Re: [DI Glossary] DeliveryUunit

I am probably responsible for creating the confusion here, as I believe 
I drafted the original definition of 'delivery unit', as it appears in 
the September 2003 version of Device Independence Principles [DIP2003]:
http://www.w3.org/TR/2003/NOTE-di-princ-20030901/

Example 2.2.1.2 in [DIP2003] suggests that a delivery unit is the 
response to a single HTTP request (whether direct or indirect in Rotan's 
description). The 'set of material' refers to the combination of 
representations that may be delivered together, such as HTML with 
embedded ECMAScript.

However, Example 2.3.1.1 in [DIP2003] says that 'a user agent may render 
the material received in a delivery unit' - implying that it consists of 
all the material needed to render it, as Rotan suggests.

This latter example, however, does make clear the distinction between a 
'delivery unit' and a 'perceivable unit', which was the original 
motivation for introducing the two terms.

Together, they replaced the term 'presentation data' used in the 
September 2001 DIP, which itself caused confusion:
http://www.w3.org/TR/2001/WD-di-princ-20010918/

As far as device independence is concerned, the point is that adaptation 
may take place server-side, producing a delivery unit, and/or may take 
place client-side, producing a perceivable unit.

Though it is not stated explicitly, the implication is that several 
delivery units may be required to produce one perceivable unit and that 
one delivery unit may produce several perceivable units.

This is also the reason that the definition states that 'Users are not 
normally aware of individual delivery units.'

Roger
-- 
roger.gimson@hp.com | HP Labs, Bristol BS34 8QZ, UK | +44 117-312-8167

Received on Thursday, 18 August 2005 17:07:21 UTC