Chapter 6. PSTN and TDM

SIP dominates much of the telephony landscape in VoIP. The SIP protocol is also probably the most covered in FreeSWITCH books, documentation, and tutorials. Despite this apparent hegemony of the SIP protocol, there are a significant number of protocols that are used in the PSTN (public switched telephony network) and in other private and public telephony networks. Some of those protocols do not even run over IP, but use rudimentary analog audio signals to setup a call. (Yikes! This sounds almost as archaic as sending smoke signals!) One of the great powers of FreeSWITCH comes from its flexibility to interconnect different telephony protocols, from bleeding edge new media and signaling protocols such as WebRTC/Verto, to protocols as old as FXO/FXS and MFC-R2. In a sense, FreeSWITCH is a powerful translator that knows many different languages and is able to translate from one language to another quickly. These different languages are the protocols, and there...