[asterisk-users] Fwd: Re: Asterisk T.38 Pass-Through doesn't work

[Joshua Colp]

Larry Moore wrote:
> On 31/07/2013 8:08 PM, Joshua Colp wrote:
>> Zoltán Fekete wrote:
>>> Thank You Larry!
>>>
>>> I have discussed with my provider. They are not able to insert the
>>> T38MaxBitRate value into the sip answer. :(
>>> https://gist.github.com/anonymous/6120148 (line 559)
>>>
>>> That means we are not able to passtrough T38 Faxes with any asterisk
>>> version at all?
>>> What do you mean? Am I able to modify and compile the source? Is it
>>> compicated? (I'm not a developer)
>>
>> Based on the SDP in your gist the remote implementation has given no
>> attributes with the T.38 stream which makes it pretty broken
>> (T38FaxRateManagement is mandatory) and fun. The two hard parts really
>> would be 1. Modifying Asterisk in a sane fashion to cope and 2.
>> Determining the exact settings to make the implementation happy.
>> Defaults as defined in the spec are fine and good, but my experience has
>> taught me to throw those out the window when it comes to actual
>> implementations.
>>
>
> It would seem that having a configurable option would be an idea for
> this scenario.

That implies it would solve the problem, which my gut and experience 
tells me... it wouldn't. I think the T.38 implementation is just cobbled 
together and without knowing exactly how it behaves getting it to work 
would likely be a nightmare (trust me, I've spent time in those deep 
dark reaches). Throwing assumptions and defaults at it to try to make it 
work is of course an option.

> My testing with Asterisk 1.8 and T.38, I obserevd that setting
> FAXOPT(minrate) or FAXOPT(maxrate)had no effect, I concluded that when
> Astrerisk is receiving it uses hard coded values - is this a sane thing
> to do?!

When Asterisk is receiving the stack implementation offers what it 
wants, with the ability to override. So Asterisk doesn't hard code those 
values, the stack provides them. What is hard coded is the default 
values if none are received.

I would even say it's a bug that the negotiation doesn't fail, since the 
remote side isn't providing a mandatory attribute.

-- 
Joshua Colp
Digium, Inc. | Senior Software Developer
445 Jan Davis Drive NW - Huntsville, AL 35806 - USA
Check us out at:  www.digium.com  & www.asterisk.org