[asterisk-dev] Asterisk RTP Jitter Buffer and DAHDI Channels

[Russell Bryant]

On Wed, Apr 25, 2012 at 11:58 AM, Kevin P. Fleming <kpfleming at digium.com> wrote:
> On 04/25/2012 09:27 AM, Fernando Berretta wrote:
>>
>> Hi Everybody,
>>
>> We recently had some problems with some SIP/PSTN gateways have not
>> properly implemented RTP Jitter buffer, and as work around, we have
>> forced a Jitter buffer in Asterisk and all worked ok.. but with extra
>> delay of course. This situation have maked us think about all possible
>> problems related with common RTP Jitter Buffer and how Asterisk handle
>> Jitter Buffer when it is working as a SIP/PSTN gateway through DAHDI
>> Channels. Could some one there please let me know if asterisk / dahdi
>> ,by default, do something about Jitter Buffer with calls which comes
>> from VoIP Network and goes to PSTN through Asterisk acting as a gateway
>
>
> DAHDI itself provides some very basic de-jittering, but it does not do
> packet loss concealment or reordering. DAHDI channels have an 'input buffer'
> that can hold a configurable number of frames of audio, and the buffer
> policy can be configured so that the buffer must be full (or half-full)
> before any audio will be played out to the channel. In this configuration,
> small variations in the packet delivery times over the IP network will be
> absorbed by the DAHDI channel, but as you say, this comes at the cost of
> introducing some delay in the audio path (as all jitter buffering does).
>
> Note that the default configuration for DAHDI channels won't help much, as
> the normal mode for the input buffer is 'immediate' mode (audio playout
> begins as soon as the first frame is stored in the buffer). If the next
> frame arrives before the first one is done playing out, then the buffer will
> accept it and it won't have to be dropped, but if the next frame is late,
> then there would be a gap in the playout. The 'half' and 'full' buffer
> policies in DAHDI allow you to decide how you want to address this problem.

And IIRC, chan_dahdi can provide a jitterbuffer with PLC and
reordering above DAHDI if you enable it in chan_dahdi.conf.

-- 
Russell Bryant