![\"Too](\"http:\/\/www.smallnetbuilder.com\/images\/stories\/wireless\/howto_wlan_collide\/too_many_wlans.jpg\" "\\"Too")
<\/p>\n<\/p>\n
Wireless networks are wonderful things – when they work! But many consumers find that even if they manage to get their WLAN (wireless LAN) working, keeping<\/strong> it up and running reliably with good performance is often frustrating and a struggle. Though flaky equipment and improper setup can sometimes be the cause, all too often the growing popularity of WLAN equipment is itself the cause of wireless woes.<\/p>\n In this article, I’ll explain the causes of problems in closely-spaced wireless LANs (and dispel some myths). I’ll then give you fixes for common problems, and even tell you what not to waste your time trying.<\/p>\n So, how do you know that you have a problem from neighboring WLANs and not just something wrong with your own equipment? Take this little test:<\/p>\n If any of these sound familiar, then you probably should read on. If, on the other hand, you live out in the boonies where your cell phone doesn’t even work and you have to drive over to see your neighbor, then this article probably isn’t going to be much help!<\/p>\n NOTE:<\/strong> Please read references to access points (AP) or wireless routers as applicable to both<\/strong> kinds of products unless otherwise noted.<\/p><\/blockquote>\n The primary causes of wireless LAN problems in high-density areas are:<\/p>\n 1) Too many users trying to use the same channel.<\/strong><\/p>\n 2) RF (Radio Frequency) interference from nearby WLANs<\/strong><\/p>\n 3) Improper wireless adapter settings<\/strong><\/p><\/blockquote>\n The first problem is a capacity<\/strong> issue, i.e. not enough bandwidth to go around. Simply put, there are too many radios trying to use the same channel (i.e. frequency) at the same time in the same area. “High density” is a relative term, but if you live in an apartment building or school dorm, you’re definitely in this category. And even if you live in a single-family dwelling, if the distance between your and your neighbors’ homes is 50 feet or so, and you know the names (SSIDs) of your neighbors’ wireless networks, you’re also in this category!<\/p>\n An 802.11g network has a best-case useable bandwidth of around 25Mbps<\/strong>. 802.11n can move this up to anywhere between 50 and 100 Mbps. But if any 11g clients are also actively using the same radio on an 802.11n router, throughput for both will be reduced by more than half. The second cause falls into the category of RF-based interference<\/strong>. Though you might think of wireless LAN interference only in terms of 2.4GHz cordless phones and microwave ovens, WLAN equipment itself is becoming another growing – and perhaps dominant – category of RF “noise”.<\/p>\n Every form of communication has to deal with two components: signal<\/strong>, which is the part that contains the desired information; and noise<\/strong>, which is everything else. Key attributes of radio receiver design are maximizing sensitivity to signal and minimizing sensitivity to noise.<\/p>\n As long as Wi-Fi products receive sufficient signal, the Carrier Sense Multiple Access\/Collision Avoidance<\/strong><\/a> (CSMA\/CA) mechanism built into the protocol coordinates their communication. This access method – similar to CSMA\/CD<\/strong><\/a> used in wired Ethernet – makes sure that only one product transmits at a time, so that the data is understood by all receivers.<\/p>\n But when the radio energy detected by a piece of WLAN gear can’t be understood – even if that energy comes from valid WLAN equipment – it turns into noise. Wireless LAN gear does a remarkable job of differentiating between signal and noise, but not all products are created equal in this area.<\/p>\n If you’re using 2.4 GHz band equipment, you probably know that your access point has eleven channels<\/strong> that it can be set to. You may not know, however, that only three of those channels should be used<\/strong>. The reason for this is illustrated in Figures 1 and 2<\/strong>.<\/p>\n The yellow shaded area in Figure 1<\/strong> represents the power from channel 2’s signal that overlaps into channel 1’s main lobe (the largest “hump” and also the frequency band that contains most of the signal’s power). Since a significant amount of channel 2’s main lobe overlaps into channel 1’s main lobe (and vice versa), communication on both channels will suffer. (Note that this effect is the same for any two adjacent channels, not just Channels 1 and 2.)<\/p>\n Contrast this picture with the situation shown in Figure 2<\/strong>.<\/p>\n This figure has the same scale as Figure 1<\/strong>, but shows signals in the “non-overlapping” channels 1, 6 and 11<\/strong>. Since the power from each signal doesn’t magically stop at the 22MHz 2.4 GHz band channel boundaries, there is still overlap between “non-overlapping” channels. But in this case, the yellow shaded area that represents channel 11’s power that is overlapping into the main lobe of channel 6 is at least 30 dB lower (1\/1000) than channel 11’s peak power. For most well-designed radios, this 30dB difference between signal and “noise” is sufficient to ensure good rejection of the adjacent channels’ signals (i.e. noise).<\/p>\n Use of overlapping channels isn’t the only source of WLAN RF interference. As I’ll explain later (What Doesn’t Help<\/strong>), some of the techniques that your might be using to “ignore” other WLANs actually can hurt rather than help your own WLAN’s performance!<\/p>\n The take-away from all this is that not only do you have to deal with possible problems from microwave ovens and 2.4GHz phones, but neighboring WLANs themselves can also become interference sources.<\/p>\n The third cause\u2014incorrect settings\u2014is just that. More on that topic later.<\/p>\n 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.<\/strong> 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.<\/p>\n NetStumbler<\/a> used to be the go-to program for getting a clearer view of your wireless environment. But MetaGeek’s inSSIDer<\/a> 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.<\/p>\n 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<\/strong>, has the lowest signal<\/strong>, is the least busy<\/strong>, or hopefully all three!<\/p>\n 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<\/strong> shows the main Setup screen for the NETGEAR WNDR3700<\/strong>, with the yellow highlight indicating the SSID<\/strong> settings for the two radios.<\/p>\n 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.<\/p>\n By the way, while client utilities can help you count APs and determine their operating channel, they won’t show you how busy<\/strong> 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<\/strong>.<\/p>\n Figure 5<\/strong> 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.<\/p>\n 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<\/strong><\/a>, 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.<\/p>\n 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!<\/p>\n 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.<\/p>\n 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<\/strong>! 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’<\/strong> APs, it will at some point try to connect to them, usually when it detects an AP with a stronger signal.<\/p>\n 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.<\/p>\n What makes this situation worse – by interfering with proper diagnosis of the problem – is that the Available Networks<\/strong> part of XP Zero Config doesn’t show multiple instances of the same network name (SSID)<\/em>. So unless you run the wireless client utility that came with your adapter and<\/strong> it shows every<\/strong> AP that it detects, you can only guess at what your wireless client is really seeing.<\/p>\n 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.<\/p>\n 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 SSID<\/strong>s, including the one duplicate.<\/p>\n 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\u2014you don’t even get channel information when you mouse over each connection.<\/p>\n Finally, for those still using Windows XP, Figure 8 shows that you won’t see the duplicate SSID.<\/p>\n Fortunately there are two things you can do to keep your wireless clients on their own network. The first is to change your SSID from its default value and one not used by nearby WLANs<\/strong>. Choose something unique and that doesn’t divulge your name or location (for security’s sake). Using only letters, numbers, underscores and no spaces<\/strong> should give you plenty of options for the SSID name.<\/p>\n The second step is to clear the Preferred Network list<\/strong> and make sure connection to non-preferred networks is disabled. Some wireless client utilities refer to this list as Profiles<\/strong>, but the concept is the same.<\/p>\n With XP, you’ll find the icon for your wireless adapter in the Network Connections<\/strong> window (Start > Settings > Network Connections)<\/strong>. Right-clicking on the icon and selecting Properties should bring up a window similar to Figure 9<\/strong>.<\/p>\n The upper portion of the window lists Available, i.e. currently detected, wireless networks while the lower section lists Preferred networks. Simply delete every network except yours<\/strong> by selecting them and clicking the Remove<\/strong> button. Then click the Advanced<\/strong> button to bring up that window, and make yours look like Figure 10<\/strong>.<\/p>\n This will prevent your card from trying to connect to Ad-Hoc networks (in the unlikely event that any are around), but more importantly prevent automatic connection to any new wireless LANs that appear in your neighborhood.<\/p>\n For Win 7, the process looks different, but does the same thing. You first bring up the Network and Sharing Center<\/strong> (Figure 11), which you can do from the Network connection tray icon. I’ve highlighted the Manage wireless networks<\/em> link that you need to click next.<\/p>\n The Manage Wireless Networks<\/strong> window lets you set the network connection priority and add and delete wireless networks. Again, make sure that your network name is unique and at the top of this list<\/em>. You might also want to delete all the other networks that may have made their way into this list. You definitely<\/em> want to delete any networks that are the same as any in-range SSID’s that you can see!<\/p>\n To reach other wireless controls in Win 7, you need to get to the Wireless Connection Status<\/strong> window. You can do this by clicking the network connection icon in the tray, right-clicking the currently-connected network and choosing Status<\/em> to bring up a window like the one in Figure 13.<\/p>\n Clicking the Wireless Properties<\/em> button will bring up the window shown in Figure 14.<\/p>\n The Connect automatically…<\/em> box shown checked in Figure 14 should be checked only<\/em> for the network you want to connect to. If it’s checked on other wireless networks that you want to keep around (Manage Wireless Networks – Figure 12) for when you travel or take your notebook to the office, that fine, as long as that network doesn’t have the same name as any in-range networks!<\/em><\/p>\n If you’re using your wireless adapter’s client utility, check to see if has a similar “preferred network” capability, and perform a similar clean-out if possible. Some utilities use “connection profiles”, which store all the settings for connecting to a particular WLAN and require you to manually<\/strong> switch among them. You shouldn’t have to perform a “clean-out” in this case, but you may need to delete unwanted profiles if your client utility automatically creates them when it detects new networks and automatically switches among them.<\/p>\n If you’re on a battery-powered device, you probably know how fast it can suck down a battery. But wireless adapters can be adjusted to use less power, mostly by shutting off their transceivers for longer periods of time. This is called powersaving mode<\/em> (PS).<\/p>\n Successful PS requires coordination by both the AP and STA (client). APs must know the power management state of all connected STAs and save frames for STAs that are in PS. They must also, however, periodically announce stations that have frames waiting for them. Mattew Gast’s 802.11 Wireless Networks: The Definitive Guide<\/a>, my go-to reference for all things 802.11, has a complete explanation in Chapter 8: Power Management in Infrastructure Networks<\/em>, which you can view via Google books<\/a>.<\/p>\n But sometimes the coordination doesn’t work well, which can lead to APs and STAs losing connection. To fix this, you may need to set your wireless adapter to stay awake longer or not use PS mode at all.<\/p>\n Unfortunately, you may have to hunt around for the settings. First stop is your wireless client adapter’s properties, where you may find a power saving mode setting among the Advanced properties. Win 7 “helpfully” moves these settings to the Power Options Control Panel<\/strong>, but then buries them a few levels down.<\/p>\n You’ll need to bring up the Power Options<\/strong> Control Panel, then the click the Change Plan Settings<\/em> link for the currently-selected power plan to bring up the screen in Figure 15. Then click the Change advanced power settings<\/em> link I’ve highlighted.<\/p>\n That will finally bring you to the Power Options Advanced Setting<\/strong> window. Scroll until you find Wireless Adapter Settings<\/strong> as shown in Figure 16 and change the option to Maximum Performance<\/em> if it already isn’t. Make sure you change the option for both On battery<\/em> and Plugged In<\/em> modes. It’s probably a good idea to reboot both the computer and the wireless router after making the change to make sure that old state information gets cleared out.<\/p>\n If this fixes the problem, but shortens your battery life too much, you can then try backing off the settings to Low Power Saving<\/em>, Medium Power Saving<\/em>, etc.<\/p>\n If you can’t find a clear channel and still have problems after locking your clients down so that they don’t go straying to other WLANs, it’s time to see if you can do something about all the unwanted signals bombarding your poor little WLAN, i.e. reduce the RF noise. This is an area where understanding the problem is especially important before implementing a solution and that means you’ll need something to measure signal strength to help you perform a site survey.<\/p>\n A “site survey” is just a fancy term for walking around with something that can measure wireless signal strength and recording what you see. You can do this with your wireless laptop, provided that its client application has the ability to show all in-range APs, their channels and some indication of signal strength.<\/p>\n It doesn’t matter whether the signal indicator reads in %, dBm, or no units at all, or whether it measures signal strength, signal quality or both, since you’ll be looking mainly for changes<\/strong> in whatever indicator you use. It’s also helpful if the client utility has a fast-responding, real-time signal indicator for the AP that it’s connected to, and even better if it can plot the signal strength over time.<\/p>\n Once again, inSSIDer comes to the rescue providing both numerical RSSI readings and nice plots of in-range networks.<\/p>\n<\/ins><\/ins><\/div>\n\n
What’s the problem?<\/h3>\n
\n<\/a><\/p>\nToo Much Noise<\/h3>\n
![\"2.4](\"http:\/\/www.smallnetbuilder.com\/images_old\/myimages\/NTK\/superg_ntk_11b_adjacent_chans_alt.gif\" "\\"2.4")
<\/p>\nFigure 1: 2.4 GHz band adjacent channel overlap<\/h6>\n
![\"802.11b](\"http:\/\/www.smallnetbuilder.com\/images_old\/myimages\/NTK\/superg_ntk_11b_1_6_11.gif\" "\\"2.4")
<\/p>\nFigure 2: 2.4 GHz band “non-overlapping” channel overlap<\/h6>\n
![\"Tip\"](\"http:\/\/www.smallnetbuilder.com\/images\/tip_hp.gif\")
Tip:<\/strong> Wireless networking management company Cirond<\/strong> argued that there are actually four channels (1, 4, 8, 11) that can be used for 802.11b and g with virtually no performance penalty. I recommend you stick with using Channels 1, 6, and 11 since they are more likely to be used by neighboring WLANs.<\/p><\/blockquote>\nChange Channels<\/h3>\n
![\"MetaGeek's](\"http:\/\/www.smallnetbuilder.com\/images\/stories\/wireless\/howto_wlan_collide\/inssider-small.jpg\" "\\"MetaGeek's")
<\/p>\nFigure 3: MetaGeek’s inSSIDer<\/h6>\n
![\"NETGEAR](\"http:\/\/www.smallnetbuilder.com\/images\/stories\/wireless\/howto_wlan_collide\/ssid_setting.jpg\" "\\"NETGEAR")
<\/p>\nFigure 4: NETGEAR WNDR3700 wireless settings<\/h6>\n
![\"AirMagnet](\"http:\/\/www.smallnetbuilder.com\/images_old\/Reviews\/images\/scrnshots\/airmag_aplist.jpg\" "\\"AirMagnet")
<\/p>\nFigure 5: AirMagnet Handheld showing APs<\/h6>\n
![\"NOTE!\"](\"http:\/\/www.smallnetbuilder.com\/images_old\/myimages\/attentionsml.gif\")
NOTE:<\/strong> Don’t bother trying to change the operating channel of your wireless client<\/strong>. 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.<\/p><\/blockquote>\nDuplicate SSIDs<\/h3>\n
![\"Multiple](\"http:\/\/www.smallnetbuilder.com\/images\/stories\/wireless\/howto_wlan_collide\/multi_wlan_inssider_550.jpg\" "\\"Multiple")
\n![\"Click](\"http:\/\/www.smallnetbuilder.com\/images\/enlarge.gif\" "\\"Click")
<\/a><\/p>\nFigure 6: Multiple APs viewed with inSSIDer<\/h6>\n
![\"Same](\"http:\/\/www.smallnetbuilder.com\/images\/stories\/wireless\/howto_wlan_collide\/multi_wlan_win7.jpg\" "\\"Same")
<\/p>\nFigure 7: Same view with built-in Win 7 Wireless Utility<\/h6>\n
![\"Same](\"http:\/\/www.smallnetbuilder.com\/images\/stories\/wireless\/howto_wlan_collide\/multi_wlan_xp.jpg\" "\\"Same")
<\/p>\nFigure 8: Same view with Win XP Wireless Zero Config<\/h6>\n
Making your client stay home<\/h3>\n
![\"Tip!\"](\"http:\/\/www.smallnetbuilder.com\/images_old\/myimages\/tip_hp.gif\")
TIP:<\/strong> Some APs allow you to set an “AP Name”, which has nothing to do with its SSID, but is used to tell multiple APs apart for management purposes. So make sure you change your APs SSID<\/strong> (sometimes called ESSID).<\/p><\/blockquote>\n![\"Too](\"http:\/\/www.smallnetbuilder.com\/images_old\/myimages\/howto\/wlan_neighbor_wzcfg.jpg\" "\\"Too")
<\/p>\nFigure 9: Too many “Preferred” networks<\/h6>\n
![\"XP](\"http:\/\/www.smallnetbuilder.com\/images_old\/myimages\/howto\/wlan_neighbor_wzcfg_adv.jpg\" "\\"XP")
<\/p>\nFigure 10: XP Advanced Wireless Network Properties<\/h6>\n
TIP:<\/strong> If your wireless client is moved to another location where wireless LANs are present, you should repeat the Preferred Network clean-out when you return to your normal location.<\/p><\/blockquote>\n![\"Windows](\"http:\/\/www.smallnetbuilder.com\/images\/stories\/wireless\/howto_wlan_collide\/win7_network_center.jpg\" "\\"Windows")
<\/p>\nFigure 11: Windows 7 Network and Sharing Center<\/h6>\n
![\"Windows](\"http:\/\/www.smallnetbuilder.com\/images\/stories\/wireless\/howto_wlan_collide\/win7_manage_wireless_networks.jpg\" "\\"Windows")
<\/p>\nFigure 12: Windows 7 Manage Wireless Networks<\/h6>\n
![\"Windows](\"http:\/\/www.smallnetbuilder.com\/images\/stories\/wireless\/howto_wlan_collide\/win7_wireless_connection_status.jpg\" "\\"Windows")
<\/p>\nFigure 13: Windows 7 Wireless Connection Status<\/h6>\n
![\"Windows](\"http:\/\/www.smallnetbuilder.com\/images\/stories\/wireless\/howto_wlan_collide\/win7_wireless_properties.jpg\" "\\"Windows")
<\/p>\nFigure 14: Windows 7 Wireless Network properties<\/h6>\n
Other Adapter Settings<\/h3>\n
<\/ins><\/ins><\/div>\n![\"Windows](\"http:\/\/www.smallnetbuilder.com\/images\/stories\/wireless\/howto_wlan_collide\/win7_power_plan.jpg\" "\\"Windows")
<\/p>\nFigure 15: Windows 7 Edit Power Plan settings<\/h6>\n
![\"Changing](\"http:\/\/www.smallnetbuilder.com\/images\/stories\/wireless\/howto_wlan_collide\/win7_wireless_power_properties.jpg\" "\\"Changing")
<\/p>\nFigure 16: Changing Wireless Adapter Power settings in Win 7<\/h6>\n
Survey the possibilities<\/h3>\n