[asterisk-users] PJSIP / T.38 - Asterisk not passing on v21 preamble and data

[Frederic Van Espen]

Hi,

On Thu, Jan 1, 2015 at 7:09 PM, Recursive <lists at binarus.de> wrote:
> 1) Did anybody test T.38 with SPANDSP? If yes, which version of SPANDSP did you use? Mine is 0.0.6 PRE 20. Should I try to upgrade to PRE 21? Or to one of the snapshots?

I remember that I was having issues with an older libspandsp library.
I simply upgraded to the latest available version which made the issue
go away. We were using 0.0.6 pre 12 before and upgraded to 0.0.6 pre
20. However, I believe (and please someone do correct me if I'm wrong)
that libspandsp is not required when using t38 passthrough, which is
what you are trying to do.

BTW, in the old thread I had asked if you saw anything in the logs
about the SIP message that was being retransmitted. Do you still see
retransmission with asterisk 13 and pjsip and a hangup after 30
seconds?

> 2) Recently, I have sent some questions about similar subjects to this list, and I have got helpful answers; people told me that I should *not* enable the fax *gateway* feature if both endpoints are capable of T.38. On the other hand, I have read (at multiple places) the the *gateway* code is responsible for detecting the v21 preamble. How does this fit together?

AFAIK, when both ends support T.38, you don't need the gateway and can
turn to passthough mode. In passthrough mode, asterisk does not need
to detect anything. Just take the packet it receives and pass the
payload to the other end of the call.

Cheers,

Frederic