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 Post subject: QoS bandwidth reservation
 Post Posted: Wed Apr 02, 2008 3:43 pm 

Joined: Sun Feb 04, 2007 1:32 pm
Posts: 225
I was curious if you had any thoughts on this:

If you're using XP Pro, and have administrator rights, you can modify the
Quality of Service http://en.wikipedia.org/wiki/Quality_of_service (QoS)
Packet Scheduler's settings. On some PCs, you'll get a boost in Internet
access, yet on other machines, you won't see any change. A few PCs show a
slowdown in Internet access.

I'm finding Web site pop on screen faster with this modification. I not
seeing any increase in downloading files.

There's no harm in trying it -- and it's easy to change the settings back to
the default.

If you do try it, lemme know the results.

1. From the Start menu, click Run and type gpedit.msc.

2. From Computer Configuration, find and expand Administrative Templates.

3. Expand Network and click QoS Packet Scheduler.

4. Double click Limit Reservable Bandwidth, make sure it's enabled, and set
the Bandwidth limit to 0.

To set it back to the default, repeat the steps and change the bandwidth to

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 Post subject:
 Post Posted: Wed Apr 02, 2008 3:49 pm 
Site Admin

Joined: Sun Feb 04, 2007 11:44 am
Posts: 6129
I think you are referring to a tip like the one posted here:

Unfortunately that tip is bogus:
http://blogs.msdn.com/oldnewthing/archi ... 28591.aspx

In addition, note that the specific numbers quoted in *both* of the above
posts are also incorrect. For example, the QoS packet scheduler normally
reserves 80% of the bandwidth for QoS-aware applications, meaning that the
restaurant in the analogy used in the latter post would normally accept
reservations for up to 80 out of the 100 seats, leaving at least 20 seats for
walk-in customers. More on that later...

For more information, see the following article:

Windows XP Quality of Service (QoS) enhancements and behavior

In fact if you read between the lines a bit, the "Clarification about the use
of QoS..." section in the above article seems to have been written in order
to specifically refute inaccurate online recommendations for changing the
default reservation setting as you describe.

To understand the implications of changing the "Limit Reservable Bandwidth"
setting, let's examine what it actually means. First, note that QoS only
applies to data that is *sent* (not received). In Windows 2000 and later,
programs using connectivity can either use "best effort" connectivity (i.e.
no QoS), or request *priority* connectivity via QoS. If no QoS-aware program
reserves bandwidth, then 100% of the available transmission bandwidth is
available to all "best effort" programs.

The default "Limit Reservable Bandwidth" setting in Windows XP is "Not
configured", which results in a value of 80% being used internally. This
allows QoS-aware (i.e. non-best-effort) programs to reserve up to a maximum
of 80% of the total bandwidth, leaving at least 20% remaining bandwidth for
programs relying on "best effort" connectivity. This insures that non-QoS
applications get *some* bandwidth, even if QoS-aware applications try to take
it all. Note that even if QoS-aware programs reserve up to 80% of the
bandwidth, if they are not actually sending enough data to use all that they
reserved, the unused portion is made available for other applications.

There is a lot of confusion about whether the default setting for this limit
is 80% or 20%. Here is the truth: In Windows XP, if "Limit Reservable
Bandwidth" is set to the default setting of "Not configured", then the limit
internally defaults to 80%. In other words, a setting of "Not configured" is
the *same* as a setting of "Enabled" with a value of 80%.

This is confusing because the Explain tab indicates (incorrectly) that the
default is 20%, and when you first change the setting from "Not configured"
to "Enabled", the initial value that appears is also 20%. In fact you would
have to change that value to 80% for it to have the same actual setting as
"Not configured"! In addition, if you change the setting to "Disabled", it
also reverts internally to a setting of 80%. I verified the true values
invoked via "Not configured" and "Disabled" by using the Tcmon.exe (Traffic
Control Monitor) included with the Resource Kit Tools for Windows XP, which
reports the actual bandwidth that is reserved. These findings are also
corroborated by Microsoft's published description of the setting, which

...the default value of the registry entry reserves 80 percent of the
bandwidth for QoS-aware applications, but this does not prevent other
applications from using more than the remaining 20 percent of the

For more information see:

QoS Tools and Settings

In any case, based on how the QoS packet scheduler function is described, I'd
say that changing the reservation limit for QoS-aware applications from 80%
to 0% will cause severe problems with multimedia applications relying on
priority connectivity, especially high transmission bandwidth QoS-aware
programs such as Media Sharing from your PC or using other media streaming
applications such as Skype in High Quality Video mode (which is awesome BTW):
http://share.skype.com/sites/en/2007/11 ... ublic.html

Bottom line: Don't do it! <g>

On the other hand, if you want to modify QoS default settings in a
potentially beneficial way, then you might consider changing QoS settings in
your router to prioritize traffic from applications that might benefit:
http://www.pcworld.com/article/id,13986 ... ticle.html


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