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chan_motif
chan_multi
cast_rtp
chan_pjsip
Purpose
Used internally by the ConfBridge() application; should not be used directly
Provides connection to PSTN cards that use DAHDI channel drivers
Provides a mechanism to treat a portion of the dialplan as a channel
Implements the Jingle protocol, including the ability to connect to Google Talk and Google Voice;
introduced in Asterisk 11
Provides connection to multicast Realtime Transport Protocol (RTP) streams
Session Initiation Protocol (SIP) channel driver
Codec Translators
The codec3 translators (often called transcoders) allow Asterisk to convert audio
stream formats between calls. So if a call comes in on a PRI circuit (using G.711) and
needs to be passed out a compressed SIP channel (e.g., using G.729, one of many
codecs that SIP can handle), the relevant codec translator would perform the
conversion.
Codecs are complex algorithms that handle conversion of analog information (sound,
in this case, but could be video as well) into a digital format. Many codecs provide
compression and error correction as well, but this is not a requirement.
If a codec (such as G.729) uses a complex encoding algorithm,
heavy use of transcoding can place a massive burden on the CPU.
Specialized hardware for the decoding/encoding of G.729 is avail‐
able from hardware manufacturers such as Sangoma and Digium
(and likely others).
Asterisk does a fairly good job of supporting codecs, but is mostly focused on the
codecs typically used by telephone applications (as opposed to codes used for, say,
music or video such as MP3 or MP4). These are listed in Table 2-6.
3 The term codec is short for “coder decoder.”
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Chapter 2: Asterisk Architecture
Table 2-6. Common codec translators
Name
codec_alaw
codec_g729
codec_a_mu
codec_g722
codec_gsm
codec_ilbc
codec_lpc10
codec_opus
Purpose
A-law PCM codec used all over the world on the PSTN (except Canada/USA). This codec (along with
ulaw) should be enabled on all your channels.
Was until recently a patented codec, but is now royalty-free. As of this writing it is still sold by Digium
as an add-on, but it can also be found as a free package. It’s a very popular codec if compression is
desired (and CPU use is not an issue), but it imposes load on the CPU, adds latency to calls, reduces
quality slightly, and will not reduce overhead in any way.
A-law to mu-law direct converter.
Wideband audio codec.
Global System for Mobile Communications (GSM) codec. Very poor sound quality.
Internet Low Bitrate Codec.
Linear Predictive Coding vocoder (extremely low bandwidth).
Intended to replace speex (and vorbis).
codec_resample Resamples between 8-bit and 16-bit signed linear.
codec_speex
codec_ulaw
Speex codec.
Mu-law PCM codec used on PSTN in Canada/USA. It’s more formally written as μ-law, but not many
people have a Greek letter μ on their keyboard, so it’s popularly written as ulaw.a This is often the
default codec, and should be enabled on all your channels.
a Spoken, you should say “mew-law,” but again, you’ll hear this pronounced “you-law” very often.
Digium distributes some additional useful codec modules:
codec_g729, codec_silk, codec_siren7, and codec_siren14.
These codec modules are not open source for various reasons. You
must purchase a license to use codec_g729, but the others are free.
You can find them on the Digium site.
Format Interpreters
Format interpreters (Table 2-7) perform a similar function as codec translators, but
they do their work on files rather than channels, and handle more than just audio. If