H.225.0 v4 white paper: ASN.1 problem

Horvath Ernst ernst.horvath at SIEMENS.AT
Tue Jul 18 08:52:08 EDT 2000


The following definition (added for support of mobile systems) is not
correct ASN.1:

   TBCD-STRING ::= OCTET STRING (FROM ("0123456789#*abc"))

The keyword "FROM" indicates a "permitted alphabet" constraint and can only
be applied to character string types, not to OCTET STRING.

A possible correct definition would be:

     TBCD-STRING ::= IA5String (FROM ("0123456789#*abc"))

The resulting PER encoding is 4 bit per character, but the digits do not
represent BCD values since # and * come first in the canonical ordering of
characters, i.e. "#" is encoded as '0000', "*" as '0001', "0" as '0010', "a"
as '1100', etc.

Question to the mobility experts who introduced this ASN.1 type: Is this an
acceptable solution?

Ernst Horvath
Siemens AG

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