A major advantage of using the IAX2 protocol is that it is designed to be more friendly to working within odd network configurations, especially working behind NAT. This makes it a fantastic protocol for softphone clients since they are often utilized on laptops that roam into many different networks, often with no control of the network itself (such as when traveling between hotel networks). The Inter-Asterisk eXchange (IAX) protocol is usually used for server-to-server com- munication; more hard phones are available that talk SIP. However, there are several softphones that support the IAX protocol, and work is progressing on several fronts for hard phone support in firmware. The primary difference between the IAX and SIP protocols is the way media (your voice) is passed between endpoints. With SIP, the RTP (media) traffic is passed using different ports than those used by the signaling methods. For example, Asterisk receives the signaling of SIP on port 5060 and the RTP (media) traffic on ports 10,000 through 20,000 by default. The IAX protocol differs in that both the signaling and media traffic are passed via a single port: 4569. An advantage to this approach is that the IAX protocol tends to be better suited to topologies involving NAT. There exist many IAX-based softphones, but not so many hardware based phones. The most pronounced reason is because IAX2 is not yet an IETF standard, yet many people have become early adopters and have reaped the benefits. An excellent IAX2 softphone is idefisk, available at http://www.asteriskguru.com‡ for free download. The authors have had excellent results with this softphone, and since it runs on Microsoft Windows, Mac OS X, and Linux, it is an excellent cross-platform softphone to write about. We will be demonstrating version 1.31 in this book, although version 2.0 was recently released (April 2007) but is not yet available for Linux. |
telephony