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I have a DSL router (dlink dsl 2640b) with 802.11 b/g support. In the last few years the DSL connection worked fine as well as the Wifi.
Recently however the wifi started to be problematic.

Apparently my neighborhood has suddenly filled up of wifi routers: every time I try to access my network I get "connection time out" from my Macs, "Error establishing connection" by windows machine, "Unable to connect" by a couple of iPhones.

I started doing some diagnosys by exclusion, here are the facts:

  • the problem is platform independent
  • the problem is independent of the encryption used (I've tried no encryption, WEP, WPA, WPA2 etc)
  • the problem is loosely dependent on the channel used: the spectrum in the location of the router is quite filled up in the channels 4-11, channel 1-2-3 seem to be fairly free
  • the problem is independent of the distance of the stations(mac,pc, iphones) from the router: I've tried from 20cm to 20m, same result
  • The stations (mac,pc, iphones) do work correctly if used in other places (eg. work) and other scenarios.
  • The problem does not only involve the router itself: using one of the iphone with the "hotspot" setting on would give the same result.
  • by staying at ~1m of distance from the router I can see my network with a RSSI of almost -53 while all the other network are all around -75.

By excluding all of the previous I assume I am in a noisy environment and I have a small confirmation:
my home has two floors and the higher one also is the last one. By placing the router here (and thus with less interference by other AP) I can connect to the wifi by using any device (mac,pc, iphones) and any combination of channel / encryption.

I have read some thread on server fault about tuning the wifi parameters to introduce fragmentation and RTS/CTS protocol. However this did not helped much. I cannot exclude some hardware fault in the router that could have caused it not to transmit at full power. However I assume an hw fault would block the connection even when in not so noisy environment as in the upper floor.

My GUESS is that the very initial association frames of the 802.11 protocol might get lost, or the fragmentation would not be applied. Do you have any suggestion as how to further investigate this problem?

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migrated from serverfault.com Mar 28 '13 at 18:08

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You've investigated, your conclusion is correct. Maybe consider a 5ghz AP? –  NickW Mar 28 '13 at 17:52
    
THAT is the short answer:yes, I have already bought a linksys X3500, but I would like to understand more about this situation: as far as a program like inSSIDer can tell I only have 7 nearby networks all in channels that I do not use. I don't understand why transmitting in a "quiet" channel does not solve the problem. –  nick2k3 Mar 28 '13 at 18:01
    
See my answer, 802.11 b/c/d frequencies overlap, if the frequencies are next to yours, there will be interference, and inSSIDer probably only "sees" Wi-Fi traffic.. not other forms of interference. –  NickW Mar 28 '13 at 18:06

2 Answers 2

There is very little you can do to resolve the problem, 802.11 2.4 GHZ has that issue as the spectrum slices are overlapping, so even two close frequencies will often cause interference. Basically what you are seeing is a fundamental limitation of wireless communication, even though you can add layers of encryption, authentication and maybe even frequency hop, antennas are analog devices, and anyone who broadcasts a signal at that frequency will end talking to your antenna. So to hear your packets, the AP has to discard lots of useless info, and possibly even radio frequencies that have nothing to do with communication (badly insulated microwaves and non DECT cordless phones being prime examples).

I would consider a 5GHZ AP, 802.11a will probably be much less used (and walls attenuate it better).

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As said in the comment of the post I am already buying a 5Ghz AP; however here is my spectrum (captured with inSSIDer): imageshack.us/photo/my-images/40/schermata20130328alle19.png However,I'll look for other sources of interf. –  nick2k3 Mar 28 '13 at 18:11

There must be some key piece of information that is missing here, because it doesn't quite add up for me. However, I wouldn't suspect your neighboring WiFi networks for the following reasons:

1) Only seven neighboring networks, especially with how cleanly your image shows them should not be problematic. By this I mean they are on the right channels to avoid overlap and I have seen plenty of networks working just fine with 20+ separate networks visible. Especially since none of them seem to be on the same channel as your network (assuming based on your description and image that yours is the one on channel 1).

2) You mention that everything works fine when you move the router to the second floor. This often increases the interference from other WiFi networks. Using a tool like inSSIDer, you will typically see more wireless networks and at stronger signal strengths when you move upward.

I usually lean towards non-WiFi interference from non-WiFi sources in cases like this, but most of those won't create the problem described. Namely interference that prevents two devices within 20cm from connecting will typically also create more wide spread problems.

If I had to guess based on the information provided, this may fall into what I call a "shower conversation" type of problem. Ever talk to someone while you were in the shower? They seem to hear you just fine, but often if they don't talk really loud or are in the next room, you can have trouble understanding what they are saying. There is a source of noise that may not be overly loud, but is constant and much closer to you.

RF can be the same way when there is a source of interference close to one end point. I would look for a source of interference close to where you keep the router on the first floor. This could be another wireless device, something that draws a lot of power, or maybe a malfunctioning electronic device (wireless or not).

A quick check that can often point to this as the answer would be if you reverse the situation and it also fails (i.e. put the router on the second floor where you say it works and try to connect to it from a laptop placed in the normal first floor location of the router). Keep in mind this is not an infallible test as there are many other factors at play (antenna signal patterns, for one) and other problems that can provide the same result.

Either way, see if things work better at the 20cm range after you remove or turn off (and unplug/remove batteries if you can) as many devices near the first floor location as possible (say within at least 3 meters - including on opposite side of walls and directly above/below). This includes things like wireless mice/keyboards and gaming controllers.

Only other thing I can think of based on this information is also one of the other cases that may fail the quick check I mention above. Depending on construction materials, you can create your own interference especially if the router is too close to the wall/corner. Try the first floor location, but move the router about a meter away from it's original location, making sure it is at least 30-60 centimeters from any wall.

Good luck and hope you get your answer soon.

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