[asterisk-biz] New faxing protocol. Good/Bad ?

[Trixter aka Bret McDanel]

On Mon, 2008-06-02 at 00:54 +0300, Dovid Bender wrote:
> Hi List,
> I was thinking the other day that even with T.38 there are still some
> issues with faxing. I was thinking of a protocol that instead of just
> sending down the fax tones an ATA or "VOIP fax machine" would get the
> entire fax convert it into some sort of image and pass it down the
> line to the receiving end. I got the idea from RFC2833. Yes I know
> that fax machines send bit by bit and get a conformation on it but
> maybe this would work a bit better. Send the entire "image" over and
> then get a response when it is done. This way if there is issues along
> the way the packets can be re-sent with out any issue.
>  
> Dovid
> (Cross posted to Biz for those that aren't on the users list - I want
> their onion too ;) ).


first t.38 is a codec not a protocol as that word is commonly
defined. :)

What you are describing is similar to T.37, which uses smtp, mime and
all that and is in essence store and forward.  

The reason for T.38 is so that the capabilities of each of the two
endpoints can be agreed upon.  With T.37 for example, the least common
matching features between the two endpoints and the gateway in the
middle are what ends up being used.  

RFC2833 lets you represent tones, you could in theory use that, however
I wouldnt recommend it as many devices do not properly support RFC2833
tones, and to convert the tones that way really provides little benefit
and is likely to trash the data.

T.38 has some redundancy built in, or at least its supposed to, so that
"this frame" and "last frame" are sent, so any single frame lost can be
recovered with the next one.  One issue is that not everyone implements
T.38 the same way, kinda like SIP, RFC2833, and some others.  If you
dont have a *really* good T.38 implementation that can identify and deal
with all the quirks you are potentially going to have issues somewhere
with some devices.
-- 
Trixter http://www.0xdecafbad.com     Bret McDanel
Belfast +44 28 9099 6461        US +1 516 687 5200
http://www.trxtel.com the phone company that pays you!