[SIPForum-discussion] Fax call flow

[Maciej Wasiel]

Hi Ridhuan,
Even if T.38 is not supported, don't be surprised to see a second G.711 re-invite from the terminating gateway once it detects a fax call. This re-invite will often suggest disabling silence suppression and changing the ptime in order to improve the line conditions for the fax pass-through call.

Maciej

From: discussion-bounces at sipforum.org [mailto:discussion-bounces at sipforum.org] On Behalf Of vidyut gupta
Sent: Monday, August 23, 2010 6:01 AM
To: discussion at sipforum.org; ridhuan yusof
Subject: Re: [SIPForum-discussion] Fax call flow

Hi ridhuan,

The SDP exchange will be different for both the call flows, in case of T.38 the SDP will have image in "m=" line and corresponding parameters in "a=" line. Generally UA tries for send the fax using t.38 but in case other party does not supports, the it fall back to pass through fax.You see T.38 SDP in second or third INVITE because initially they might have established the audio call and after that fax would have initiated. have a look at the following draft, it will surely help you :

draft-ietf-sipping-realtimefax-01

--Vidyut


--- On Fri, 8/20/10, ridhuan yusof <mridhuan at gmail.com> wrote:

From: ridhuan yusof <mridhuan at gmail.com>
Subject: [SIPForum-discussion] Fax call flow
To: discussion at sipforum.org
Date: Friday, August 20, 2010, 9:08 PM
Folks,

I really not understand which are the correct call flow for fax..in my case i have 2 different codec i.e. using g.711passthrough and t.38. Can u explain for me how to differentiate those fax flow... Sometime i noticed that the SDP carried T.38 in 2nd and 3rd INVITE msg..Any reference/standard can be referred?

Regards,
Ridhuan

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