTER 7

Replacing Call Signaling with VoIP

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 sig-

naling. 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 repli-

cated 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 net-

work; it also describes the ways these standards compete with and complement one

another.

VoIP Signaling Protocols

A signaling protocol is a common language spoken by telephones and call-manage-

ment servers, the PSTN, and legacy