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Fax over VoIP

By Greg on June 4, 2018 | Blog, Newsletter, Tech Tips

How to fax over VoIP.

VoIP Faxing: Faxing over VoIP can be a significant challenge, especially for a business that depends on numerous multi-page faxes.  If your business depends on faxing, like a mortgage company, attorney’s office or medical provider we recommend that you keep a dedicated copper line (POTS line) for your fax machine or use an online Internet Fax service.  If you are an occasional user of faxing then with a few configuration changes on your current fax machine, you may find fairly reliable faxing over your VoIP Internet connection.  For those of you that fall somewhere in between we suggest a cost effective reliable recommended Internet faxing service, which offers additional benefits that most businesses find appealing.

An explanation of the problems that exist with faxing & voip.

Faxing ( Facsimile Transmission) has been a standard for years with most of the faxing protocols having been written with the intent of sending the faxing signals over traditional phone circuits using sounds.  Those sounds were then turned back into data by the receiving fax machine.  The receiving fax machine was designed to expect a constant, steady transmission of data, without any loss.  If loss of data occurred then the receiving fax machine would shut down the transmission.  VoIP behaves differently in that voice is first converted into data packets and then sent over the Internet connections to their final destination.  These voice data packets may take slightly different times to their arrival point.  When that happens some packets may get discarded, but the end result is that the receiving VoIP device has enough packets to make a clear and understandable conversation.

Fax machine settings for VoIP faxing.

We recommend the following settings for best voip fax results.

  • G711 or a Non-Compressed Codec
    Make sure that your VoIP fax connection (the line you use for faxing) is set to G711, which is a non-compressed codec. G729 uses compression which will make faxes fail.  If your VoIP provider has what may be called a “Bandwidth Saver” setting, then this setting would be a compressed codec.  Make sure this setting is not enabled. You may have to talk with tech support for your VoIP service who will verify what type of codec your connection uses and to make sure your connection is set for G711.
  • Slow the transmission speeds on the fax machine settings.  Both the Tx and Rx settings.  This is called the BAUD rate of the fax machine.  The best and fastest setting should be 9600 bps.
  • Disable ECM (Error Correction Mode).  ECM is usually on by default on most fax machines.  This setting needs to turned off.

Why ECM should be disabled: 

Fax machines (also multi-function machines) set ECM as a default to on. T his setting causes the receiving fax machine to analyze the received data frames and detect any data that may be corrupted.  Any noise, lost packets, or poor signal strength can cause part of the the transmission to be lost which then triggers a retransmit signal to be sent until an error free frame is received.  This causes retransmits more often over VoIP due to some packet loss, latency or jitter which exist on an Internet connection.  These retransmits increase the call duration, quickly adding to the instability of the fax, requiring additional retransmits which then cause a failure with a communication error reported by the receiving fax machine.

Disabling ECM, will result in the receiving fax machine to accept a transmission with some frames that may have bits of information missing, but still resulting in a completed fax, which should be of acceptable quality.

Faxing over voip – when other alternatives are not available

Companies that depend on multiple page faxes inbound and outbound should first and foremost use a dedicated copper phone line.  A great alternative later technology choice would be to use an Internet fax service.

Where technology is taking faxing.

Better fax protocols for transmissions over IP have been in existence for some time, but this has not seemed to alleviate many users major issues with faxing over VoIP.  Anything but traditional copper lines are still suspect when it comes to faxes for heavy users.  Too many businesses, like attorneys, real estate offices and medical facilities depend on a stable full proof fax environment and will continue to do so for the foreseeable future. Unfortunately there is no one best solution


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