Subject: P1394> units and logical units definition

From: Atsushi_Nakamura <[email-masked]>
Date: Mon, 16 Mar 1998 20:58:11 +0900

I have a questions about the current profile.

I am still wondering if the usage method of SBP-2
logical units(and "unit"s) in the printer profile 
matches with the meaning of logical units 
described in SBP-2.

In the SBP-2 spec, it says ;

  "A logical unit is part of the unit architecture and is
   an instance of a device model e.g., mass storage, 
   CD-ROM or printer"

In section 17:Model Configuration of document 0.1d -PWG 
printer profile, it says ;

  "Each unique combination of the above information represents
    a unique service provided by the device and is identified as a 
    separate Unit within the device. Zero or more instances of
    a unit may exist within the device at any given time. 
    All instances of a unit have identical functionality."

It may be my lack of mis-understanding, but doesn't the 
2 statements conflict ? 
SBP-2 considers logical units as instances of "device" models
whereas the printer profile considers logical units as instances
of a "logical channels".

If there is 1 instance of a printer device with multiple 
SBP-2 logical channels, how do I address this device as a
unit architecture following both rules ?

1 "printer" unit with 2 logical unit directories?... 
but then,a logical unit has to be "an instance" of a 
device model,so there should be only 1 logical unit ????
 
Ats

-----------------------------------------
Atsushi Nakamura
-----------------------------------------

BJ Technology Develpoment 22,
Canon Inc.

53 Imai Kami-cho
Nakahara-Ku, Kawasaki-shi
postal no. 211

tel:+81-3-44-733-6111(ext.5593)
    +81-3-44-739-6634(direct)
fax:+81-3-44-739-6756
email(1):[email-masked]
email(2):[email-masked]
-----------------------------------------

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From: Greg Shue <[email-masked]>
Date: Mon, 16 Mar 1998 09:57:26 -0800 (PST)

> It may be my lack of mis-understanding, but doesn't the 

The two statements do not conflict.  The PWG profile is more
restrictive than the SBP-2 proposal.

Note that SBP-2's concept of a printer certainly does not match
well with our devices.  Disks are easily modeled.  Scanners are
fairly easily modeled.  Printers definitely are not.  (They would
have fits trying to deal with the degree of flexability in the
Printer MIB!  :-)

> whereas the printer profile considers logical units as instances

Almost.  The PWG profile considers a logical unit as an instance
of a service.  The login establishes an instance of a
communications channel to the associated service.  If you think
of the service definition as being it's own device model, then
everything should line up.

> device model,so there should be only 1 logical unit ????

The confusion comes from trying to model a printer as "1" device.
There are really many independent services, which is why
everybody wants Multiple Independent Channels.

In my mind, a "printer" is composed of (at least):

  1+ Print Job interpretors (PRINTING service) running concurrently
  1+ Printer Status monitors (DEVICE STATUS service) running concurrently
  1+ Printer Control services (DEVICE CONTROL service) running concurrently

There are a number of ways in which this could be arranged according
to SBP-2, and fewer according to the PWG proposal.  Let's take the
case of a "printer" which can simultaneously manage accepting and
parsing 2 print files, servicing N different remote queries, and
allows 1 client control access at a time:

The SBP-2 proposal allows this to be modeled as:
    1 unit with:
      2 logins to the Print Job logical unit
      N logins to the Printer Status logical unit
      1 login  to the Printer control logical unit

  Similarly, since each of these services operate independently
  of each other (and may be used by other protocol stacks), SBP-2
  allows for them to be modeled as:
    2 + N + 3 units with 1 logical unit each
  or:
    1 Print Job Unit Directory with 2 logical units (instances)
    1 Device Status Unit Directory with N logical units (instances)
    1 Device Control Unit Directory with 1 exclusive logical unit 

The current PWG proposal restricts the options to the last one.
For the "Disk & Media Changer" example, we would have to ask the
question "Do the 2 functions operate independently of each other?"
I don't know.  If they do, then they would be modeled as 2 units
with one logical unit per supported connection to each.

Does this help?

Just to check my assumptions:
  Am I incorrectly modeling a "printer"?

-- 
Greg Shue
Hewlett-Packard Company
Office Products Division			[email-masked]
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