Asterisk VoIP News

Friday, March 10, 2006

Development news :: T38 passthrough support

Friends in the community,

There is a lot of cool stuff going on in Asterisk development, things that will change Asterisk and make it work better in your organisation, make it easier to sell in your area or give you more consulting oppurtunities - in short, functionality that will make a lot of sense for you users.

However, developers can't really get anywhere without a dialog with the users. You know what you need, you know what is missing and how you would like to
make Asterisk a better choice.

I am planning to send out a description of new features now and then, to inform you about what is going on, but also to get some feedback. The bug tracker is not only a tool for developers, but also for testers and users to react to changes and contribute.

*** ITU T.38 -- Fax over VoIP

Fax over VoIP is a hot issue. VoIP service providers encourage people to switch to VoIP but often forget to mention that faxing over VoIP is like russian
roulette. On a local LAN, it might work if you pick a clear channel codec like G.711. Steve Underwood, member of the Asterisk developer team, has writen a good article about the problems involved and the solutions for it on his web site, the URL is:

T.38 is an ITU standard for fax over VoIP. To simplify, the idea is to decode the fax audio stream at the ingress point, convert it to a data stream that is not
sensitive towards jitter or delays and encode it into audio again if needed at the other end of the call - if you can't convert it to an image somewhere in the middle and print it directly, or send it by e-mail.

*** T.38 PASSTHROUGH in Asterisk

Steve is the main contributor behind the work for T38 support in Asterisk. He's also
the author of spandsp - the fax application that many use in Asterisk. The first part
is to be able to send T38 calls to your Asterisk PBX and make Asterisk recognize
this and forward the data stream to another endpoint that supports T.38.

Asterisk won't be an T.38 endpoint, but will handle T.38 calls properly, regardless
if the T.38 was offered in the original call setup, or if the caller suddenly sends a fax in the middle of a call (a re-invite). The requirement is that the incoming channel and the outbound channel both supports T38. If not, the call will be
declined in a proper way.

When this is tested and stable, work will continue to see if we can make Asterisk an T.38 endpoint.

This is a very important addition to Asterisk. There is code for testing available.
If you are interested, please check this URL in the bug tracker:

I think this is a big step for Asterisk. Do you? If so, don't forget to say "thank you" to Steve Underwood - Coppice!

Have a nice weekend!