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New Member

Fax transmission levels

THE PROBLEM IS FAX TRANSMISSION LEVEL

setup of faxe machine is

FAX--VG248 ---2651XM--PRI--PSTN

We encountered the usual problems in getting good fax transmission during the implementation of this customer.

In Australia Telstra (the Telco) provide a service that can be used to test the quality of fax transmissions

A single page fax is sent to a special number,Telstra then send back a report about the quality of the received fax.

Telstra say the ideal transmission level of a fax arriving

at the exchange should be between -15.0dBm and -17.0 dBm and the majority of fax problems are caused by the transmission level being set incorrectly.

sending a fax via the vg248 and 2651xM...the transmission level was -9dBm which telstra said was too loud. adjusting the gain at the VG248 port made no

difference, adding output attenuation of 6 to the PRI Voice port of the 2651 brought the fax into spec(-15dBm) so the Telstra report said the transmission level was excellent and overall the fax transmission was excellent...The problem then was voice was too quite.

What is the way round this problem?

does the VG248 dSPs ignore the transmission level of a fax ( because only interested in tones not volume) and put the output at a uniform level.

How is the transmisison level for a fax transmitted in the Cisco Fax Relay stream

Being able to get the fax transmission level right will fix a lot of fax problems in the AVVID scenario

by the way the fax used when connected directly to a pstn line had a level of -15.5dBm

2 REPLIES
Cisco Employee

Re: Fax transmission levels

It all depends upon the mechanism used to transmit the fax over the VoIP section of the call. With the VG248 you can either do fax passthrough or cisco fax relay. Fax passthrough means that we just encapsulate the fax tones as an RTP stream and send them across the VoIP portion as g711. With fax relay, the DSP at one interprets the fax tones and sends the actuall fax signaling across to the other GW DSP where the fax tones are regenerated by the DSP. With passthrough, the gains setting on the VG248 will have an effect on what telestra is seeing for the level, with fax relay I don't think the VG248 gain wil have an effect (like you are seeing). More specifics are needed to diagnose further but have you tried ather fax machines and are they too loud as well? Can you configure the TX level on the fax machine? You can always use passthropugh as a workaround, but if you see that all fax machines are too loud in this setup with fax relay, please open a case with TAC to find out why.

New Member

Re: Fax transmission levels

I had to use fax passthru on the vG248s it is the only way the audio levels are transfered through the VOIP network.

The only thing I can say about this is it goes against Ciscos recommendation of using Fax Relay!

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