[Sipping] Unofficial communication from ITU-T on T.38

[Glenn Parsons <[e-mail]>]

Folks,

I am not sure if the official communication from ITU-T SG16 on T.38 has been
sent/received on what is now draft-ietf-sipping-realtimefax-00.txt (it was
draft-mule-sip-t38callflows-02.txt.  )

As a result, in order to ensure that the group is aware of this before the
meeting next week, I have attached a copy of the communication below.

Cheers,
Glenn.


_____________________
COMMUNICATIONS STATEMENT
TO:    IETF SIPPING WG
APPROVAL:    Agreed to at SG16 meeting (Geneva, 5-15 February 2002)
(proposed)
FOR:    Action
DEADLINE:    April 2002
    
CONTACT:    Mr. Tom Geary
Rapporteur Q14/16         Tel:[phone]
 Fax:[phone]
E-mail : [e-mail]
_____________________
1. Introduction

At the ITU-T SG16 held 5-15 February 2002, the facsimile experts of Q14/16
had the opportunity to review the proposed call flow examples of
draft-mule-sip-t38callflows-02.txt.  We have a number of general and
specific comments that we would like to present.

2. General Comments

Q.14/16 is interested in collaborating with the SIPPING WG on the T.38 SIP
call flows work.  We would appreciate if future comments on this Internet
Draft could be copied to our experts mailing list to allow ITU-T fax experts
an opportunity to comment.  The group¹s mailing list is:  [e-mail]

Also, Q.14/16 would like to point out that at our meeting held 5-15 February
2002, a revised Recommendation T.38 was consented (similar to IETF last
call).  As well as fixing minor issues, this revised Recommendation
incorporates all of the pieces of T.38 into one document.  If it is approved
(that is, no negative technical comments are received), this Recommendation
will be available on the ITU-T website in May 2002 at the following URL:
http://www.itu.int/rec/recommendation.asp?type=folders&lang=e&parent=T-REC-T
.38

IANA has finally registered the requested T.38 SDP extensions.  They can be
viewed at: http://www.iana.org/assignments/sdp-parameters.  Note, however,
that the addition of the image/t38 MIME type is still in process as it
requires the approval of an RFC documenting the registration  (see
draft-parsons-itu-t38-reg-00.txt ).

Q.14/16 would like to suggest adding the call flows of this Internet Draft
to Rec. T.38.  Unfortunately, the ITU-T process will not allow the
referencing of a non-ŒStandards Track¹ IETF RFC in an ITU-T Recommendation.
As a result, we would suggest referencing this document in a future
informative Appendix to Rec. T.38 (similar to the current Appendix
describing H.248 call flows).

We note from the SIPPING WG status web page that the intent is to include
call flows for modems over IP (MoIP) in this document.  V.MoIP is currently
under study in Q.11/16 (the group¹s mailing list is:  [e-mail]).  As
this work is expected to be significantly different, Q.14/16 recommends that
SIP call flows for MoIP be detailed in a separate document.  It would make
sense in this separate document to describe the SIP call flows of V.34
half-duplex T.38 as well as V.MoIP as both will require V.8 control signal
analysis for call discrimination.  This would allow the immediate
publication of the current draft for the benefit of the industry.

2. Specific Comments

We reviewed text on call discrimination in section 4.  We agree with this
process for pre-V.34 implementations.  As a result, the section title should
indicate ³Šand fax detection up to V.17 (14400bps)².  SG16 is currently
working on extensions to T.38 to support V.34 fax, as well as a new
Recommendation to support modems over IP.  We therefore suggest that the
following sentence be added to the end of section 4.1.:

Note that V.34 half-duplex fax (up to 33600bps) over T.38 will require use
of more complex call discrimination methods that will require analysis of
V.8 control signals (that are also used for V.34/V.90 data).  An Amendment
to Rec. T.38 to support V.34 fax, as well as a new Recommendation V.MoIP to
describe data modem over IP are under study in ITU-T SG16.

Further, we confirm that fax detection (of V.21 flags) by the receiving
gateway is the preferred method.  This was our original intent and we have
modified Rec. T.38 section D.2.2.4 to reflect that.  In addition , we have
also indicated that both Œvoice and fax¹ and Œfax replaces voice¹ modes are
possible.  We note that you recommend Œfax replaces voice¹ and would be
interested in understanding the rational for this choice.

In section 4.4 (and at least example 6.2) it is indicated that when falling
back to Œpass-through¹ mode several characteristics should be followed.  You
should note that Q.11/16 is defining specific characteristics for a pass
though mode in V.MoIP that is called Œvoice band data¹.  For your
information, the current definition in V.MoIP is:

For Voice Band Data (VBD) mode of operation, the path between G1 and G2
remains in a voice configuration. The modem signals are encoded using an
appropriate speech codec suitable for the task and the samples are
transported across a packet network.

VBD should:
o Use a voice codec that passes voice band modulated signals with minimal
distortion.
o Have end-to-end constant latency.
o Disable Voice Activity Detection (VAD) and Comfort Noise Generation (CNG)
during the data transfer phase.
o Disable any DC removal filters that may be integral with the speech
encoder used.
o Be capable of tone detection, including mid-call DTMF, as well insertion
of tones, announcements, voice prompts etc.

VBD should consider the appropriate application of:
o The use of echo cancellers on a VBD channel.
o Forward Error Correction (FEC) (e.g. RFC 2733) or other forms of
Redundancy (e.g. RFC 2198).
o Voice packet loss concealment techniques and algorithms that may not be
suitable for VBD.

While SG16 agrees that G.711 is a valid codec that can be used for this, we
are concerned about your the choice of enabling the echo canceller.  You
will note that we have not made a recommendation for enabling or disabling
echo cancellation in VBD.  In T.30 transmission, however, echo cancellers
may be turned off (by the appropriate answer tone) for a more effective fax
transmission.  Hence there are varying perspectives on the most applicable
setting for echo cancellation amongst ITU-T fax and modem experts.  We would
appreciate understanding your rationale for this recommendation.  In
addition, you should note that the presence of adverse network conditions
(notably jitter and delay) will have a significant effect on whether the fax
connection will be successful.

Section 5.1 indicates that the T.38 Annex D is a temporary document ­ this
is incorrect.  T.38 Annex D should be introduced earlier in this document.
However, in this case, we suggest that a more appropriate description would
simply delete everything after the comma:

The mechanism for supporting T.38 in SIP & SDP is detailed in T.38 Annex D
[9].

In section 5.2, contrary to section 4.1 (and the terminology of Rec. T.38),
the term Œterminating gateway¹ is used instead of Œreceiving gateway¹.  The
document should be consistent and use the terms emitting gateway and
receiving gateway (as defined in Rec. T.38) throughout.

In section 3.1 and before F11 of section 5.2.2 and 5.3.2 it notes that the
CNG (and other tones including V.21 flags) can be sent in-band.  You should
note that CNG and these other tones sent in-band in any codec besides Œtoll
quality codecs¹ like G.711 A or mu-law (and especially in compressed codecs)
will not be recognizable by the receiving fax machines.  This should be
explained in the text.

In section 7 (and in many of the examples in section 5), values of Œ0¹ are
given to the boolean SDP attribute names.  This is incorrect, the attributes
are defined as boolean without values.  That is, their presence on an a=
line indicates that they are true for the session.  As a result, this
informative table requires modifications.  Further, many of the examples
will also need modification.  For example, the SDP in the first example in
section 5.1.2 should be:

F1 INVITE  I.FAX UA -> PROXY
    
      INVITE sip:[e-mail];user=phone SIP/2.0
      Via: SIP/2.0/UDP ifax.here.com:5060
      From: sip:[e-mail];user=phone;tag=ab111
      To: sip:[e-mail];user=phone
      Call-ID: [e-mail]
      CSeq: 17 INVITE
      Contact: <sip:[e-mail]>
      Content-Type: application/sdp
      Content-Length: 320
    
      v=0 
      o=ifaxgw1 2890846527 2890846527 IN IP4 ifax.here.com
      s=Session SDP
      c=IN IP4 ifaxmg.here.com
      t=0 0 
      m=image 15002 udptl t38
      a=T38FaxVersion:0
      a=T38MaxBitRate:14400
      a=T38FaxFillBitRemoval
      a=T38FaxTranscodingMMR
      a=T38FaxTranscodingJBIG
      a=T38FaxRateManagement:transferredTCF
      a=T38FaxMaxBuffer:72
      a=T38FaxMaxDatagram:316
      a=T38FaxUdpEC:t38UDPFEC
      a=T38FaxUdpEC:t38UDPRedundancy

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