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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