Faxing over VoIP Lines

Faxing over VoIP Lines

Faxing over VoIP Much is provided on a best effort basis.  Faxing over VoIP is not supported by VoIP Much representatives. 
We have compiled some VoIP faxing details below to assist, if you wish to try faxing over VoIP.


 First some background on faxing and VoIP. 

Faxing has been around for years and most of the protocols were written with the intent of sending those signals over traditional phone circuits using sounds.  Those sounds were turned back into data by the receiving fax machine, which expects a constant, steady transmission of data, without any loss.  If there is some loss of data the receiving fax machine will shut down the transmission.  

The problem is that the codecs used by VoIP Carriers are designed to compress voice, not the analog signals sent and received by modems.

In a VoIP Internet world, voice is first converted into packets and then they are sent over the connections that make up our our vast Internet.  They may take slightly different times to arrive at their destination.  In doing so some packets may be discarded, but the end result is that the receiving VoIP device has enough packets to make a clear and understandable conversation.

We suggest these settings on a fax machine for faxing over VoIP;
  • slowing the transmission rate down and allowing the machine to continue receiving the transmission even though a few bits of data were lost, then faxing over VoIP can become more consistent. 
Our suggestions in many cases can resolve issues that prevent faxing over a VoIP connection, but not in all cases.  If after trying and making all the VoIP fax changes we suggest you still cannot fax over your connection then you may wish to try an Internet Fax service.


 G711 or Non-Compressed Codec 

First make sure that your VoIP fax connection (the line that you fax on) is set to G711.  This is a non-compressed codec.  G729 uses compression and will make the fax fail. This is typically a setting contained within your ATA or SIP device.


You will want to make these changes:

  • Slow down the transmission speed.  Set the BAUD rate of the fax machine to 9600 bps or even lower (7200 bps would be the next slower setting).
  • Turn off or disable ECM (error correction mode), a setting, usually defaulted on, that many fax machines have.
Some fax machines, especially Brother, have an Overseas Mode, which can be enabled before a fax. This setting makes temporary changes to the fax tones to accommodate noise and static and should be set before each outgoing fax.

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