Subject: Re: [Sipping] Question about SIP support for real-time fax

From: [email-masked]
Date: Fri, 2 Aug 2002 15:59:21 +0530

Hi  Eiji,

If i may attempt to respond, i think what you've actually tried out is the
scenario in section 5.2
where a voice path has to exist for receiving end to detect the fax tones
and then setup a SIP connection. See section D.2.2.4 of T.38(04/02).

However section 5.1 refers to a case where a voice path does not exist
(only fax) and negotiation  is done by passing relevant information in the
SIP SDP. And therefore, there is no need to allow voice path first for
tones to pass and then detect fax.
(In fact negotiation in both cases is actually done using SIP SDP)

Please see  section D.2.1.1 and D.2.2.3 of ITU T.38 specs(04/02).

Scenario in Section 5.1 does appear to be a valid scenario from T.38
perspective.

Section 4 of the referenced draft says:
"The Internet telephony gateway only supports T.38 real-time
   fax communications (by design or by configuration).  In this case,
   the Internet fax gateway SHOULD initiate the SIP session with T.38
   SDP capabilities (this is typically the case of Internet fax
   terminals, also called Internet-aware fax devices or the case of
   gateways statically configured to support T.38 fax calls only)".

regards
Harsh Bhondwe
hughes software systems

Eiji Tomimura <[email-masked]> on 08/02/2002 01:31:37 PM

To:   [email-masked]
cc:

Subject:  [Sipping] Question about SIP support for real-time fax

Dear all,

I resubmit this question since it has got no answer yet.

A about draft-ietf-sipping-realtimefax-00.txt,

In section 5.1 "Internet fax device  fax only support" call sequence, it
shows a diagram to establish T.38/UDPTL session from the beginning. We
tried
this scenario and realized the following thing:

--
(1) In case both SIP-UA are telephony gateway which are configured to
support fax only, T.38/UDPTL session are established as the section 5.1
represents.

(2) FAX machines tries to negotiate their capability by exchanging CNG and
CED tones.

(3) As T.38/UDPTL session basically doesn't have a capability to pass
through CNG or CED tones, this negotiation fails.

(4) The Calling FAX machine terminate fax call as it cannot get CED
response
from the callee side. Then, FAX fails.

---

So, if FAX machines need CNG/CED exchange to start fax, this call scenario
is not appropriate. Instead, we should always use section 5.2 call
scenario,
which use voice stream to exchange CNG/CED first.

So, my questions is that, is section 5.1 is appropriate from fax machines'
and T.38 session's perspective?

If not, I would like to recommend to remover section 5.1 of
draft-ietf-sipping-realtimefax-00.txt, and allows only the procedure
described in section 5.2.

Regards,
- Eiji Tomimura

--
Eiji Tomimura
Senior Engineer
Systems Technology Dept., Information Technology R&D Laboratories
Sumitomo Electric Industries, Ltd.
Phone: +81-6-6466-5609  Fax: +81-6-6466-5732  Email: [email-masked]

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_______________________________________________
Sipping mailing list  https://www1.ietf.org/mailman/listinfo/sipping
This list is for NEW development of the application of SIP
Use [email-masked] for questions on current sip
Use [email-masked] for new developments of core SIP

---

From: Eiji Tomimura <[email-masked]>
Date: Sun, 04 Aug 2002 16:51:17 +0900

Hi Harsh,

Thank you very much for your information. I agree with you that from T.38
spec's perspective, section 5.1 procedure should be supported.

My question is, as David mentioned, that this section 5.1 is really
applicable for a PSTN gateway or not.

I understand that a incoming PSTN gateway could establish T.38 session
instead of Voice path first. But, in that case, traditional fax machines
connecting to the gateway might fails because it might not be able to
accomplish CNG/CED handshake between calling fax machines.

That's why, I think we need more clarification how scenario 5.1 is used, and
the section 5.1 is really applicable for a PSTN gateway or not.

Regards,
- Eiji Tomimura