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How would you explain, troubleshoot (and solve) the following problem?

Wifi ADSL modem router D-link 2640R installed in living room at about 1.8m height. Working fine, synchronising and getting/serving stable internet connection.

First situation:

  • Laptop 01 in other end of the house, let's say in room01 southern to the living room, distant by about 15m. Getting stable signal of good to very good quality. No disconnection.

  • Laptop 02 in room02 opposite to room01 (5m West) which makes it almost at the same distance and direction from the router located 15m North. Getting stable signal of good to very good quality. No disconnection.


Second situation:

  • Laptop 01 moved to room03 Northern to the living room (actually just 3m behind the wall where the router lies). Getting stable signal of excellent quality. No disconnection.

  • Laptop 02 still in room02 but now experiences frequent disconnections (actually almost impossible to get the Internet even though the signal level is still very good. Either no Internet with the wifi icon appearing connected to access point or no connection established at all which happens every 2 minutes and that means virtually no Internet at all as I can just get a timeframe of 1 minute or so to load any website or even get to the router's web based control panel.

If Laptop 01 is completely shut down or its wifi adapters shut down or even still working but its wifi MAC address forbidden, then Laptop 02 has no problem at all. If Laptop 02 is moved to a nearer location to the router, in the living room for instance, then no connection problem occurs even if Laptop 01 is also connected. And also if we move back Laptop 01 to its original location (room 01), then no problem as well.

I'm completely lost and don't know how to address this issue. I tried to change the Wifi channel and even tried the auto channel scan but that didn't solve it. I know that the problem is probably coming from Laptop 01 being in its new location or some sort of interference as the problem occurs only under the described condition but I have no idea how to solve it!

I also scanned the neighborhood for wifi jam using InSSIDer, there are few other access points but they don't seem to affect the situation.

Any ideas about the steps to follow or tools to use ?

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Weird. I suppose it could be an extreme version of the "hidden node problem". In the second situation, laptop 01 might not be able to see laptop 02's transmissions, so it might accidentally be transmitting at the same time and clobbering laptop 01's transmissions. You might want to force your devices to use RTS/CTS (crank down your RTS/CTS Threshold on your AP, and also see if your laptops' advanced settings for their wireless drivers allows you to force RTS/CTS) and see if that helps.

Under RTS/CTS operation, the device that wants to transmit must first ask permission from the AP by sending an RTS (Ready To Send) message. Then the AP gives it clearance by sending a CTS (Clear To Send), which the other devices of the AP will all see. The CTS from the AP contains a "duration" field that lets all the other devices in range know how long the medium will be busy. So Laptop 01 would see the CTS from the AP to laptop 02 and it would know not to transmit during that time.

If this actually fixes your problem, it may be the first time I've ever heard of tweaking RTS/CTS settings making a real difference for someone.

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Yes now I'm sure that's exactly the problem you described. I tried to lower the RTS threshold on the AP and use RTS/CTS mode in the Intel wireless adapter of Laptop01 that has plenty of options but there is no way to use that option on the Atheros adapter of Laptop02 :( –  Sidou Apr 11 '12 at 11:27
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