Subject: [Megaco] Definition of mediaDescriptor in ABNF and ASN.1

From: "Anil Jangam" <[email-masked]>
Date: Fri, 25 Oct 2002 13:45:45 +0530

Hi,

Look out for the definition of the MediaDescriptor in both, ABNF and in
ASN.1. In ABNF, it is defined that either streamParam(s) or
streamDescriptor(s) but not both. In ASN.1, the definiton says that streams
could be either "oneStreamParms" or "SEQUENCE OF StreamDescriptor" (its
choice between the two).

The question/doubt is:-
What is the significance of the "streamParm(s)" in ABNF?? Does it (the
factor '(s)' in the text) mean that there could be multiple streamParm in
the list of mediaParm?? If yes, then ASN.1 says something different, which
is "oneStreamParms", means that only one streamParam. Isn't it contradicting
with the definition of the ABNF. What is the consistency between these
two.....??

------------
mediaDescriptor  = MediaToken LBRKT mediaParm *(COMMA mediaParm) RBRKT

; at-most one terminationStateDescriptor
; and either streamParm(s) or streamDescriptor(s) but not both
mediaParm   = (streamParm / streamDescriptor /
                           terminationStateDescriptor)

MediaDescriptor ::= SEQUENCE
{
      termStateDescr TerminationStateDescriptor OPTIONAL,
      streams     CHOICE
      {
         oneStreamParms,
         multiStream SEQUENCE OF StreamDescriptor
      } OPTIONAL,
      ...
 }

====

Regards,

Anil Jangam

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From: Terry L Anderson <[email-masked]>
Date: Fri, 25 Oct 2002 04:46:22 -0400

The issue is simply differences in the style of definition in ASN.1 vs 
ABNF.  streamParms in ASN.1 is a sequence including upto one each of: 
localControlDescriptor, localDescriptor and remoteDescriptor, while in 
ABNF including these was done by iteration so inclusion of two or three 
of these requires two or three instances of  streamParm (hence the 
omission of the 's' in the name).  The restriction is the same either 
ASN.1: a single instance of streamParms or SEQUENCE OF StreamDescriptor 
(in order to include the streamID); in ABNF: either 1 or more instances 
of streamParm (at most once per item so really 1-3 instances) or 
streamDescriptor (which is a sequence of the same with streamIDs).

-- 

------------------------------------------------------------
Terry L Anderson          mailto:[email-masked]
Tel:+1 908 766 4463
http://www.gti.net/tla

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From: Terry L Anderson <[email-masked]>
Date: Fri, 25 Oct 2002 04:46:22 -0400

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