in order of the questions asked -
- The default is when a user/extension owner email is not specified. That person may not be the same as the PBX admin. People change jobs, but the DID for the fax remains, so it offers a catch-all when a DID no longer has an email at the extension level.
- I have not experienced this, as I have faxed spec sheets. I assume that a “normal” pdf file is one that might be editable whereas a scan is an image PDF. I have faxed scanned PDF images, but perhaps the scan resolution or color may have a bearing. PM me with one such failure and I will test to see if it will go.
- Yes, you can increase it to 14.4k, but more in a bit.
4, You don’t. The UCM will receive the Invite at a voice level initially. Once accepted, a re-Invite
for t.38 will occur and if not possible a fallback to g711 if a re-Invite is allowed and assuming the response is not a 488 not acceptable here
- It is simply an option where you get to set the number of retries and the interval between same. However, I suspect like you, that I have not seen the retry function in action. If I send and it fails, it fails and I must manually invoke another attempt. I wonder if the retry is only applicable in the event the other fax is busy? Maybe someone else knows as I do not use the inbuilt server that much except for testing.
The issue with fax (FoIP) is that it is taking a conventional TDM from of communications and converting it to a stateless (UDP) packet switched mode of communications. As a result the reliability of the transmission is dependent on the ability of the Internet to deliver the signal from point A to B in a timely and error-free manner…or as much as possible. Additionally, the providers along the way on the Internet must all support t.38 for it to reliably work and while t.38 is designed to overcome some of the issues associated to FoIP, it is not a guarantee and when it can’t handle the job, then there is not much you can do about it. If the provider does not support t.38, then the fallback g711 codec is used, which does not have the robustness of t,38 and the odds increase for failure.
The longer the fax, the odds increase for failure
The more complex a fax is in its content, the more time is sent having to render the image and therefore the longer the time to transmit, thus increasing the odds.
The faster the speed, the odds increase for failure
The more hand-offs (carriers) along the way, the odds increase for failure,
Use of g711, the odds increase as the use of a voice codec has no provision for error correction.
If your provider claims to support t.38, most likely they have a statement somewhere in their small print that says something to the effect that while they support t.38, that due to the nature of FoIP, they do not offer troubleshooting support for fax issues.
Error correction? I have tried it both on and off and honestly have not been able to tell a difference, but I suspect that this is more due to my ISP capability than anything else. Of course, my observation are merely subjective and lack any technical testing or empirical data to back it up. My suspicion is that it should be left off, but with the caveat that it may depend on which codec is used. ECM does a level of handshaking to determine if the page sent was received as expected at the other end and if not, the page is resent again. ECM requires CPU cycles. If using g711 and given the connection is stateless, it occurs to me ECM is pointless. The stream might be completely unintelligible as the resent packets arrive in the middle of packets in process. With t.38, there are some forms of EC in place within the protocol and while the EC methods are different, the addition of ECM to what is already in the protocol makes the CPU use even more so and to some users redundant. However, I have read some article that indicate that if you have t.38, then ECM is definitely worth it. As mentioned I have tried it both on and off with the providers I have that are known to support t.38 and I have not been able to tell a difference subjectively. Maybe others have more insight.
I have tried GS, Planet, CIsco, Net Gen Communications, Patton M-ATA, MediaTrix, Yeastar, Welltech, and others over the past years all in a quest for the Holy Grail. They all had some success, but never to the point that the installation of one was solid to the extent that service calls always cropped up sometime later,and frequently, if the client was a heavy user. I have since ceased their use for the purpose.
Your mileage may very if you are fortunate enough to live in an area with a number of ISPs with fiber. We are only starting to fiber in limited distribution and while I have clients with fiber and they seem to have better results, you still have to consider that this provider is probably not the end-to-end provider and the fax will traverse circuits that still raise the odds for failure.
Nevertheless in the US, I have been playing around some, not much, with trying out a few providers’ claims and I can say that some do appear to be pretty robust. My playing around however is not the same as clients who live and breath by the (dreaded, despised and unwanted) fax that they send hundreds of pages a day.
To me, its like going to Vegas and rolling the dice…knowing the odds are in the house’s favor.