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I have set up at wireless LAN network in a small office, connected to a ADSL connection.

Using a wired connection between a computer and the router, I get the expected ~8.5 Mbps down, and ~1 Mbps up. Using wireless connection, I get ~400 Kbps down, and ~90 Kbps up.

I have tried different clients, I have tried different wireless LAN routers. I have tried running 802.11g and 802.11n. I have tried resetting everything and starting over.

I can't run 5GHz, because some of the clients don't support it.

The experience is consistent across all devices.

I am convinced that I am dealing with an interference problem, but how do I find the source? I don't want to buy a $200 kit to find out, so I wonder if there is some software available that can do this.

I have a Mac, iPhone and iPad available, but I don't think there is much of this kind of software available on iOS and Mac OS X. I do have a laptop that I can install Linux or whatever onto, but I rather not have to install Windows, if I can avoid it.

How do I determine whether or not interference is causing my performance issues?

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Product recommendations are off-topic per the faq. I've answered your question since it's a simple answer, but please read the faq before you ask another question. This is a great community, but you need to follow the rules to keep it on-topic and useful. –  MDMarra Jul 24 '12 at 14:58
    
Sorry about that. –  hstr Jul 24 '12 at 19:04
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3 Answers

up vote 3 down vote accepted

You can start cheap. You can always shell out for equipment later if you are really desperate. (And $1000 to $2000 is cheap in this case. I got $15.000 of measuring equipment in my office just for Wifi. )

Start with inSSIDer. The OSX version is in the Appstore.

That should give you an idea how much other Wifi LAN's are visible in your location. You may have to change the channels you are using to channels that have less overlap with the neighbors.

That might not be possible. The Wifi radio-channels are the garbage dump of the radio-spectrum. That is exactly why they are free to use without needing a license.
That also means that everything but the kitchen-sink uses it. (And I wouldn't be too sure the kitchen-sink doesn't...)

If inSSIDer shows nothing else in the vicinity, there may be other (non-Wifi) radio-pollution on the channels. Possible culprits are baby-phones, old (analog) wireless phones, badly shielded microwave ovens, wireless links for security cameras, remotes for lights/window-blinds, wireless cable-extenders for audio or TV sets. Just about anything you can think of.

Some really weird things that can affect Wifi performance badly:
Plasterboard walls/ceilings and relatively high humidity. (Causes the walls to act as a signal-dampener.) An office with a lot of plants or indoor garden may have s similar effect.
Wire-mesh worked in the wall or ceiling (often used as a base for plaster) or windows with UV or infrared coating on the glass. The coating (or mesh) either blocks signal or scatters/reflects the signal so it becomes noise.

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It was something like that I was looking for. Thanks a bunch! –  hstr Jul 24 '12 at 19:04
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I am convinced that I am dealing with an interference problem, but how do I find the source? I don't want to buy a $200 kit to find out, so I wonder if there is some software available that can do this.

Nope. Scanning for interference requires specialized hardware. $200 honestly isn't a lot. I've paid upwards of $1k for hardware to do this very task and that was considered cheap.

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I agree $200 is not a lot for a tool that you can put to good use, but I don't do this for a living, and will probably never need to tool again. –  hstr Jul 24 '12 at 19:01
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Proper hardware for detecting this kind of interference is simple expensive.

Alternatively you could hire someone who already has the hardware to do this for you.

As for recommended tools: start with inSSDider. It does not detect all interference. It can only report what the windows network driver tell it. Still, it is a start.

For proper detection you need specialised hardware, and the cheapest I found it US $199. (Which is probably the same as your $200 in the OP).

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