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I have two computers on my network - a desktop and a laptop. Both are hardwired to my router. Yesterday I started a 500MB Windows update on my laptop.

My desktop was suddenly very unresponsive: the mouse was delayed, switching tabs in Firefox took 10-15 seconds, programs would take a minute to launch, etc.

It occurred to me that it might have something to do with my laptop downloading files, but didn't think that was possible. However, this morning I did one last update for my laptop. The second the download started, my desktop slowed down and the second it ended my desktop returned to normal.

Why would network traffic on my laptop affect the speed of my desktop?

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Is there any software running on that desktop that might care about the state of the network? – Shinrai Dec 14 '11 at 16:23

This is something that should not happend, but if both computers are connected to the same ethernet hub(not a switch) all trafic frames will be delivered to both machines, so the destination machine will filter the frames that belongs of not to it. This is the scenario that I could imagine that would be happening with you, you are connecting both computers to the same hub and both computers are receiving the same ethernet frames, but the "rejection" of the unwanted ones by your desktop are causing some performance delay. Try to use a switch instead a hub, this would solve the problem(assuming that my opinion is right).

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Is there a way I could detect this situation? – Jason Dec 14 '11 at 16:44
I don't know if you can see the "unwanted" trafic from your desktop with a sniffer, I mean, as far i know, the ethernet frames are discarted on your ethernet adapter(phisical OSI layer). However, you could try to install a sniffer(such as Wireshark) and analyze if you are receiving frames that dont maches with your ethernet MAC. Another and easyer way to test is using a switch(insted your hub) and check if the same problem happend again. – Diogo Dec 14 '11 at 16:49
Do the activity lights on the router give any indication of activity on the desktop's port? Also, brand/mode may help with identifying the type of hardware. – Tevo D Dec 14 '11 at 17:20

Processing power on one machine should not be affected by network traffic on another machine. I suspect there is something more going on behind the scenes. Perhaps a virus?

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Ran my antivirus and nothing came up. I haven't noticed any other signs of infection. – Jason Dec 14 '11 at 22:57

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