[SIPForum-discussion] FAX Call Identification

[Ronald del Rosario]

Ashoke,

 

Please review the ITU Specification for T.38 (fax over IP) and IETF's
SDP Extensions for Fax over IP Using T.38
<http://tools.ietf.org/html/draft-ietf-mmusic-sdp-t38-00> .

 

All your questions will be answered there, well almost J

 

Best.

Ron

http://packetboyperseus.blogspot.com

 

 

 

From: discussion-bounces at sipforum.org
[mailto:discussion-bounces at sipforum.org] On Behalf Of Ashoke Kumar Ghosh
Sent: Wednesday, January 05, 2011 8:20 PM
To: discussion at sipforum.org
Subject: [SIPForum-discussion] FAX Call Identification

 

Dear All,

 

We have SIP Application server platform that works in B2BUA mode. The
platform handles the prepaid charging as well.


The Application Servere receives SIP request from the NGN Elements,
applies account validation and initiates one new SIP Dialog with NGN for
routing towards B party. The Application Server maintains two separate
dialog for A party and B party.

 

We have observed that Re-INVITE message is received for normal voice
calls for heart beating purpose for both legs of the call. The
Application Server sends 200 OK on receipt of Re-INVITE message without
SDP.

 

PROBLEM STATEMENT:

 

We are having one problem in terms of identifying the fax calls. Sample
traces taken show that for fax calls are being set up initially like
normal voice calls. After dialog confirmation, Re-INVITE message is
being received from A party side with modified SDP. Since Application
Server considers Re-INVITE as heart beating message, it is simply
generating 200 OK without SDP. As a result, fax calls are failing.

 

QUESTIONS:

 

I would like to receive suggestions/references for handling fax calls.

 

1. Is there any RFC that proposes how fax calls should be handled.

 

2. How to identify the fax call/normal call. This is critical from
charging aspect. Unless we are able to identify the call as fax call, we
can not apply separate charging for fax calls.

 

3. We have raised the issue with our NGN vendor. They have suggested to
compare the SDP parameters and for all Re-INVITE messages and if there
is any difference observed from the initial SDP, then pass the modified
SDP to B party leg.

 

I am not sure if Application Server as a B2BUA should inspect the SDP
parameters and apply such validations. This is not only going to have
lot of overheads, it is not going to resolve the issue of charging as
fax call identification will still be required.

 

Would appreciate a quick response from the experts.

Thanks & Regards...

 

Ashoke Kumar Ghosh 

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