[Freeswitch-users] Fax T.38: How to deal with NSF (NSS)?

[Steve Underwood]

On 03/20/2017 05:19 PM, Peter Steinbach wrote:
> Hello,
>
> we have a number of fax machines attached via a Grandstream ATA to a
> Freeswitch. Fax devices are Brother MFC. Freeswitch works in
> t38-passthru mode.
>
> In the last weeks we see (maybe due to software update?), that the fax
> devices detect, that they are siblings with extended feature sets, and
> then one side switches to "non standard facilities" mode by sending an
> NSF signal (resp NSS in wireshark). The opposite side then answers with
> "failed to train" (FTT). This always breaks the fax transmission. We
> also asked Brother to send us an advice, how to disable NSF in the fax
> device, but these codes did not work. The fax devices are continuing to
> send NSF. Besides that we have no control of the opposite fax device,
> who are initiating NSF.
>
> So I am asking here: How are you dealing with NSF? How did you solve
> this issue?
>
All T.38 gateway implementations, including the one in FreeSwitch, edit 
the NSF messages so they will not be recognised by the receiving end. If 
your ATA is not doing that, its badly broken. If Freeswitch is operating 
in pass through mode it will not be doing any editing. That is the job 
of the gateway between the analogue telephone line and the IP world.

Regards,

Steve