Fax/T.38 issue

[Rick C]

Hello,

Over the last couple weeks I've been working on getting our fax machines to work with OpenUC via an ATA device (specifically an SPA 112) and have been having some odd issues. I'd prefer to get it working with T38 since G711u is kind of unreliable for faxing and dependent on network conditions. 

I've had no issue registering the device on the server. But what I've found is that if I enable T.38 on the device, only outbound faxes work. Inbound faxes seem to fail to negotiate for some reason (the RTP stream doesn't even start) and the call drops after 30-40 seconds; I've tried turning ReInvite on/off in the SPA settings and no dice. If I disable T38 inbound faxes work. 

The call path is fax>SPA 112>Phone Server (15.12)>Audiocodes Mediant 1000. 

So I'm just trying to figure out whether the issue is the ATA, the server, or something on the gateway screwing up the T38 negotiation, and why it's not falling back to G711 if that's the case and figured I'd see if anybody has any input on this. I've attached a PCAP (though it doesn't give much info) and my SPA config. Definitely a frustrating issue. Fax machines registered on the Audiocodes gateway via phone line work fine. For reference, .70 is the phone server, 168.10 is the gateway, and 73.187 is the SPA 112. Thanks in advance for any input.

[Tony Graziano]

Did you ensure the fax machine is set for TX/TX 9600/baud and that the ecm is set to off for both Rx and TX?

[Rick C]

On Saturday, March 19, 2016 at 6:48:25 AM UTC-4, Tony Graziano wrote:

Did you ensure the fax machine is set for TX/TX 9600/baud and that the ecm is set to off for both Rx and TX?

I did. 

In this case I don't think it's an issue with the fax speed/ECM simply because I'd expect G711 to be more sensitive to that than T.38. But in my case G711 works and T38 fails and the RTP stream doesn't even start. Has to be some sort of T38 negotiation issue. Problem is, I'm not sure if it's the Audiocodes gateway, the server, or the ATA.  

[Rick C]

And as an addendum to that, the issue is strictly with receiving. I can send fine with T.38 enabled on the ATA. It's only incoming faxes that fail.

[Joegen E. Baclor]

One thing I see suspect is that the audiocodes re-offered (re-invite) a totally new stream to fallback to T38 but it never changed the session identifier.  It only incremented the version number.  Since T38 is a brand new offer and is not simply a modification of the previous stream, it must have a new session id.    Check if you can disable G711->T38 fallback in audiocodes. 

On 20/03/2016 6:27 AM, Rick C wrote:

And as an addendum to that, the issue is strictly with receiving. I can send fine with T.38 enabled on the ATA. It's only incoming faxes that fail.

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[George Niculae]

Try to change following setting to true in System > Voicemail page and see if it improves anything

Enable T.38 ReINVITE true false (Default: false)Send T.38-ReINVITE when fax detected by tone detection

George

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