Recommended Fax Settings for Greandstream Gateways and ATA

Fax Settings for Grandstream ATAs and Gateways


This article describes the recommended fax settings for Grandstream Gateways and ATAs. The suggested parameters may not apply to all environment setups.


  1. Make sure the devices have the latest firmware version
  2. Fax Mode – Select Pass-Through if you connect it with a physical fax machine; select T.38 if your Service Provider supports it
  3. The fax machine baud rate is set to less than 14400 or 9600bps
  4. Use First Matching Vocoder in 200OK SDP: Yes
  5. Disable Line Echo Canceller (LEC): Yes
  6. Preferred Vocoder (in listed order): set all to PCMUs only
  7. Re-INVITE After Fax Tone Detected: Enabled 
  8. Jitter Buffer Type: Fixed
  9. Polarity Reversal: No

Click Update and Apply, followed by a reboot.

Troubleshooting and Logs

On the Advanced Setting tab, please configure:

– Syslog Server: IP of the PC with Wireshark  
– Syslog Level: Extra DEBUG  
– Send SIP Log: yes

– Save and Apply, followed by a reboot. The full instructions can be found here.

Understanding Jitter Buffer

A jitter buffer is used to overcome network instability, eg. sometimes the audio packet does not arrive consecutively, so the jitter buffer gathers the packet and then rearranges them. It applies to all RTP traffic passes through the devices such as UCM. We recommend having the setting configured at default (disabled).

When there are any audio delays at the customer site, verify if the jitter buffer is enabled. If it is, disable it entirely or lower the buffer size. Adjust it until audio latency and quality are acceptable. Excessive use of jitter buffer may introduce echo or robotic/synthetic voice due to compensation of packet loss.


Understanding T.38 Fax Media

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