[asterisk-users] TDM Fax

[Lee Howard]

On 08/17/2012 04:58 AM, Steve Underwood wrote:
> On 08/17/2012 06:08 AM, Eric Wieling wrote:
>> Has anyone experimented with increasing the DAHDI chunk size in 
>> improve fax reliability?  If so, did it help, hurt, or not make any 
>> difference?
>>
> I haven't found issues related to the DAHDI chunk size. The main thing 
> which used to hurt FAXing with Asterisk before Digium launched their 
> own FAX software was the timing within Asterisk, which they refused to 
> fix at that time (although independent patches were available). With 
> the launch of FFA they changed chan_dahdi so on a FAX call the 
> buffering should change to make the flow of transmitted audio a lot 
> more elastic. People just tolerate some hiccups in voice calls, but 
> hate latency. Modem signals must be rigidly timed, but a bit more 
> latency is OK. This change fixed the main issue affecting all the FAX 
> solutions around. If that switch in the buffering mode is not 
> happening on your system for some reason it can badly affect the 
> reliability of FAXes.

I'm uncertain of exactly to which changes you're referring.  Your 
comments seem to fall in-line with the notion behind the DAHDI "buffers" 
feature for the channel as well as the DAHDI fax-detection "faxbuffers" 
feature, but I'm seeing no noticeable improvement, AND I'm uncertain how 
to implement the CHANNEL(buffers) feature due to:

     -- Executing [4628160 at fax-outbound:1] Set("IAX2/ttyIAX99-584", 
"CHANNEL(buffers)="12,half"") in new stack
[Aug 18 20:12:40] WARNING[6381]: func_channel.c:530 
func_channel_write_real: Unknown or unavailable item requested: 'buffers'
     -- Executing [4628160 at fax-outbound:2] Goto("IAX2/ttyIAX99-584", 
"outbound,4628160,1") in new stack
     -- Goto (outbound,4628160,1)
     -- Executing [4628160 at outbound:1] Dial("IAX2/ttyIAX99-584", 
"DAHDI/g0/4628160") in new stack

On some installations there are occasional instances in most outbound 
calls where Asterisk creates what otherwise would be considered jitter 
on the DAHDI channel.  Generally these do not cause much real-world 
trouble, but I'm a stickler for perfect audio quality on all-digital 
calls.  I've seen this on Asterisk versions 1.4, 1.6, and 1.8.  On other 
installations there never is any such trouble noticeable.

Would you mind being a bit more specific on the Asterisk changes to 
which you refer and how they should be implemented in the configuration?

Thanks,

Lee.