Aquí les dejo una interesante referencia sobre el codec G.729 de Digium.
Rather than the 64kbit/s required for a standard, uncompressed G.711 PCM audio data stream, the G.729 codec compresses the payload to 8kbit/s. Bandwidth calculations for a VoIP call should consider signaling and packet overhead as well, which varies according to network topology. In a typical Ethernet environment and utilizing the SIP or IAX signaling protocols, a G.711 call will consume about 87.2kbit/s while a typical G.729 compressed call will consume about 31.2kbit/s.
A practical example is the number of calls that may be carried across a standard 1.5 megabit/s T1 link. When using uncompressed G.711 audio, one can expect 18 concurrent calls across a T1. And, when using G.729 compression and Digium's IAX2 Trunking, instead of SIP, signaling protocol, one can expect about 140 concurrent calls across the same link.
Digium's implementation of the G.729 Codec in software allows Asterisk to transcode (compress and decompress) audio to and from formats other than G.729. Many business-class IP telephones and VoIP gateways include support for G.729. With the Digium G.729 Codec for Asterisk, those devices can now exchange calls with Asterisk directly.
Without the capability to transcode G.729, Asterisk can only pass-through G.729 data between endpoints. This means that basic station to station calling can be made to work, but the advanced PBX features of Asterisk such as Call Conferences, DTMF digit collection, Call Recording and more will not work without Digium's licensed G.729 Codec.
For high-density applications, users should consider Digium's TC400B. The TC400B is a PCI card that removes the transcoding burden from the host CPU. As a hardware product with dedicated Digital Signal Processors, the Digium TC400B can process 120 G.729a streams (and also supports G.723.1). Unlike Digium's software G.729 Codec, the TC400B does not require any per-channel licensing fees.
Digium's software G.729 Codec utilizes the power of the host system's CPU to perform its transformations. Therefore, the transcoding capacity, in terms of simultaneous channels/transcodes, is determined by the performance of the host server. Digium's internal testing indicates that 60 concurrent G.729 calls/transcodes require a system equivalent to a dual Intel Xeon at 1.8GHz. Further testing indicates that 80 concurrent G.729 calls/transcodes require something equivalent to a dual Intel Xeon at 2.8GHz.
Multiple versions of G.729 are defined according to industry standards. Asterisk, and Digium's G.729 implementation support G.729 Annex A, or G.729a. Aster isk and Digium's G.729 implementation do not support G.729 Annex B, or G.729b.
Digium's G.729 Codec for Asterisk is licensed on a per-channel basis. A channel is defined as a single connection from an endpoint to an Asterisk applicatio n, or a bi-directional call between two endpoints attached to Asterisk. Customers may use the licensed G.729 Codec in conjunction with Asterisk and any combination of Digi um telephony interface boards.
The G.729 Codec is provided with support from Digium's Technical Support organization for Linux x86 and x86_64 environments. Digium also provides builds for other platforms, but without support.
For specific download and installation instructions, read the Asterisk README file.
Note: The Digium G.729 codec module and Digium product registration tools are supported on the Linux operating system, on x86-32 or x86-64 (and compatible) processors only.
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