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New Member

Fax over IP (Bandwidth/Compression issue)

I have a question about Fax transmission over IP networks. If I configure as the following, what will be the FoIP packet size? I mean, what will be the compressed fax_data packet size?

Let the default codec is g729a (8Kbps>>12Kbps in real). And the fax call comes from voice port 1/0/0. I assume that the voice port adaptively decide on the incoming calls that the call is a voice conversation or the call is a fax message. Correct??

**************************************

!

hostname NM_A_3640

ip subnet-zero

!

voice service voip

fax protocol t38 ls-redundancy 0 hs-redundancy 0

h323 call start fast

!

!

dial-peer voice 10 pots

destination-pattern 2222

port 1/0/0

!

dial-peer voice 20 voip

destination-pattern 4444

fax rate 144400

session target ipv4:10.1.1.100

!

**************************************

Thanks,

Onur DC

2 REPLIES
New Member

Re: Fax over IP (Bandwidth/Compression issue)

The router/gateway will act differently based on if you configure Fax Relay or Fax Passthrough. Don't forget to check the ability of all the gateways involved. Either way you have to use G711 for Faxes. Faxes will not work using G729a. Search CCO for information on Fax Passthrough. I've found that to work fairly well.

Just to warn you, Faxing is a trial and error process. Make sure you talk to TAC about which software versions work best for your hardware. You will get them working pretty good, but doen't expect things to work perfectly with every fax you send. One concept that Sales-people forget to mention is that analog devices such as faxes and modems are known to have problems in the VoIP world. As expected with the difference in technologies. Don't get me wrong, things are getting better as new loads and fixes become available.

New Member

Re: Fax over IP (Bandwidth/Compression issue)

In your case you have enable on the configuartion T.38 which is the fax relay function.

The Fax transmission rate is determine by the 'fax rate command' which is set to 14.4kbps max.

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