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Asterisk: The Future of Telephony by Leif Madsen, Jared Smith, Jim Van Meggelen

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VoIP Protocols

The mechanism for carrying a VoIP connection generally involves a series of signaling transactions between the endpoints (and gateways in between), culminating in two persistent media streams (one for each direction) that carry the actual conversation. There are several protocols in existence to handle this. In this section, we will discuss some of those that are important to VoIP in general and to Asterisk specifically.

IAX (The “Inter-Asterisk eXchange” Protocol)

The test of your Asterisk-ness comes when you have to pronounce the name of this protocol. Newbies say “eye-ay-ex”; those in the know say “eeks.” IAX[58] is an open protocol, meaning that anyone can download and develop for it, but it is not yet a standard of any kind.

In Asterisk, IAX is supported by the chan_iax2.so module.

History

The IAX protocol was developed by Digium for the purpose of communicating with other Asterisk servers (hence “the Inter-Asterisk eXchange protocol”). IAX is a transport protocol (much like SIP) that uses a single UDP port (4569) for both the channel signaling and Realtime Transport Protocol (RTP) streams. As discussed below, this makes it easier to firewall and more likely to work behind NAT.

IAX also has the unique ability to trunk multiple sessions into one dataflow, which can be a tremendous bandwidth advantage when sending a lot of simultaneous channels to a remote box. Trunking allows multiple data streams to be represented with a single datagram header, to lower the overhead ...

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