IAX

IAX is the Inter-Asterisk eXchange protocol used by Asterisk, an Open Source PBX server from Digium. It is used to enable VoIP connections between Asterisk servers, and between servers and clients that also use the IAX protocol.

IAX now most commonly refers to IAX2, the second version of the IAX protocol. The original IAX protocol has been deprecated almost universally in favor of IAX2.

Basic properties

IAX2 is very robust and full-featured yet simple as far as protocols go. It is agnostic to codecs and number of streams, meaning that it can be used as a transport for virtually any type of data. (This capability will be useful as videophones become common.)

IAX2 uses a single UDP data stream (usually on port 4569) to communicate between endpoints, both for signaling and data. The voice traffic is transmitted in-band, making IAX2 easier to firewall and more likely to work behind network address translation. (This is in contrast to SIP, which uses an out-of-band RTP stream to deliver information.)

IAX2 supports trunking, wherein a single link carries data and signaling for multiple channels. When trunking, data from multiple calls are merged into a single set of packets, meaning that one IP datagram can deliver information for more than one call, reducing the effective IP overhead without creating additional latency. This is a big advantage for VoIP users, where IP headers are large percentage of the bandwidth usage.