[Foip] FoIP-TG: Issues with V.8/V.34/V.152 call flows.
Paul E. Jones
paulej at packetizer.com
Fri Jun 26 22:15:05 EDT 2009
Gerard,
RFC 3407 support would be good, but few do it today. Even so, I would
support making that a recommendation and/or perhaps combining that with what
Albrecht is working on. Those are the kinds of things that have long-term
benefit and do not (in theory) break anything deployed today.
Paul
From: gboulay.ext at orange-ftgroup.com [mailto:gboulay.ext at orange-ftgroup.com]
Sent: Thursday, June 25, 2009 4:12 AM
To: Michael.Chen at lsi.com; paulej at packetizer.com; Neil.Weldon at dialogic.com;
foip at sipforum.org
Subject: RE: [Foip] FoIP-TG: Issues with V.8/V.34/V.152 call flows.
Hi,
Some comments about Paul's questions :
1) Do we always start out with audio and then switch to fax?
The answer is yes at least for some services. Most of the VoIP services
providers use PSTN/IP GWs that are shared for telephony and fax (+ modems).
When a call arrives (eg from PSTN) , the gateway does not know whether or
not the call concerns an audio communication or a fax communication. It's
the reason why the GWs starts the communication in audio mode and then
switches in T.38 mode upon the detection of the fax communication (V.21
preamble)
2) Do we want to allow (or require) fax to be advertised as a supported
capability at the start of the call
I have the feeling that it should solve most of the problems that have been
identified, especially if we could send not only the capability to manage
T.38 but also some T.38 parameters (eg the T.38 version). And there is for
example a RFC that allows (with examples dealing with T.38) to advertise the
T.38 capability : RFC 3407. So, why not request to take into account this
RFC ?
Gerard
_____
De : foip-bounces at sipforum.org [mailto:foip-bounces at sipforum.org] De la part
de Chen, Ximing M (Michael)
Envoyé : jeudi 25 juin 2009 06:52
À : Paul E. Jones; 'Neil Weldon'; foip at sipforum.org
Objet : Re: [Foip] FoIP-TG: Issues with V.8/V.34/V.152 call flows.
Paul,
Some of the high level issues you raised, such as UDPTL vs TCP, T.38 with
SRTP, etc, probably should be mentioned in TG's problem statement and
investigated in specific working groups. Other issues (latent capability,
syntax for t.38 parameters, etc) may be categorized into "SDP negotiation"
issues. Neil has put the latest draft of problem statement out for reviewing
and signing off. I am OK with spending a little more time to discuss/review
problem statement as these are important issues. We can also switch to study
the "SDP negotiation" issues first if it makes more sense to everyone. We
agreed to tackle the "V.8/V.34/V.152 call flows" first during the last
conference call because everyone seems to believe that it is an "easier (?)"
and "well-defined (?)" problem.
The issues I presented are about the fallback from V.34 to V.17. I was
hoping that it would not involve too much SIP negotiation problems; after
all, people have tolerated SIP negotiation issues for so long for both
T.38/V.17 and T.38/V.34. The question is really how the existing T.38/V.17
GWs (with existing SIP format/procedures) should handle a V.34fax call.
Actually, it is a purely T.38 question regardless signaling protocols (SIP,
H323, H,248, etc) used.
Michael
_____
From: Paul E. Jones [paulej at packetizer.com]
Sent: Wednesday, June 24, 2009 6:05 PM
To: Chen, Ximing M (Michael); 'Neil Weldon'; foip at sipforum.org
Subject: RE: [Foip] FoIP-TG: Issues with V.8/V.34/V.152 call flows.
Michael,
Even before we can employ T.38, we have to have a reliable means of
negotiating its use via SIP. This can get a big complex, with lots of
different options – and I’ve seen a number of situations where fax “fails”
simply due to this.
And, it’s not as if there are not a few options we can consider. But, it is
not clear to me that the industry has agreed upon a way to signal the use of
fax.
Some examples:
1) Do we always start out with audio and then switch to fax?
Traditionally, that’s how it was done for voice gateways and there are
millions of such devices out there that expect that today. (And, so far,
that seems to be what we’re talking about primarily.)
2) I asked (1), because the next question is “how do we handle IP fax
devices that have no concept of audio”? Today, those would not work with
many GW devices.
3) Do we allow a SIP device to offer both voice and fax?
a. If yes, and the recipient can only handle one flow at a time, which
shall it accept and which shall it reject?
b. If no, and we want to transition from voice to fax, exactly what
should those flows look like?
4) Do we want to allow (or require) fax to be advertised as a supported
capability at the start of the call, even if it is not employed right away?
(That is, do we want to require that T.38 and/or V.152 be advertised as
latent capabilities in the initial INVITE?)
5) There are a few syntaxes used for the T.38 parameters. T.38 seems
to be pretty consistent in form, but I’ve seen implementations vary. Is
there anything we need to say about the syntax of the m= line and T.38
parameters? Over the years, I’ve seen parameters on multiple lines, a
single line, semicolons used and not, etc. I’m not sure if this is still a
point of confusion or not, honestly.
Also, UDPTL seems to be universally supported, though there was discussion
of using TCP. Do we want to mandate UDPTL? And, what about secure fax?
UDPTL is not secure, so we introduced audio/t38 as a means of securing T.38
with SRTP. As a part of this effort, do we want to address that?
In any case, the issues you presented are important, but it seems like some
of the decisions cannot be made until we also discuss the SIP signaling.
For example, a decision to switch from G.729 to T.38 or VBD cannot be made
without knowing whether either is even possible. And, since virtually all
implementations do not advertise latent capabilities, the move from one form
of media is really “trial and error”, leading to greater possibility,
perhaps, of fax failures.
Perhaps what scares me even more about some of the decision making is that I
know this gets tied up with modem relay and TTY tone detection, too. Not
being a DSP guru, I’ll have to yield to others to provide guidance in that
area. But, how much lower-layer communication has to occur before one
determines that the device is a fax or a modem? And, at such time as the
determination is made, what timing constraints do we have for SIP signaling?
Paul
From: foip-bounces at sipforum.org [mailto:foip-bounces at sipforum.org] On Behalf
Of Chen, Ximing M (Michael)
Sent: Monday, June 22, 2009 10:46 PM
To: Neil Weldon; foip at sipforum.org
Subject: [Foip] FoIP-TG: Issues with V.8/V.34/V.152 call flows.
Hi all,
Instead of looking into the call flows for all possible system/network
configurations (which will be many), it probably makes more sense for us
to concentrate on a few cases where clarification/guidance is needed from
TG. Attached are brief descriptions of two possible cases. Both of them are
related to fallbacks from V.34 to V.17, which are not specified in T.38.
Please comment on these two cases and/or add any other cases that you
think TG should look into and provide recommendations.
Regards,
Michael
_____
From: foip-bounces at sipforum.org [foip-bounces at sipforum.org] On Behalf Of
Neil Weldon [Neil.Weldon at dialogic.com]
Sent: Thursday, June 18, 2009 5:41 AM
To: foip at sipforum.org
Subject: [Foip] June 17th Meeting Minutes.
Hi all,
Minutes from yesterday are attached. I’ll follow-up with the updated
Problem statement.
Next meeting will be called by Michael Chen, via this list. Please ensure
you review material that Michael will provide over the coming weeks so we
can gather some momentum for dealing with the identified problems.
Regards,
Neil
Neil Weldon
Director of Technology
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Dialogic Distribution Limited
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