Switching to VoIP

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Chapter 7: Replacing Call Signaling with VoIP

signaling families is that IAX doesn’t implement RTP as the packetizing mechanism.

Instead, IAX has its own way of packaging encoded voice.

IAX is also NAT-proof, so dozens or hundreds of simultaneous calls from behind a

masquerading firewall will function correctly, just as HTTP does.

IAX is implemented in a far-simpler and less application-exhaustive manner than SIP

and H.323. It is really intended just for telephony applications, while H.323 and

especially SIP, include far more extensibility. IAX is therefore much more compact;

complete implementations have been done with as little as 64 kb of object code.

While a complete cycle of registration, call signaling, voice transmission, and tear-

down can use several TCP and UDP ports and connections with SIP or H.323, IAX

handles all of these functions using a single UDP port. When the IAX client (end-

point) registers with the IAX server or proxy, this UDP port is utilized. When a call is

placed, this same port is utilized. When voice transmission occurs, this port is uti-

lized once again. The way IAX distinguishes between registration, signaling, and

voice packets is by including headers and metadata in each packet that defines what

the packet’s purpose is and whether it has a payload ...