SIP has become the standard VoIP protocol used across the world but is it better than alternatives such as Inter-Asterisk eXchange (IAX) in a trunk environment? This article highlights the benefits of using IAX (now IAX2) over SIP.
Bandwidth usage
IAX is more efficient than RTP in a trunk using any codec and for any number of calls made. This is mainly due to the fact that IAX uses binary messages instead of text messages (which SIP uses). IAX will reduce the size of the message headers as well. The result is more channels for your bandwith especially when using G.729 compression.
Avoiding Service Providers blocking VoIP
In certain countries the service providers will block SIP to prevent their clients bypassing traditional PSTN billing processes. IAX is a good way to deliver VoIP to these countries because it is comparitively less well known and uses a specific port (4569) which is not normally blocked.
NAT traversal
IAX uses port 4569 to send both signalling and audio, it multiplexes them over a single UDP. This is different to SIP which uses port 5060 and then 10000-50000 for RTP streams. Having a single port to unify audio and signalling allows an IAX client to transparently navigate NATs and to run through a network environment without having to know anything about it.
Security
The firewall administrator needs only to open port 4569 to allow IAX through. For example in a SIP environment 100 simultaneous voice calls will require 200 RTP ports and port 5060 for SIP signalling. IAX would only require one port, 4569. Security can be unsured by limiting UDP to and from the host IP address through this port.
Reliability
IAX runs very reliabily due to the separation of layer 2 and layer 3 elements which have their own clearky defined states. This means that if one end of the session terminates suddenly the call will be shut down quickly even no further audio or signalling is recieved. SIP does not have such a mechanism so its relibility from a signalling perpective is poor in comparison.
