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Fax settings - Linksys ATA

When having difficulty faxing over VoIP make these adjustments in your Linksys ATA.

There are some configuration settings which can be made inside the Cisco/Linksys ATA which will help faxing.  The first step in configuring these settings would be to access the Linksys PAP2t or other Linksys ATA device itself and have access to the Advanced Settings.  Some VoIP providers may "lock" the device, in which case you may not have the ability to make these advanced fax adjustments.

Typically, faxing can be disrupted or act negatively to silence suppression and  to echo cancelling techniques, so these variables would want to be disabled in many cases, as well to packet loss, which can quickly cause a fax to fail. 

Access your ATA device and click Switch to  Advanced View.

The Linksys PAP2T has an Advanced View which is where many settings can be made.


Next choose the Line that is being used for faxing. 

Network Jitter is network induced delays of varying amounts for received packets (RTP). The Linksys ATA is capable of keeping a reserve of RTP samples which is known as the Jitter Buffer. The larger the buffer the more jitter the ATA can absorb, but it also will increase delay. A larger Jitter Buffer setting on the line that is used as a fax line will help absorb larger amounts of jitter and help keep the fax from failing.

Look for Network Settings and Set Network Jitter Level to Very High.

Set the Network Jitter Buffer to Very High on your Linksys ATA for VoIP faxing.

Note: You may find a extremely high setting.  This setting could also be considered.

Next Go to the Audio Configuration section.

Use the ATA configuration settings for better VoIP faxing.

Make the following Configuration Settings in the Audio Configuration section in your Cisco/Linksys ATA device.  These settings will help resolve issues with difficulty when trying to fax over your VoIP connection.  (See the note below with regard to FAX Disable ECAN.)

  • Preferred Codec to G711a
  • Set Silence Supp to No (default setting).
  • Set Echo Supp Enable to No.*
  • Set Echo Canceller to No.*
  • Set FAX Passthru Method to NSE.
  • Set FAX Passthru Codec to G711u.
  • Set Use Pref Codec Only to Yes.
  • FAX Disable ECAN to Yes.*

Fax Disable ECAN is a configuration in some Linksys ATAs.*Note: The Fax Pass-through Mode is triggered by NSE events, which we suggest that you choose as the setting of choice.  If available in your configuration settings, the setting "FAX Disable ECAN" can be set to Yes, which will trigger the echo canceller to be automatically disabled and both call waiting and silence suppression to be automatically disabled for the fax pass-through.  Additionally, out-of-band DTMF Tx is also disabled.

Setting the RTP Packet Size.

You may find that the setting for the RTP Packet Size is 0.03 (which is default setting), in which case a lowering of this setting would be more advantageous for faxing.  Real-Time Protocol (RTP) Packet Size choices are typically 10 or 20 or 30 ms with a typical setting of being 20 ms or 0.020.
RTP Packet Size in seconds provides lower latency, but is less efficient, in that more bandwidth will be used for the connection. In general smaller packet sizes would use more bandwidth, however this would be good for faxing since latency is decreased.

First Go to the SIP tab

Then look for the RTP Packet Size Setting.

On most Cisco/Linksys VoIP ATAs ypu mau need to set the RTP packet size for better faxing.

Make this setting to the standard setting of 0.20 which will help improve faxing success.