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Fax woes


joeh
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We use non-T.38 ISDN gateways with our PBXnSIP deployments. We have a customer who has a fax number slap bang in the middle of a DDI range that is (now) routed to their PBXnSIP. I attached a Linksys ATA to that extension, fixed G.711u, disabled echo cancellation\jitter buffers etc. Faxes from 'good' lines receive and send ok, faxes from international numbers or poor quality lines tend to fail.

 

Their old PBX basically had what I guess is a cut down TA - which they then plug their analogue fax into. They then route a DDI number to this interface.

 

My question is - what is the best way of supporting faxes on PBXnSIP installations? Is it best to avoid it entirely, stick with T.38 Gateways and ATAs or suggest a seperate analogue line entirely?

 

I would be interested to here what people use in the form of gateways and ATAs to support faxes.

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The problem is packet loss. T.38 compansates packet loss. That is the reason why T.38 has been specified. Unfortunately, they made it very complicated, so that the product support is still quite poor.

 

The PBX has the additional problem that it has to maintain a constant RTP playout rate. If the input side has too much jitter, the PBX starts to insert packets on its own. For voice that is great, but for Fax it is desastrous.

 

All in all, the bottom line is that you cannot transport Fax over the public Internet using G711. BTW the same is true if you are using compressing codecs.

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  • 3 weeks later...

T.38 only solves the problem for traffic between the two T.38 endpoints. If you have international carriers, they might tunnel the voice traffic through VoIP and don't deal with packet loss. In that case, you will not be able to receive faxes, no matter what you do on your side.

 

Generally speaking, if your RTP traffic is just in the LAN (where you have no packet loss) and your PSTN termination comes from a PSTN gateway, using T.38 or just G.711 does not make a difference. T.38 makes only sense if you want to send faxes over links that might loose packets.

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T.38 only solves the problem for traffic between the two T.38 endpoints. If you have international carriers, they might tunnel the voice traffic through VoIP and don't deal with packet loss. In that case, you will not be able to receive faxes, no matter what you do on your side.

 

Generally speaking, if your RTP traffic is just in the LAN (where you have no packet loss) and your PSTN termination comes from a PSTN gateway, using T.38 or just G.711 does not make a difference. T.38 makes only sense if you want to send faxes over links that might loose packets.

 

I've done tests using G.711U with different combinations of jitter and echo variables - with the same results. Faxes to national fax numbers work ok 50-75% of the time, international ones fail most, if not all of the time. Plugging the same fax into a vanilla analogue line works fine.

 

This is when both VoIP endpoints are on a uncongested 100Mbps LAN (the ATA and PSTN Gateway). Searching through Google suggests that faxing using G.711 may work sometimes, but isn't ideal - even on a LAN. My understanding was that T.38 may improve things. That said, the problems I am experiencing could be due to the interaction between the ATA and the analogue fax (I have tried different ATA vendors).

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  • 3 weeks later...
I've done tests using G.711U with different combinations of jitter and echo variables - with the same results. Faxes to national fax numbers work ok 50-75% of the time, international ones fail most, if not all of the time.

Whenever we sell a softswitch we tell the customer to buy analog lines.

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  • 1 month later...

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