Fax only works sometimes

[Peter Richardson]

Hi all,

I'm setting up a fax line for the first time for a customer. I host the PBX and their extensions connect through an SBC running on a Raspberry Pi. I have set up a Cisco SPA112 for the fax machine. I can get the extension to register with 3CX, I can get the fax machine to call my mobile phone, I can get the fax machine to send a fax to my virtual fax provided by efax.com.au but when testing it with the client I ask him to send some faxes to the people he would normally fax, but it does not work. The fax machine does not give any error, it just keep retrying.

I have done a lot of reading and there are plenty of people saying that it won't work, it can't work, etc etc, however, it worked perfectly for the customer every single time when he had it plugged directly into the voice/fax port (the UNI-V port) of the NTD (the customer has fibre and the NTD is the box on the wall that the router and/or phones/fax plug into, depending on configuration). If Telstra can make it work over their UNI-V port, then surely we can make it work through a Cisco SPA112.

Our SIP provider supports G711a, G711u, G722, and G729.

From anyone who is actually using a fax through 3CX, what codec are you using, what ATA are you using and what other tricks did you use to get it working reliably?

 

[lneblett]

Reliably, not likely.

The fax is dependent on the quality of the carriage. It is extremely sensitive to timing. The previous set-up was likely a "controlled" circuit where Telstra managed the end-to-end carriage and may have had some synchronizing circuitry such as may be found in some PRI gateways.

Both 3CX and the ATA will work best when using the t.38 protocol, which was developed in an attempt to overcome some of the issues of FoIP. The call starts off using a voice codec g711 and will then attempt to negotiate to t.38. However, not all providers support t.38 so there may be cases when the fax will be sent using g711 which is back (fall-back) to a voice attempt or some form of transcoding occurs mids-stream. In the voice realm, the odds of successful fax delivery are greatly reduced.

The following link may provide some help, but very,very few have been very successful getting a highly reliable fax function using an ATA.
http://goughlui.com/2013/02/13/telstra-fax-on-line-diagnostics-folds-test/

 

[JasonNadeau]

When it comes to faxing at this point i only use Mediatrix - highest success rate for me.

 

[Peter Richardson]

@lneblett

Thanks very much for that. So is it possible to find out whether my Cisco SPA112 is using T.38 or G711?
Do I need to find out whether my SIP provider supports T.38? What if the receiving party's provider does not?
The tricks that you mentioned that Telstra use, are we able to replicate them?

@JasonNadeau

Thanks for this, which model do you use? And where do you buy them? Not many results on Google and only 1 used one on ebay from US.

 

[leejor]

The latest Cisco ATA's will use the G711 Codec (that is pretty standard on all SIP devices), and t.38 (that is sometimes optional). The previous generation (SPA-2102) supported t.38, and they usually tend to add features in later generations, not remove them. Easily checked on most vendors sites that list specs. You don't need to be concerned about the far end. If they use a fax machine on PSTN, then it is handed off to the PSTN network by your provider, or another. If they use VoIP, then it is up to them to have a provider that supports it or it won't work for them at all.

 

[lneblett]

Yes, the spec sheet for the SPA112 indicates t.38 support. You may be able to use Wireshark to see if the provider supports and presumably you can call them.

There are a number of ATA manufacturers that support FAX. I have used Planet, Cisco, Welltech. Grandstream, Net Gen Communications, Patton and others. Subjectively, some have appeared to be better than others, but my experience is that it is a hit or miss proposition and never 100%. ATAs, providers, carriers, etc., all play a part and it only takes one not wanting to tango that makes it hard,

I have since transitioned to HTTPS devices, but still have a couple of clients that resist the change (more expensive) and continue to stay with ATAs. I have one that I am currently struggling with as I can get some faxes to work to some numbers but seemingly not others, Then, while I can receive from a location, the receiver cannot send back to the same one that sent. These are negotiating to t.38 at 14.4K. Ugh!

Oh well, more wireshark captures for me.

 

[leejor]

Some "newer" fax machines seem to have difficulties slowing to a speed that works (somewhat) reliably. Some actually have an option for use when faxing over VoIP.