I have seen a number of technical articles and newsgroup stating that Windows always reserves 20 percent of the available internet bandwidth for their own purposes and QoS (Quality of Service). They also describe techniques to get that back and improve internet connection speed. But, according to Microsoft, these claims are absolutely incorrect.
Windows does reserve 20% bandwidth under QoS service, but, it is not being blocked or consumbed always.
QoS is the "Networking Quality of Service" that refers to a variety of operations Windows operates across a network connection. QoS is used by various programs installed in computer to communicate with the network. Qos allows bandwidth to such programs, up to a maximum of 20% (so called reserved bandwidth) of total bandwidth available.
But this "reserved" bandwidth is still available to other programs unless the requesting program is sending or receiving any data. If the program that reserved the bandwidth is not using sufficient amount of it, the unused part of the reserved bandwidth is available for other data flows on the same host.
So 100% bandwidth is availing to you and I, personaly do not advice to change this.
If you still desire to alter this allocated bandwidth, you can do so by following these steps:
Open gpedit.msc using Run command box.
(available only in Professional and Ultimate versions of Windows)
Go to Computer Configuration -> Administrative Templates -> Network -> QoS Packet Scheduler
Double click on "Limit reservable bandwidth".
A new windows will appear. It will say it is not configured, but system uses the default value of 20%. Here bandwidth allocated can be changed by selecting enable option button. By default, the Packet Scheduler limits the system to 20 percent of the bandwidth of a connection, but you can use this setting to override the default.
If you enable this setting, you can use the "Bandwidth limit" box to adjust the amount of bandwidth the system can reserve.
If you disable this setting or do not configure it, the system uses the default value of 20 percent of the connection.
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