H.225.0 Annex G and Mobility

Douglas Clowes dclowes1 at OPTUSHOME.COM.AU
Wed Jun 13 09:08:25 EDT 2001


H.323 imports ASN.1 constructs from H.248. If I am correct, these
elements must be encoded as OCTET STRINGS. However, specifying
elements in these way does not make it clear what is supposed to go
in the OCTET STRING, unless this is added as a comment in the ASN.1
or as explanatory text somewhere else in the document. Recently, I
was introduced to the TYPE-IDENTIFIER notation and it would seem
that its "&Type part" could help to make the ASN.1 a bit clearer.

E.g. the StimulusControl structure currently looks like:

StimulusControl ::= SEQUENCE
{
  nonStandard  NonStandardParameter OPTIONAL,
  isText       NULL OPTIONAL,
  h248Message  OCTET STRING OPTIONAL
}

According to the text in Annex L/H.323, the h248Message element
may contain any H.248 command. Assuming that this refers to the
"Command" element from the H.248 ASN.1, the above can be re-
written as:

StimulusControl ::= SEQUENCE
{
  nonStandard  NonStandardParameter OPTIONAL,
  isText       NULL OPTIONAL,
  h248Message  TYPE-IDENTIFIER.&Type(Command)
}

This makes it clear that what goes in the h248 message, is a (h.248)
command. The same can be applied to the following definitions:

H248PackagesDescriptor  ::=     PackagesDescriptor
H248SignalsDescriptor   ::=     SignalsDescriptor

I.e.

H248PackagesDescriptor ::= TYPE-IDENTIFIER.&Type(PackagesDescriptor)
H248SignalsDescriptor  ::= TYPE-IDENTIFIER.&Type(SignalsDescriptor)

Any ideas?

Regards,

Frank

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