Knowledgebase: IP Fax
T.38 Fax Overview
Posted by Albert Diaz, Last modified by Gabe Olano on 30 August 2022 01:41 PM

How Does T38 Faxing Work?

Fax machines on the PSTN communicate using the T30 protocol. T38 encapsulates the T30 transmission in real-time and uses data redundancy methods to ensure the complete delivery of a fax. By sending the same packets up to 5 times, most issues with jitter and packet loss on the last mile do not affect the fax completion rate.


Is T38 Faxing Reliable?

In short, YES. Infact we send 40-60 page faxes with it all the time. Even, non T38, 711 fax pass-though transmissions work very well.

However, for most people, their prior experiences are usually not the same as ours and it's for for a couple reasons; bad carrier implementations, bad services and or bad hardware.

Infact, most T38 and 711 fax pass-through failures are blamed on poor last mile internet connections with jitter, latency and packet loss as the usual suspects. However, FoIP failures are much more commonly related to 2 other issues known as ‘late-arriving T.38 re-Invites’ and ‘PCM Clock Sync’ problems.

Late-arriving T.38 re-Invites sent after a fax transmission has already begun will cause the fax to drop; and PCM clock sync mismatches, found on 711 pass-through transmissions, steadily grow per fax page also causing the fax to drop. Both problems result in the all too common complaint of “1-2 page faxes work fine but big faxes fail”.

To address these problems, RingLogix  has partnered with Netgen, a manufacture of Fax Aware analog telephone adapters (ATA). Netgen has developed a patented solution to address the ‘late-arriving T.38 re-Invites’ and ‘PCM Clock Sync’ problems mentioned. 

To resolve the first issue the ATA monitors the line and listens for fax transmissions. If a fax is already in progress and a late-arriving T.38 re-Invite is received, the ATA will reject it and continue the transmission in 711 pass-through mode. To address the PCM clock sync mismatch that occurs on larger faxes, the ATA will re-sync the PCM timer’s in-between each page, essentially treating a multi-page fax as several single page faxes. This is done without delaying the original transmission.


How to purchase Fax-Aware ATA's

From Netgen Directly:

Recommended Models - Configurations:
  • HX402E - 2 FXS Ports
  • HX404E - 4 FXS Ports
  • HX402G - 4 FXS Ports
  • HX404G - 4 FXS Ports

Click here for a configuration guide.