[SIPForum-discussion] FAX Call Identification

[Ashoke Kumar Ghosh]

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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