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We have a B/G/N wi-fi network on D-Link DIR-620 rev. D1.

It works fine for 10+ clients, but when a particular laptop (HP Pavilion DV3 on Kubuntu 13.04) connects - some clients' ping 8.8.8.8 (or even to a router) skyrockets from 40ms to 2-3s or more.

The slowed clients are - one Linux laptop (Dell 7720/Ubuntu 13.04) and one Windows PC w/TP-Link USB wi-fi card.

The offending laptop doesn't load the network at all, as per iftop.

ifconfig shows no lost packets even when disconnected.

1st channel was pretty busy. Moving to a free 13th channel didn't help.

How do I diagnose the problem and how can I fix it?

*I know of similar cases fixed by replacing a network card, but this doesn't look like a network card problem, rather a compatibility issue. And it's not the first guest laptop this happens to.*

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Maybe this laptop starts network-heavy tasks when connected to the internet ? check with tools like iftop –  Kwaio Aug 30 '13 at 9:19
    
It doesn't, verified. And the mere presence of that laptop raises ping inside network - no network load can do that. –  Victor Sergienko Aug 30 '13 at 10:09
    
Sounds like a broken or misconfigured network card driver. That's the problem with multiple-access networks, one bad actor can torpedo the whole thing. –  Greg Askew Aug 30 '13 at 11:45
    
Many Wi-Fi advanced features requires that all hosts associated to the network support it. It could be that your laptop does not support some of those so the AP degrades the quality for every one else. Would be interesting to see on a separate Linux laptop (it does not have to be associated) the output of iw dev XXXX scan ssid YOURSSID when that problematic machine is associated and when it isn't. –  BatchyX Aug 30 '13 at 11:49
    
Did it: gist.github.com/singalen/6410527 . Thanks. The network we're conflicting on is "Torba Lab". –  Victor Sergienko Sep 2 '13 at 8:30

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