On July 19th, I wrote a blog about Fax and FoIP and the cloud, and I got a few questions in regarding which codec to use when FoIPing, T.38 or G.711. And since Dialogic just wrote a white paper called “Considerations for Using T.38 versus G.711 for Fax over IP.” I was ecstatic that I’d have a short answer.
Before we get to the simple answer, it’s important to understand a few things. Since FoIP goes over VoIP networks, which are optimized for voice traffic, there are some complications which arise when using FoIP. The two best fax transmission options are T.38 Fax Relay and G.711 fax pass-through.
T.38 was specifically designed to carry fax data over IP networks and as such, results in excellent utilization of bandwidth and is good at dealing with packet loss. The downside of T.38 is that since it is specialized, there may be parts of the network that don’t support T.38 so transcoding might be required somewhere in the stream as it traverses the network.
G.711, which is an audio codec, is an option since it is an uncompressed format, so it’s akin to faxing over the PSTN network in that the digital fax data is converted to a PCM stream and then sent as G.711 RTP packets. The benefit of G.711 is that it’s a simple codec and there likely won’t be interoperability problems in the network. However, it is not optimized for fax and does not support packet redundancy, so fax errors might occur if the network is degraded.
So which to use? Well, the white paper I mention above is excellent at going through everything and you will need to read it because the definitive answer depends on a few things! Yes, it depends on the network so you have to read the paper! I’m not saying the paper wimps out because indeed it does depend on the factors in your network and the speed you want to fax with. T.38 has a higher tolerance than G.711. However, if the network is properly optimized then G.711 can also work fine.
The paper is definitive about one thing though - just use Dialogic Brooktrout SR140 for FoIPing and you’ll be fine because it supports our long fax field-proven history, interoperability testing, and support of all the different speeds and protocols.