asterisk/doc/rtp-packetization.txt

Overview
-------
Asterisk currently supports configurable RTP packetization per codec for 
select RTP-based channels.

Channels
-------
These channel drivers allow RTP packetization on a user/peer/friend
or global level:
    chan_sip
    chan_skinny
    chan_h323
    chan_ooh323 (Asterisk-Addons)
    chan_gtalk
    chan_jingle

Configuration
-------
To set a desired packetization interval on a specific codec,
append that inteval to the allow= statement.

Example:
allow=ulaw:30,alaw,g729:60

No packetization is specified in the case of alaw in this example,
so the default of 20ms is used.

Autoframing
-------
In addition, chan_sip has the ability to negotiate the desired
framing at call establishment.

In sip.conf if autoframing=yes is set in the global section, then
all calls will try to set the packetization based on the remote
endpoint's preferences.  This behaviour depends on the endpoints
ability to present the desired packetization (ptime:) in the SDP.
If the endpoint does not include a ptime attribute, the call will 
be established with 20ms packetization.

Autoframing can be set at the global level or on a user/peer/friend
basis.  If it is enabled at the global level, it applies to all
users/peers/friends regardless of their prefered codec packetization.

Codec framing options
-------
The following table lists the minimum and maximum values that are
valid per codec, as well as the increment value used for each.
Please note that the maximum values here are only recommended
maximums, and should not exceed the RTP MTU.

Name		Min		Max		Default		Increment
g723		30		300		30		30
gsm		20		300		20		20
ulaw		10		150		20		10
alaw		10		150		20		10
g726		10		300		20		10
ADPCM		10		300		20		10
SLIN		10		70		20		10
lpc10		20		20		20		20
g729		10		230		20		10
speex		10		60		20		10
ilbc		30		30		30		30
g726_aal2	10		300		20		10

Invalid framing options are handled based on the following rules:
  1.  If the specified framing is less than the codec's minimum, then
        the minimum value is used.
  2.  If the specific framing is greater than the codec's maximum, then
        the maximum value is used
  3.  If the specificed framing does not meet the increment requirement,
        the specified framing is rounded down to the closest valid
        framing options.
                example allow=ulaw:33 will set the codec to 30ms framing
  4.  If no framing is specified in the allow= directive, then the
        codec default is used.