# Asterisk Project : RTP Packetization

Added by mdavenport , edited by mjordan on Aug 08, 2012

# 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
- chan_motif (Asterisk 11+)

##### 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 | Minimum (ms) | Maximum (ms) | Default (ms) | Increment (ms)
--- | --- | --- | --- | ---
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.