When having discussions about fax over IP, I’m often asked what the differences are between T.38, T.37, and G.711 and which is the best choice in a FoIP deployment. The short answer is that T.38 wins. The longer answer… well, I’m not a network engineer, so I figured I’d poke around and find some good resources that reinforce my position of T.38 being the best choice for FoIP.

I recently stumbled across a great article written by David Hanes of Cisco that discusses the reasons for T.38 being adopted as the standard for fax over IP.

Real-Time FoIP

T.38 is a real-time protocol compared to T.37 which uses a store-and-forward approach. T.37 also relies on SMTP which gives rise to a few concerns, most notably delivery of time-sensation information, confidentiality of information, etc.


This is the biggie. Faxing in general is rather time-sensitive. The digital data contained in fax transmissions can be affected by things like packet loss, jitter, and latency. This is one of the reasons why G.711 fax passthrough is not the best choice. Sure fax passthrough is real-time, but there’s no measure for redundancy with G.711. This is really highlighted by David Hanes when he talks about redundancy lab tests. Tests using fax passthrough showed that even packet loss below 1% can result in unreliable fax transmissions, whereas T.38 with full redundancy demonstrated it can tolerate packet loss levels in excess of 10%. That’s a substantial difference.

Standardization and Interoperability

While there were past initiatives to standardize fax passthrough, it never really caught, resulting in interoperability issues. On the contrary, T.38 is an ITU-T standard that offers multi-vendor interoperability.

Imecom’s Useit Fax Server includes support for two IP fax drivers – Brooktrout SR140 by Dialogic and XCAPI by TE-Systems – to deliver reliable, compatible T.38 FoIP. Our FoIP solutions have been tested for interoperability with popular VoIP systems and equipment from the likes of Cisco, Avaya, Alcatel, Mitel, ShoreTel, AudioCodes, and others.

If you’re in search of a complete reliable T.38 FoIP solution, we invite you to learn more about our handSIP T.38 Fax SIP Trunking. handSIP T.38 Fax SIP Trunks offer affordable, reliable faxing over SIP with exceptional success rates and performance.