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You are here: Wireless Wireless How To When Wireless LANs Collide: How To Beat The Wireless Crowd - Change Channels, Duplicate SSIDs

When Wireless LANs Collide: How To Beat The Wireless Crowd - Change Channels, Duplicate SSIDs

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Change Channels

Now that you understand the primary mechanisms at work, you're ready to move on to ways to combat them. A simple first step is to change your access point's channel. Since Windows XP's Wireless Zero Configuration utility is no help in determining the channels that neighboring wireless LANs are operating on, you'll need to fire up the wireless client utility that came with your wireless adapter card.

NetStumbler used to be the go-to program for getting a clearer view of your wireless environment. But MetaGeek's inSSIDer seems to have now taken the lead. It's free, works with Windows XP, Vista and 7 (32 and 64-bit) and uses any wireless network card.

MetaGeek's inSSIDer

Figure 3: MetaGeek's inSSIDer

One you know the lay of the airwaves, the countermeasure is simple. You'll just need to choose a channel - 1, 6, or 11 - that is used by the fewest neighboring APs, has the lowest signal, is the least busy, or hopefully all three!

Changing channels is easy, but you have to know how to access your access point or wireless router's setup screens (this info is in your product's setup guide and user manual). As an example, Figure 4 shows the main Setup screen for the NETGEAR WNDR3700, with the yellow highlight indicating the SSID settings for the two radios.

NETGEAR WNDR3700 wireless settings

Figure 4: NETGEAR WNDR3700 wireless settings

You'll probably want to change both settings - I'll explain the how and why of the SSID setting later. Be sure to Apply, Save or whatever your product has you do to make the settings stick after you change them.

By the way, while client utilities can help you count APs and determine their operating channel, they won't show you how busy each of those AP's are, i.e. how many clients are associated to them. For that, you need a handy tool like my personal favorite - AirMagnet.

AirMagnet Handheld showing APs

Figure 5: AirMagnet Handheld showing APs

Figure 5 shows just one of the many views that AirMagnet can provide of all in-range wireless equipment. This tree-type view shows access points (the little towers) and their associated clients (the little laptops). You can see that there are plenty of idle APs, and with a few stylus taps, AirMagnet can show the channel they're operating on, too.

Unfortunately, AirMagnet and similar wireless LAN analysis tools are not intended for consumers and are priced accordingly ($3000 and up). If you're handy with Linux, you can try Kismet, but otherwise you'll have to make do with counting APs and looking at signal strength to guide you in your choice of new channel.

NOTE!NOTE: Don't bother trying to change the operating channel of your wireless client. The operating channel of Infrastructure-based wireless LANs (those that use access points or wireless routers) is determined by the AP, not the client. All you need to do is change the AP channel, and its associated clients will follow.

Duplicate SSIDs

Finding unused airspace will solve most neighboring LAN problems. But if that's not an option (or you've tried it and you still need help), you might just need to tell your laptop to not go wandering and stay home!

In its zeal to make wireless networking as easy and automatic as possible, the default behavior of WinXP's built-in Wireless Zero Configuration utility is very, well, promiscuous. Once you use it to successfully connect to a wireless LAN with a particular name (i.e. SSID), it automatically considers that a "preferred network" and will connect your wireless computer to it whenever it comes within range.

This convenient feature becomes a problem, however, in areas where there are multiple access points with the same name, but that are not part of the same network! As far as your wireless laptop is concerned, APs with the same name are part of the same network (this is how wireless LANs with multiple APs are set up, actually). Since your laptop has no way of knowing that those other APs with the same SSID as your AP are actually belong to your neighbors' APs, it will at some point try to connect to them, usually when it detects an AP with a stronger signal.

But if your neighbor's AP happens to have WEP or WPA encryption enabled, or is using MAC address filtering (association control), your laptop won't be able to connect. What you'll see is your wireless connection dropping, then (maybe) reconnecting to your own AP (you may have to rescan for networks and manually reconnect). You may think that your wireless network has gone haywire, but in truth, your laptop's wireless card is just trying to do its job and keep you connected to the best signal available.

What makes this situation worse - by interfering with proper diagnosis of the problem - is that the Available Networks part of XP Zero Config doesn't show multiple instances of the same network name (SSID). So unless you run the wireless client utility that came with your adapter and it shows every AP that it detects, you can only guess at what your wireless client is really seeing.

The good news is that Windows 7 seems to have addressed this issue and shows multiple instances of the same SSID. And many manufacturers have helped by having APs and wireless routers generate unique SSIDs, usually with part of the router's MAC address concatenated with the manufacturer name.

Figure 6 shows inSSIDer's view of a few wireless routers that I just fired up. The only change I made was to set two of the SSIDs on one EnGenius dual-band concurrent router to "EnGenius". I ran this scan from a notebook with an Intel WiFi Link 5300 dual-band N client card, so you can see SSIDs in both bands. InSSIDer found seven SSIDs, including the one duplicate.

Multiple APs viewed with inSSIDer
Click to enlarge image

Figure 6: Multiple APs viewed with inSSIDer

Figure 7 shows a Win 7 notebook's view of the same set of SSIDs. While all SSIDs are shown, the display is less informative—you don't even get channel information when you mouse over each connection.

Same view with built-in Win 7 Wireless Utility

Figure 7: Same view with built-in Win 7 Wireless Utility

Finally, for those still using Windows XP, Figure 8 shows that you won't see the duplicate SSID.

Same view with Win XP Wireless Zero Config

Figure 8: Same view with Win XP Wireless Zero Config



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