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New Member

Transmission Volume on a FXS Port


We have a fax machine connected to an FXS port in a 2851 (CME 4.1). Running tests on the machine reveal that the transmission volume is set too high, causing distortion on faxes and an inability to fax to some destinations.

Is there a way to turn the transmission volume down at the port-level? Apparently it cannot be done on the fax machine (? this sounds odd to me...but the technician for the machine is adament)



Cisco Employee

Re: Transmission Volume on a FXS Port

This depends if the faxing is being done via fax relay or fax passthrough.

If fax relay is being used, the fax traffic originates from the egress voice port connected to the PSTN, not from the FXS port that has the fax machine on it. The output level of the fax relay codec is fixed - at this stage it is not possible to alter the level of the outgoing audio on the 2800/3800 ISR's.

If you use fax passthrough, the audio is transmitted via a G711 codec. You can use the voice port 'input-gain' or 'output-attenuation' command to boost or reduce the levels in different directions.

If the level FROM the CME system towards the PSTN is too high (for example 3dB), you would use the command 'input-gain -3' on the FXS port to reduce the inwards audio level by 3dB, so the level SENT to the PSTN is correspondingly reduced by 3dB.

Most fax machines can set the outgoing audio levels - they may have a special config menu that is not normally directly accessible by users.

New Member

Re: Transmission Volume on a FXS Port

Is fax-passthrough the default? The config does not specify either...

Cisco Employee

Re: Transmission Volume on a FXS Port

Fax relay is default for fax operation.

If you are doing a local hairpin connection (FXS to PRI/BRI), you could try making fax pass-through the default faxing mode -

voice service voip

fax protocol pass-through g711ulaw

Don't worry about the ulaw being specified as the passthrough codec if this is connecting to a E1 interface - the DSP will do an lossless and automatic ulaw to alaw conversion for the PSTN side of the connection.

Might help to see the full configs if you are still seeing problems.

New Member

Re: Transmission Volume on a FXS Port

Thanks for your responses. I will check it out again when back in the office after the weekend. Will post results after that.