IAX
IAX is the Inter-Asterisk Exchange protocol that establishes connections between clients and Asterisk servers or between two Asterisk PBX units. The current version is IAX2. Asterisk is a free open source call server (IP PBX) licensed under the GNU General Public License (GNU GPL). Users can run Asterisk on Linux, Solaris, FreeBSD, and Windows. You need special hardware (a PCI expansion card) for interfacing phone lines and T1 lines. Asterisk also supports SIP and H.323 and works with SIP hard phones.
IAX works much like SIP, but it is designed to address the problem that SIP has with Network Address Translation (NAT) by sending both session setup/teardown and the voice message over one User Datagram Protocol (UDP) stream. It also uses less bandwidth because it combines the data from more than one call into one group of packets.
IAX2 usually operates on port 4569 (the first version of IAX used port 5036), and can handle streaming video as well as voice. It also supports RSA public key cryptography to authenticate users.
Other proprietary protocolsOther vendors have developed their own protocols. Nortel created UNIStim for handling the communications between their own IP phones and Nortel Call Servers. Nortel has some low-cost (under $500) PBXs for small businesses that rely on the UNIStim protocol. Mitel uses a proprietary protocol called MiNet (its products also support SIP) for call signaling. The Mitel protocol provides extra security, with traffic going into the Mitel controllers encrypted with Secure Sockets Layer (SSL) and Secure Shell.
SummaryAlthough SIP and H.323 are the best known VoIP protocols, they are by no means the only ones. One of the biggest challenges to widespread adoption of VoIP today is the lack of standardization. The fact that many VoIP vendors use proprietary protocols, resulting in a confusing array of products that don't interoperate with one another and a maze of protocols to choose from when planning a VoIP deployment, reflects this lack of standardization.
There are some advantages to proprietary protocols: Those that are closed source can provide a kind of "security through obscurity" that open standard protocols don't enjoy. Vendors can build in features to address specific problems, as IAX has done to make it easier for VoIP to work through firewalls and overcome incompatibilities with NAT devices. Manufacturers can enhance performance, as Cisco has done with Skinny, and use unconventional models, as Skype has done with the peer-to-peer concept.
On the other hand, many in the industry are in favor of standardization, and while it appears that SIP is emerging as the winner of the VoIP protocol race, there are still a number of proprietary protocols in use. Be familiar with them and be well equipped to make the best choice for your own VoIP implementation.
Play video
Nice article. I would enjoy seeing some sample calls, i.e. packet captures using these protocols, particularly the non-propietary ones and the sccp and iax.
Posted by Steve on Saturday, May 19 2007 05:55 AM