inter-asterisk exchange (VoIP) (IAX)

Inter-Asterisk Exchange (IAX) is a communication protocol for Asterisk-based PBXs. The IAX protocol is used to establish sessions for voice transmission in IP networks. It supports control signals and streaming media transmission and can transmit all media such as audio and video, preferably voice. The IAX protocol is used to transmit VoIP data streams between servers and VoIP adapters or VoIP phones.

IAX in the IAX2 version transmits the voice and signaling data over the same port, usually port 4569. For the transmission of voice data over IP networks, IAX2 uses the connectionless User Datagram Protocol( UDP), which has a lower overhead compared to the TCP protocol and thus operates with less delay. IAX2, unlike Session Initiation Protocol( SIP), H.323, or Media Gateway Control Protocol( MGCP), can easily pass firewalls and Network Address Translation ( NAT) routers.

IAX2 supports trunking and multiplexing of multiple channels over a single connection. Trunking combines data from multiple calls into a single stream between endpoints, reducing IP overhead without creating additional latency. This is beneficial for VoIP transmission because the IP header takes up a larger portion of the total data anyway.

IAX was originally developed for Linux, but it also runs under the operating systems FreeBSD, Mac OS X, Solaris and others. The revised IAX protocol is described in version IAX2 in RFC 5456.