Subject: WG: [Sipping] WGLC for Q.SIP-SIP mapping draft

From: Luis Emilio Martin <[email-masked]>
Date: Wed, 13 Aug 2003 11:07:44 +0200

Hello,

> The QSIG SETUP message may optionally transport HLC and LLC Information elements which are not mentioned
> in the draft. You might want to add something like into section 10:
>
>"The gateway SHOULD (or SHALL(?)) not include a Low layer compatibility in the QSIG SETUP message. The 
> gateway SHOULD not include a High layer compatibility information element in the QSIG SETUP message.
>
>NOTE: Inclusion of a HLC in the QSIG SETUP message might be useful, if the gateway knows prior to 
> sending the QSIG SETUP message that fax is being transmitted (e.g., because of a corresponding
> SDP offer), it may include in the QSIG SETUP message a High layer compatibility information element 
> indicating group 3 fax."

It is discussed the possibility to include HLC IE in the QSIG SETUP if
an INVITE
was received with fax transmission information (i.e. T38 only), so that
fax
detection at destination does not depend on processing audio data. It
possibly is a matter of
configuration in the gateway.

Sorry I am not very used to T.38 procedures and Fax Group 2/3 methods
but we are testing some gateways
with T.38 and automatic fax detection capabilities based on DSPs and
experiencing some strange 
behaviours in these gateways, so that SETUP messages including HLC
produce INVITEs offering G.729 or 
G.711 and a bit later new INVITES offering t38.

In reverse direction, can a SIP gateway, upon reception of a SETUP
Message with Bearer Capability
3,1 kHz Audio and an HLC indicating Fax Group 2/3, to map the HLC field
into an INVITE message
with an SDP offering T.38 only (not any other audio codec). The example
is
taken from chapter 5.1.2
http://www.watersprings.org/pub/id/draft-mule-sip-t38callflows-02.txt :

      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:0
      a=T38FaxTranscodingMMR:0
      a=T38FaxTranscodingJBIG:0
      a=T38FaxRateManagement:transferredTCF

It should be the most straightforward procedure to map fax but is my
interpretation correct?...
or the reception of a setUp with such IE values does not have enough
information to 
include an SDP offering with T.38 only, so it is required to wait for
CNG/V21 detection 
to pick all the information required, and a renegotiation is thus
required after call is
stablished?

Thank you very much in advance

Best Regards

Luis Martin

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