Fax use G711 and voice use G729

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Mar 27th, 2007
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I am going to have 2821 connecting to PBX T1 for about 10 sites. GK is used to manage the call routing and bandwidth. The codec we choose is G729. Question is: can we meantime use G711 for the fax even H323 dialpeer is G729? I know G711 passthrough is much better. Fax is on the PBX too.

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dgahm Tue, 03/27/2007 - 19:11
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Yes, that is called fax upspeed, and works if you have enough bandwidth to support the max number of fax calls running G711 plus your G729 voice calls.


You could use fax relay, and let the fax rate drop to 7200. This works well, but faxes do take longer.

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ciscoforum Wed, 03/28/2007 - 09:55
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Thanks. I have seen a lot of document talking about fax passthrough and fax relay individually. Do you see there is doc that specify the pros and cons between them and what Cisco recommended? thanks

dgahm Wed, 03/28/2007 - 11:24
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Here is a rather old document (but still mostly valid) which says fax relay is the preferred method.


This is a good document that will help you understand the issues encountered with faxing over IP.


The right answer will depend upon the specific requirements in your network. In my experience, faxes and modems are an area that can be challenging, and that may require trial and error to get it right.

As an example, we decided to use fax relay everwhere (some low speed WAN links don't have the bandwidth for upspeed), but ran into a very isolated problem with PSTN faxes between our Right Fax server and an internal fax board in a AS400 server at another company. All faxes everywhere else worked great, but this would always fail. Switching it to fax passthrough was the fix, so we now use fax relay for all faxing, except for RightFax to and from the PSTN, which is passthrough.

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ciscoforum Thu, 03/29/2007 - 10:34
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Hi Dave

thanks very much for the input. But I am just wondering why Cisco prefers fax relay? Just because it uses less bandwidth,or it is more reliable, less delay, loss tolerate?thanks

dgahm Thu, 03/29/2007 - 15:51
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I don't know for sure, but perhaps fax relay is preferred because it works well in a mixed G711 and G729 environment with no special config. Passthrough with G729 requires the upspeed config, and is not feasible if bandwidth is very limited. On the other hand fax relay is not an option with ATAs, so it may not always be a viable choice.

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skelian Fri, 03/30/2007 - 08:47
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Hi guys,

Just wondering if you can guide me to configure my ATA 186 port 1 for Fax pass-thru?




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