Using T38 for Fax

How to add T.38 Fax Capabilities in Asterisk

Advantages and Disadvantages of T.38

Data is transmitted directly without conversion to an audio stream employing T.38, which uses less bandwidth. T.38 also helps mitigate the effects of packet loss, which can be deadly for faxing, by using controls and data redundancy.
Some of the disadvantages when using T.38 are that gateway support for both V.34 transmission speeds and ECM (Error Correction Mode) is not universal. Additionally, in a mixed environment of packet-switched and PSTN, T.38 increases its transcoding overhead, which can add latency.
All in all, with a clean network, low latency, no packet loss, and low jitter, enabling T.38 can make for a better fax environment, which should lead to good success.

how to configure Asterisk to use T.38

  • Start by going to Setting, then Asterisk Sip Settings, then Chan Sip.
  • Set the following parameters:
  • T38 Pass-Through: Yes
  • Under Advanced General Settings add the line t38pt_udptl = yes
  • Then use the following: Setting -> Fax Configuration
  • Error Correction Mode= ON
  • Minimum transfer rate= 9600
  • Maximum transfer rate= 14400
  • Reboot and then test by faxing a few documents.