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How Do I find out if my neighbours have a wireless network that interferes with mine? Friday, 02/12/2005 by nik

Are you having trouble with your wireless (WIFI) network at home? For example, does it "drop out" (disconnect) a lot, or not seem to have much range/distance? Some simple diagnosis might help you on your way.

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This airspace ain't big enough for the two of us

At the time of writing, for over two years I've merrily been running a wireless network at home without any trouble. I was actually quite surprised that I was the only one in the immediate neighbourhood that seemed to have one - but then I am a bit of a techie, and I do work for a telco - so, it's natural for me to have two broadband connections and a wireless network to accompany :-)

However, that all changed a week ago. I was getting frustrated by the fact my brand new laptop was being very fussy about connecting to my network - and if it did, inevitably it would drop in and out. Previously it had been exemplary in its behaviour. This sort of co-incided with a trip to one of my relatives where I had reconfigured things and connected to his network. My suspiscion that this was the culprit increased when I got home and my laptop was still trying to connect to a network called "NETGEAR", which was the name of his network.

So, I removed the setup for this network from windows. (You can get there from Network Connections -> Wireless -> Properties -> Wireless Networks).... Windows XP has a nice/nasty habit of "remembering" networks you connect to and adding them to the list of networks you "prefer". Funny thing was, it didn't really solve the problem.

A few days later I was sitting downstairs with the laptop, having trouble connecting to my network again, when up pops "NETGEAR" as an available network. What's more, it was listed as an unsecured network - meaning I could just connect to it with any security. What's more, there was some signal strength (there wouldn't have been if it was my relative's - his network is 250 miles away!)

The light went on. Someone in the neighbourhood had bought a wireless router and set up their own wireless network. How Rude! I thought... :-)  What's more - they hadn't secured it at all, hadn't changed its name, or hadn't even changed the admin password on their router. Now that is stupid!

I could have freely used their internet connection instead of mine. And I could have reconfigured their router and secured it so that only I could use it!!  But, I'm not that stupid - after all we have a computer misuse act in the UK, so be warned about this kind of thing, tempting though it may be!

But the real point: was it interfering with my network?

A bit of background

There are two widely adopted standards for wireless networking - 802.11b and 802.11g. (There's also an "a" - but it works in a different radio band, and seems to be less widely used)

"g" is a more modern version of "b" - it's faster. But it's also backwards compatible - a "b" device can connect to a "g" network. On a modern "g" network this has no detrimental effect (although on older "g" networks it would bring the speed of the whole network down...)

Oh  - and the speeds, I nearly forgot: 11b delivers up to 11Mbps (Mega bits per second) and 11g is up to 54 Mbps.

However, the important bit is the bit about "Channels". The available radio spectrum is divided into channels, and in Europe you get as many as 13 Channels available. The number that are available are down to local licensing laws for the radio spectrum. For example, in the USA there are 11 channels, and in Spain there are only 2! Looks like the UK is better off for once!

The channels are conveniently numbered from 1 to 13 (where available) and the basic premise is that you could have up to 13 separate networks all working in the same area, by giving them a different channel. (Conversely - if two networks in the same area have the same channel, then they will interfere with each other and could cause various problems, from reduced bandwith to perhaps not working at all.)

Thing is, it's not quite as simple as that, because the channels actually overlap somewhat. It's a bit like being in a swimming pool. You could divide a swimming pool up into 13 lanes, but you would find that a swimmer in a particular lane still causes ripples in the other lanes, although they are less strong, and get weaker the further away.

So it is with the wireless channels - any particular channel interferes somewhat with its neighbouring channels, and less so with those further away.

It is often quoted that channels 1, 6 and 11 (think of lanes 1, 6 and 11 in the pool) are the only ones that don't overlap. Actually this is not strictly true - they do to some extent, but by such a small amount that it's below a significant threshold. Many wireless devices actually come pre-configured to work on channel 11 or 6 for this reason.  So, it stands to reason that if there are several networks in your neighbourhood, you want them to be configured according to this type of pattern.

back to reality

Of course, what I had discovered was that my neighbour's network was transmitting on channel 11 just as mine was. Whereas I had configured my router and set up security and made a conscious decision to keep it on 11, obviously my neighbour hand't bothered to do anything except take his Netgear router of out of its box and plug it in - without even checking what networks were already in the area.

Hence, it seemed my network might be being scuppered - but how best to fix the problem. I tried setting my Channel to 13, and it wasn't that good. And then 6 and it wasn't that good and then 1 and that wasn't very good either.

Then I stumbled across a natty piece of software called, ironically, "NetStumbler". (See www.netstumbler.com for downloads, www.netstumbler.org for forums and help and www.stumbler.net for other information).

This tidy piece of windows software sniffs out what is happening in/on your airwaves and reports such things as what networks exist, their names, their channel, their strength and so on.

Please note, it's NOT a WIFI cracking or hacking tool, although there are plenty of these freely available on the Net. It's not the most sensible area to get into, as there have already been prosecutions in the UK.

To my amazement, when I ran netstumbler, I discovered as many as 3 other networks (over and above mine) running in the neighbourhood! What's handy though is the fact it reports the channel and the signal strength, so you can start to take some steps to make sure your network channel is as far away from the others as possible.

I came to the conclusion there's a bit of a black art on figuring out the best channel to choose. For example, although I detected another network on channel 6, when I set mine to channel 6 it worked really well, depsite the fact I expected interference! I was able to get excellent link quality at a full 54Mbps. Probably didn't do the other chap any favours though!

On the other hand choosing channel 1, even though it was furthest away from all the other channels, left me with a measly 11Mbps data rate at best. Next best was 4 or 8, even though these were fairly close to the network on channel 6.

The point is, you need to try it. If necessary go through every channel one by one. Configure your router's wireless channel to the appropriate number and then allow your PC to connect. Check the signal strength and data rate by holding your mouse over the wireless icon in your system tray or double clicking on it. Better still run netstumbler and let it tell you all the facts and figures in detail. Use netstumbler as a guide to where the "gaps" are, and start with these for your network.

(If you have a windows mobile device you can use mini-stumbler or even better, WiFiFoFum.)

Hopefully you'll find a channel where the interference is sufficiently low that you'll be able to get a reliable connection and maximum data rate. However, don't get overly worried if you can't get maximum data, as long as you get close. It's rare to be able to saturate the link flat out, even if it appears that way. Bear in mind that higher data rates are more prone to errors and all the various bits of interference that exist not just from other WIFI networks, but from DECT phones, Microwave ovens, Video senders etc. - so sometimes a slightly lower data rate actually works just as well, in terms of real world usage, such as downloads and surfing.

Now get sniffing! :-)

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