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From: cs4cg@hatfield.ac.UK (Simon Jaffer)
Newsgroups: comp.protocols.iso.dev-environ
Subject: (none)
Message-ID: <6640.9003211642@unix1.hatfield.ac.uk>
Date: 21 Mar 90 16:42:27 GMT
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Fellow ISODE-ites, I wonder if you can help me,

My question concerns the representation of application data structures within
the ISODE system.  I'm using version 4.0 (soon to be replaced with the newest
version though).

My understanding of the OSI model is that the presentation services, amongst
other things, take the application data structures, and a definition of these
structures (in ASN.1 for example) and produce an on-the-wire representation
using some standard encoding rules.

However, I understand that within ISODE, users structures are first transformed
into presentaion elements, and it is in this form that they are passed to, not
only the presentation services, but also some of the application service
elements (such as RTSE).  I assume that the presentation services then 
transform these presentation elements into a transfer syntax.

I am confused as to why this initial transformation into presentation elements
occurs, as it seems to add an extra step into the whole transformation process.

I would be very grateful to anyone who can mail me here at Hatfield and explain
why the strucures are manipulated in this way.

Thanks in advance,

Simon Jaffer.