[asterisk-dev] AST_FRAME_DIGITAL

[Russell Bryant]

Klaus Darilion wrote:
> What about G3 fax. It is rather the same as 3G video:
>   fax delivers images over a telephone line
>   3G video deliver audio and video over a telephone line
> 
> In case of G3 fax, the image data is modulated
> In case of 3G video, the audio/video is multiplexed with H223.
> 
> Currently fax is handled inside an application. Taking your argument, 
> the fax decoder should be in the channel driver and deliver 
> AST_FRAME_IMAGE (maybe one per page) to Asterisk's core. But I think the 
> current implementation which is a single application is much better then 
> adding modem functionality to each channel driver.
> 
> IMO it is much more flexible only add support for generic data 
> (AST_FRAME_DIGITAL) to channel drivers. Then, new services and features 
> can be added as application without touching the code of channel drivers.

Faxes are passed into Asterisk using the VOICE frame type, indicating that it is
audio.  In the case of T.38, it uses the AST_FRAME_MODEM type.

Another reason that this must be handled using the VOICE and VIDEO frame types
is bridging with channels that are not ISDN channels.  What if you want to
connect a voice/video ISDN call to SIP?  The way that you propose, it will not
be possible.  This is why _every single channel driver_ in Asterisk decodes the
technology specific stream of voice and video and passes them into Asterisk
using the primitive frame types.

-- 
Russell Bryant
Software Engineer
Digium, Inc.