VoIP Protocols - https://www.chinaroby.com

 

VoIP Protocols

The mechanism for carrying a VoIP connection generally involves a series of signaling transactions between the endpoints (and gateways in between), culminating in two persistent media streams (one for each direction) that carry the actual conversation. There are several protocols in existence to handle this. In this section, we will discuss some of those that are important to VoIP in general and to Asterisk specifically.

8.2.1. IAX (The "Inter-Asterisk eXchange" Protocol)

The test of your Asterisk-ness comes when you have to pronounce the name of this protocol. Newbies say "eye-ay-ex"; those in the know say "eeks." IAX[*] is an open protocol, meaning that anyone can download and develop for it, but it is not yet a standard of any kind.

[*] Officially, the current version is IAX2, but all support for IAX1 has been dropped, so whether you say "IAX" or "IAX2," it is expected that you are talking about the Version 2.

In Asterisk, IAX is supported by the chan_iax2.so module.

8.2.1.1. History

The IAX protocol was developed by Digium for the purpose of communicating with other Asterisk servers (hence "the Inter-Asterisk eXchange protocol"). IAX is a transport protocol (much like SIP) that uses a single UDP port (4569) for both the channel signaling and Realtime Transport Protocol (RTP) streams. As discussed below, this makes it easier to firewall and more likely to work behind NAT.

IAX also has the unique ability to trunk multiple sessions into one dataflow, which can be a tremendous bandwidth advantage when sending a lot of simultaneous channels to a remote box. Trunking allows multiple data streams to be represented with a single datagram header, to lower the overhead associated with individual channels. This helps to lower latency and reduce the processing power and bandwidth required, allowing the protocol to scale much more easily with a large number of active channels between endpoints.

8.2.1.2. Future

Since IAX was optimized for voice, it has received some criticism for not better supporting videobut in fact, IAX holds the potential to carry pretty much any media stream desired. Because it is an open protocol, future media types are certain to be incorporated as the community desires them.

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