[SIPForum-discussion] Sip Compatible Fax Machines

[Kevin P. Fleming]

On 05/16/2011 02:11 PM, Mike Coffee wrote:

> This has to do with outbound calls from an ATA.  As we know, FoIP calls to a
> provider begin as G.711 calls, and the SIP-peer/off-ramp gateway is responsible
> for issuing the re-Invite, if any, to T.38.  Too often, the signaling gets
> delayed while the media streams between the two endpoint fax terminals are up
> and running.  This means the state machines in those terminals can pass the
> "point of no return."  But the re-Invite finally arrives at the ATA, sometimes
> several seconds beyond the onset of modem training.  If the ATA accepts the
> re-Inivite, the fax transaction will be killed.
>
> Happens alot.  We've not been able to get to the root cause, as both the gateway
> and SBC guys say they aren't causing it, but we've developed technology that
> solves the problem when it does happen.

Also, the (hopefully soon) to-be published next revision of the T.38 
recommendation also mandates that the off-ramp gateway mute the audio 
path in both directions as soon as it determines that it wants to switch 
to T.38... this will keep the T.30 session between the endpoints from 
progressing too far. Unfortunately it will require that gateway vendors 
upgrade their software to (at least) incorporate this improvement and 
that new software be deployed to the units in the field; no small task, 
to be sure.

-- 
Kevin P. Fleming
Digium, Inc. | Director of Software Technologies
Jabber: kfleming at digium.com | SIP: kpfleming at digium.com | Skype: kpfleming
445 Jan Davis Drive NW - Huntsville, AL 35806 - USA
Check us out at www.digium.com & www.asterisk.org