FAX speed over ISDN/PRI

Unanswered Question
Aug 4th, 2008

I have a couple of FAXes connected to FXS ports on a UC520 that send FAXes out over an ISDN/PRI connection. I was getting spotty results with some FAXes going thru and others failing. Troubleshooting the problem I found that the problem seems to be that my FAX is trying to connect at what it calls "Super G3" speed of 33.6kbps. When I lowered the FAX to "G3" speed of 14.4kbps most, if not all, FAXes are now going thru. Is there some configuration change I can make to get the 33.6 speed to work? Right now these are the only FAX specific commands I have on my box.

network-clock-participate wic 3

network-clock-select 1 T1 0/3/0


voice-port 0/0/0

compand-type a-law

timeouts ringing infinity

description FAX

I'm I missing something?



I have this problem too.
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Michael Hanes Tue, 08/05/2008 - 07:50

Hi Diego,

The answer to your question may be somewhat complicated because of the methods used by Cisco voice gateways to handle calls between interfaces. If I understand your topology correctly, you have fax devices connected to FXS ports on a UC520 that then go out a T1 PRI connection on this same UC5520. Is that correct?

If that is the case, then your fax calls can be handled in one of two ways, with a DSP or without a DSP. The ideal method is that the DSP sets up the call and then drops out. A TDM connection is then set up between the interfaces in what is commonly called "TDM hairpin". Assuming that a TDM backplane is present on the voice gateways and compatible interfaces are being used, a TDM hairpin will allow SG3 fax calls to wok successfully.

The other method involves a DSP processing the call for its duration. Because the DSP processes the call, including the media, it faces restrictions that a normal VoIP call would face. In fact, a protocol such as T.38 is used internally between the interfaces to relay the fax information and Cisco's T.38 implementation does not currently offer support for SG3. Therefore, if this method of transport is occurring on your UC520, then SG3 will be forced to back down to G3 speeds, which on occasion can be problematic.

I am not sure if the UC 520 supports TDM hairpinning, which would allow support for SG3. The best way to find out would be to use the show voice dsp command. WIth this command you can see DSP channels being allocated during the initial call setup. If the DSP channel is unallocated and the call remains up, then you know that the DSP has been dropped. If the call remains assigned to a DSP channel for its duration, then you know that the support for SG3 is not present. I hope that this makes sense.

Also, please make sure that your T1/PRI interface is free of errors such as slips as this is the most well-known cause of fax problems.



DIEGO ALONSO Fri, 08/08/2008 - 14:28


Using the "show voice dsp" command you suggested allowed me to determin that the DSP is on for the duration. Due to this I set all my FAXes to use G3 only and things seem to be working out.

Thank you very much.



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