Path: utzoo!attcan!uunet!bu.edu!bbn.com!mckenzie
From: mckenzie@bbn.com (Alex McKenzie)
Newsgroups: comp.protocols.iso
Subject: Re: Question on the OSI 7 layer model?
Message-ID: <60412@bbn.BBN.COM>
Date: 29 Oct 90 16:28:00 GMT
References: <28843@boulder.Colorado.EDU> <9010291205.AA05432@gjetost.cs.wisc.edu>
Sender: news@bbn.com
Reply-To: mckenzie@labs-n.bbn.com (Alex McKenzie)
Organization: Bolt Beranek and Newman Inc., Cambridge MA
Lines: 29

Marvin,

You ask why the Presentation Layer functionality is also included in
ACSE.  I admit that I haven't read the ACSE documentation recently, but
my guess is that the functionality isn't also included in ACSE, just the
access to that funcionality.

The reference model says that all communication functionality is made
visible to a user application program through the "top" of the 7-layer
stack.  This is supposed to make it easier to redesign the way
communication functions are actually provided (freedom to ISO to
redesign the layers) and make user application programs slightly more
portable (a single interface point).  But that means that if a function
is provided in the Session Layer (e.g. checkpoints) that the user
application program needs access to, then the Presentation Service has
to offer an identical*  function to the Presentation User, and then the
ACSE has to offer an identical function to its user.  These services are
known in the ISO committees as "pass through" services, because the
user's service request is "passed through" without change as a service
request to the next lower layer.  

Regards,
Alex McKenzie

* Of course the service provided by an upper layer need not be identical
to the service provided by a lower layer, and not all services are.  But
in some cases the upper layers have nothing to add - the service
offering might as well be identical.