Fax Version

[Soli]

Hi
what Values: 0, 1, 2, 3, 4 in fax version parameters stands for please?
The default is 0

 

[eddv123]

Hi soli,

Please advise as to where you are seeing this setting. In parameters on 3CX the closest I see in regards to these sorts of values is "FAXDIRECTSDP" = modifies behavior of SDP negotiation when the FAX extension is the destination.

 

[Soli]

T38_VERSION Supported T.38 protocol version. Values: 0, 1, 2, 3, 4

It on setting and then parametars

 

[eddv123]

Found it, I needed to search T.38 not FAX.

T.38 does have different versions like SIP does (our current SIP version is v2) this page talks about T.38 v3 https://www.qualitylogic.com/2013/04/29/version-3-of-t-38-super-g3/

I would imagine it means this. It wont be related to FAX analogue tones as they go v17, v29 v27 v34 etc.

 

[Soli]

I need to Know for sure
i guess 0 is pass thtough and 1 is T.38 but i want to know for sure

 

[eddv123]

No the different versions for FAX T.38 outline information carries in the INVITE message ie: SDP body capabilities - codec capabilities, image transfer, T38 UDP Redundancy, Cpar attributes etc.

T 38 FAX version 0,1 = ASN.1 syntax 1998 supported.
T 38 FAX version 2 = ASN.1 syntax 2002 supported.
3 = ASN.1 syntax 2003 support + V.34 (Super G3)
http://www.allfax.com/blog/10041/G3-Fax-Over-VoIP

If there is not version number it is assumed to be v 0 - you would see this in a Wireshark capture.

 

[Soli]

Thank you for that.
So is there a way that i can choose to use T.38 or pass through in 3cx?

 

[eddv123]

Yes, you can do this under the settings in the FXS gateway - see FXS/DECT >> "the gateway in question" and the setting "FAX Transmission Mode".

Here your options are T.38 or G711 Pass-through (if using T.38 ensure your provider supports this also).