T.38 fax over SIP trunk instead of G.711 Modem PassThrough 1

[crac74]

Hi there,

I'm wondering how to use T.38 protocol for sending faxes overs SIP trunks between our PBX instead of G.711 Modem PassThrough.

Tests between our main office in rls 4.5 and office in rls 5.5 shows that faxs are transmitting well indeed, but DspMode on VGW shows ModemPT G.711-20

I try a SIP fax server (XMedius) which negociates only with T.38 and sending faxs is unsuccessful over SIP trunks.
Sig Server says No RTP map received / RFC 2833 support set to NO / fax switchover rejected

However, locally and using very old VGMC 32 ports IP Line rls 4.5 it's ok with T.38 it works. Amazing that old products on old versions are working better than VGW in rls 5.5.

So problem is only on SIP trunks. T.38 codec is mandatory so it's enabled on all the parties involved in the call.

Any idea ?

thanks.

 

[allenmac]

Make sure your patches are up to date on all elements, and if that doesn't work, get your vendor involved.

 

[crac74]

I've found what was wrong.
On analogic TNs when CLS MPTA is activated then the VGW accepts only Modem Pass Thru G.711 transmission. It rejects T.38 and others.
It allows then to transmit at 33.6 kpbs. Usefull when you want your fax to transmit or receive faster with PSTN connexions. Or for modems. But if your SIP gw accpets only T.38 then there is no way to transmit !
CLS available since rls 5.5 I think.

 

[allenmac]

Nice catch.

Thanks for the update. Star.