[WXC/VFX] Faxing over VFX issues, feedback & technical discussion

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coffeebaron

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#23779 8-Jul-2008 21:16

Ok, after a couple of issues with faxing over VFX, I thought I'd kick this off to see what the general experience is like out there with faxing over VFX. Also some good technical discussion on T.38 & G711 codecs etc would be great too.

I've had a couple of customers that I've had to revert back to a land-line for faxing, as faxing just turned out to be very unreliable. These were on below average ADSL lines, and maybe that is the issue. Possibly with ongoing troubleshooting, this may have been resolvable, but when the customer is loosing business due to fax issues, there is not much time to troubleshoot.

Anyway, here's my observations:
- outgoing faxes, these seemed to have a lot less issues
- incoming faxes, this was very unreliable. Lots of multiple faxes, missing lines etc.
- voice calls on other VFX line often cut off with incoming fax
- codecs where hit and miss. sometimes G711u, other times T.38

So a couple of questions:
- what factors are at work here?
- are some fax machines better than others? Which ones?
- why sometimes T.38, other times G711u?
- can you lock codec to T.38?
- is T.38 suppose to make faxing work reliably?
- how does T.38 work (technically) over WxC (or other VoIP) networks?

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nate

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#145137 8-Jul-2008 23:03

This is an interesting topic, my recommendation is that fax machines have been tested with Telecom's phone network so if they work well, there is no point running over VoIP.

Having said that we run pure VoIP faxing in oru office and don't have any issues, but we do have the odd client who it just doesn't work for. It seems there are too many different types of faxes and scenarios to account for them all.

Faxing is such old technology, it's much easier to just scan to PDF and send via email.

grant_k

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#145151 8-Jul-2008 23:16

coffeebaron: Anyway, here's my observations:
- outgoing faxes, these seemed to have a lot less issues
- incoming faxes, this was very unreliable. Lots of multiple faxes, missing lines etc.

I have never bothered with Incoming Faxes over VFX, so it's interesting that you have found them unreliable. Instead I use a Fax-to-email service which costs me US$1 per month.

Outgoing Faxes to NZ numbers are generally fine, although occasionally I have to send it a second time. International Faxes -- forget it.

Recently I have signed up for Supra Phone which is an alternate SIP-based VoIP service offering free calling anywhere in your local region (for me that means anywhere north of the Bombay Hills). Because of the calling plan, it has advantages for our family as regards Voice Calls. But for Faxing, it's completely hopeless. Fortunately this doesn't matter to me now, but a few months ago, it would have been a real pain.

maverick

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#145219 9-Jul-2008 08:08

Good questions Coffeebaron,

There is a lot to this as Fax is as I have said before the most difficult of all services to run over IP networks, generally fax requires very good connections , any type of slippage or packet loss can and will have effects on faxing especially during the negotiation phases of a FAX call and possibly this is why you see either T38 or G711.

Basically for most devices 2102, WRP400, WAG54Gp2 they will by default try to use T38 for both national and International PAP2T does not support T38 (you may want to test your international again GrantK as some changes have been made here) if T38 isn't capable of being supported it will fall back to the Voice Codecs this is handled by the signaling negotiation, you can not lock it to T38 if the other end doesn't support it as it will be asked for (T38) but responded to with a not supported message so will then only allow Voice Class codec support to be used, ie the call will be treated as a voice call.

Generally T38 should be more reliable as it is actually a protocol specifically designed for FAX where voice codec's rely on negotiation and passing of the traffic via the voice path, these fax calls can be more susceptible to failures due to lost packets , latency jitter auidio distortion etc.

Also there are a number of other issues like the types of Faxes, some fax machines may try to offer properirty information within the Fax and if this is supported you can get failures

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coffeebaron

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#145237 9-Jul-2008 09:07

maverick: , you can not lock it to T38 if the other end doesn't support it as it will be asked for (T38) but responded to with a not supported message so will then only allow Voice Class codec support to be used, ie the call will be treated as a voice call.

What is it at the other end that does not support T.38? Is it the other fax machine, or the other Telco's network?
Where are the analogue ->> T.38 & back to analogue conversions taking place?

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maverick

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#145256 9-Jul-2008 09:39

it occurs during after the call is answered but supported services start before that in call setup parameters, fax information is passed between all of the equipment as it is contained in the actual messaging and depending on which end originated the call and the type of fax will depend on how the negotiation will work, ie some faxes will wait to send fax tones , others will send it straight away, it is not a one case fits all sorry ... I wish it was

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coffeebaron

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#145611 9-Jul-2008 18:43

Are there any tricks / tweaks etc that can help? E.G. fax parameters, speed, shorten or lengthen rings before pickup? Are there any particular model of fax machine, or fax specifications that give better results than others?
What other solutions are in the pipeline, e.g. store and forward etc?

Sorry for all the questions, but this quite a significant barrier for SOHO switching to VoIP.

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Fraktul

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#145641 9-Jul-2008 19:04

Some faxes allow you to alter the connection rates, eg you could specify its prefferential speed during handshake is 9600bps this *may* help with connection issues. It may also lower T.38 bandwidth usage if T.38 is in use (should have no effect on G.711u however) which in turn might help things out if there is bandwidth contention issues, given quality issues present on other calls at the time of faxing this is a consideration.

Edit - of course lowering connection speeds obviously makes fax transmissions slower :P longer calls=greater call charges.