Subject: [Megaco] Local and Remote descriptor handling in the context of T.38 strict control

From: Carsten Waitzmann <[email-masked]>
Date: Wed, 06 Apr 2005 10:00:49 +0200

Local and Remote descriptor handling in the context of T.38 strict 
control (ITU-T T.38 04/2004; T.38 MGC transitioning method; Annex 
E.1(a), E.2.2.1; Appendix III.2.1):

Assumptions:
A1) An MG is implicitly prepared for VBD mode, beside voice and T.38. 
The MG has all means to transition autonomously to VBD. Autonomous 
transition to T.38 is excluded.
A2) Voice mode may support compressed and uncompressed codecs.
A3) ReserveGroup=OFF.
A4) The originating MG supports T.38; the terminating MG does not.
A5) Voice call is initiatially setup with subscription for fax events
A6) Basic steps for MGC controlled transition to T.38:
step1) MGC is notified about fax event accordingly by the originating 
MG. The originating MG transitions autonomously to voiceband data.
step2) MGC modifies the originating MG to switch to T.38
step3) MGC modifies the terminating MG to switch to T.38 (not successful)
step4) MGC modifies the originating MG to undo the switch to T.38

Based on such a scenario, a couple of questions came up with respect to
Local and remote descriptor handling within a MG.

Related to A6); step2):

The MGC may send the modification by just specifying a Local Descriptor
asking for T.38 (unspecified remote descriptor):
According to H.248.1v3; 7.1.8  "An unspecified Local or Remote
descriptor is considered to be a missing mandatory parameter. It
requires the MG to use whatever was last specified for that descriptor."
The MG is expected to proceed according to the rules for ReserveGroup is
False.

Q1): Is it correct to say that in this case the originating MG is not 
able to choose an alternative in Local for which it is able to support 
at least one alternative in Remote?

Q2): Is it correct to say that the originating MG is able to support at 
least one Local and one Remote alternative, thus it does *not* return 
Error 510 (Insufficient Resources)?

Q3): Is it correct to say that the originating MG shall stay in its 
current state (=voice), but returns the T.38 local descriptor, 
indicating towards the MGC, that resources for T.38 have been allocated 
(but a transition towards T.38 cannot be done, due to the *temporary* 
mismatch between local and remote descriptor)?

Q4): Is there any dependency foreseen within H.248 on the LocalControl
mode property setting and the MG rules (ReserveValue/Group=False) on how
to evaluate Local and remote descriptors? E.g. in case of RecvOnly, is
the MG obliged to evaluate the remote descriptor (even in case
previously specified)?

Q5): One can consider the T.38 "call setup" as a procedure within an
existing context. To allow a similar procedure as during the inital
context setup, is it then perhaps mandatory that the MGC sends a modify
command using an empty remote descriptor?

Related to A6); step3):

Q6): Is it correct to say that the terminating MG receives a Local and 
Remote descriptor asking both for T.38, but in case the terminating MG 
cannot support T.38, it returns Error 510 (Insufficient Resources), as 
the MG is unable to support at least one Local and one Remote alternative ?

Q7): Is it correct to say that although the terminating MG returns Error 
510, the terminating MG stays in the previously requested voice mode 
(propably switched autonomously to voiceband data)?

Related to A6); step4):
Q8): Is it correct to say that the MGC is supposed to send to the 
originating MG a local descripor asking for voice mode again (remote 
descriptor optional)?

Generic Questions on ReserveGroup/Value = False rules:

Q9): Is the understanding correct that a match between local and remote
descriptor alternative is not mandatory in order to let a MG to continue
without error? I.e. even in case of "no match", no error is returned in
case at least one alternative in the particular descriptos can be supported?

Q10): Is the understanding correct that a match between local and remote
descriptor alternatives, which does not represent a supported
alternative within the MG, does not prevent to continue without an
error, in case at least another alternative (not matching) in the
particular descriptos can be supported?

Q11): Is the support of an assymmetric e.g. voice context (PT=0 Rx and
PT=4 Tx) is explicitly allowed by these mechanisms?

Q12): Is the understanding correct that the MG shall always "return its
selected alternative in each of Local and Remote", even in case a
descriptor may be returned unchanged.

Q13): Why are the rules for RV/RG false denoted as optional procedures 
("SHOULD"), where as RV/RG true rules are not? Isn't it essential for 
the H.248 protocol that such rules are mandatory requirements?

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