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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:




Onur DC

New Member

Re: Fax over IP (Bandwidth/Compression issue)

When the call is turned to fax mode will use fax ... settings.

For fax relay through G729 codec (8k) the rate must be according this codec. I use fax rate 7200 for g729r8 codecs.

Also i use the followin parameters to ensure fax quality:

fax rate 7200 ! speed setting g729

fax nsf 000000 ! Disable NonStandar negotiation

fax-relay ecm disable ! Disable Error Correction Mode


Ezequiel Colombo

New Member

Re: Fax over IP (Bandwidth/Compression issue)

Thanks for your aid about quality parameters, I will try.

If the voice port is capable of differentiating in between the fax_call & voice_call, speed of 7200 will seem to work as you mentioned.

Have a nicer day,

Onur DC

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