Problem sending faxes through VoIP

Hello every one!

I have a problem sending faxes through VoIP

This is what we have: PBX with VoIP Card --- Cisco --- E1 --- (Internet VPN) --- T1--- Cisco

--- PBX

The cisco router has activate the QoS, to priority the communication saving 512 kbps of bandwidth, for the matching VoIP ports.

The voice and faxes work fine, but when the bandwidth is over use, the voice is not affected but the faxes can not communicate even when the QoS is saving bandwidth for them. I been trying with traffic-shape, fax passthrough , fax relay and other options, but no one fix the problem.

I figure out that the cisco router is working only as data gateway not as voice gateway, because the VoIP card in the PBX is making job, so may I?m look to solve the problem in the wrong place.

The VoIP card is configured by a provider, but I try changing the communication options between G.729, G723 and G.711, when use G.711 the voice is better, but the same happen with the faxes.

The question is:

Do I have to correct the fax problem in the VoIP card configuration or is possible to fixed in the cisco Router?

If it can be in the Cisco router, I will appreciate any suggestion.

Thank you very much, regards.

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Voice more than likely is taking priority in this situation.

I been trying with traffic-shape, fax

What is your actual bandwidth??

Yes of course G711 offers better quality. You may want to see if you can change your sample rate to a value such that voice quality does not suffer, but you send less traffic per VOIP call.

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QoS is not a substitute for bandwidth. If you have 512k, line and you have

400k of priority traffic, and 120k of regular traffic, you have a problem. QoS only gives PRIORITY to traffic, it does NOT guarantee that the traffic is going to be delivered. The only solution is to increase the bandwidth, or decrease the VoIP traffic.


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What I tried with success and read in many places is to reduce the baud rate to 9600 on the fax.


Thomas See review at

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Hello every one!

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When you use G729 what kind of codec are you using? What kind of ports are you using, E&M, FXS or FXO ?

try to send me info about config in your router and QoS and if posible for your ISP QoS send me: sh run sh voice port sum sh call active voice brief sh call active voice

send me all info from A-->B i mean source and destination.

Maybe the problem is with codecs, some PBX tell cisco not to change the codec and when is compresed on cisco faxes has so many problems.

When you use G711 REMEMBER!!! you are using 64K for every voice call, also for faxes! So i am sure that if you doesnt have any voice call on place, faxes pass OK.

Send me all the info please.

stay in touch.

mramos wrote:

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