xml-dev - Re: Another Question

[tpassin@home.com]

Olivier DUBUISSON came in on the ASN.1 thread,
> John F. Schlesinger wrote:
> >
> > Mike Kay wrote:
> > "Think OSI 7-layer model. XML is layer 6, adxml is layer 7."
> >
> > I agree with Mike (I'm enjoying your XSL book by the way) that XML is a
> > layer 6 (presentation) kind of thing. So are BNF and ASN-1.
>
> Encodings associated to ASN.1 are on layer 6 (BER is the oldest one, but
> there are the packed encoding rules, PER, that are much more
> interesting).
>
> > That raises the
> > question, why is XML better than ASN-1 or BNF?
>
> Or, the other way round, why is ASN.1 better than XML? (Your question
> seems a bit biased ;-)
> Or more precisely: why is BER better than XML (they can be compared
> because they are both on the same layer)?
> Or why is ASN.1 better than XML Schemas? (these two can be compared
> because they allow the definition of datatypes.)
>
Yes, this is well put.  ASN.1 is actually pretty nice for schemas.  But it
gets very interesting to try to come up with a set of XMl encoding rules,
especially if you limit yourself to defining the XML schema with DTDs.  I
think the proposed XER rules are weak and not very well put together from an
XML point of view.

I'm involved right now in XML encoding rules for a subset of ASN.1 (for a
particular problem domain).  Since I have only a slight reading acquaintance
ASN.1, it's been even more interesting.  One of the interesting issues is
that ASN.1 has both named data types and named identifiers (an element has
both a type and an identifier), but in XML you have only element names
(basically representing complex data types).  So do you use identifiers or
data type names, and what do you do with the other of the pair?  There are a
lot of possible solutions.

Well, we're making progress.

Tom Passin


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