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Wireless Basics


Fortunately, there are other tools available if you don't have a decent wireless client utility. In fact, even if you have a client utility that shows channel, signal strength, all the in-range networks and even actual data throughput, there are other tools worth considering.

NetStumbler is the grand-daddy of network survey tools and used in countless wardrives, flies, trains and walks. Created by Marinus Milner back in 2002, it's still the tool of choice by many today, despite the fact that active development stopped on it back in 2005.

NetStumbler is free (although donations are appreciated) and comes in versions for Windows 2000 and XP and PocketPC 3.0, PocketPC 2002 and Windows Mobile 2003. It has not been tested on Vista.

TipTip: The other most popular wireless network detector is Kismet. It uses a command-line interface and non-graphic display, so isn't recommended for those who are lost without a GUI. Although it is available in a form that will run on Windows, it's best run in Linux.

If you're game, the easiest way to go is to use the BackTrack live CD, which contains Kismet and a host of other network tools. But since ease-of-use is the watchword here, we'll be doing our site surveying with NetStumbler.

Note that Kismet can detect both APs and wireless clients (also referred to as Stations or STAs). This is helpful for some wireless surveys, but not required for our simple needs.

The NetStumbler readme file says, "The requirements for NetStumbler are somewhat complex and depend on hardware, firmware versions, driver versions and operating system. The best way to see if it works on your system is to try it."

It turns out that this is pretty good advice. But you can first save yourself some time by checking this list of cards that have been reported to work with NetStumbler 0.4 (the latest and last version) on Windows XP. The list is pretty inclusive and includes cards using Atheros, Broadcom and Intel chipsets. Chances are that if you have a wireless card supporting 802.11b or g, NetStumbler will work with it.

Updated 2/10/2010: Added MetaGeek inSSIDer
You may find MetaGeek's free inSSIDer a better option than NetStumbler, which is now very long in the tooth and hasn't been updated in a very long time. While you're downloading it, you might as well buy a Wi-Spy 2.4i for $99 to complete your wireless troubleshooting kit.

So download and install NetStumbler (or inSSIDer) and launch it. It should automatically select your wireless adapter and go into capture mode. If for some reason it doesn't start, choose Device from the menu bar and select another adapter or another driver. Make sure the green "Play" button (next to the Disk "Save" icon) is depressed, or click on it anyway!

Assuming that you have an active access point nearby, you should see it appear in NetStumbler's right-hand-pane. Figure 4 shows a display with four APs found. Let's see what we can find out about this wireless environment.

Example of NetStumbler displaying info on nearby wireless networks.

Figure 4: Example of NetStumbler displaying info on nearby wireless networks.
  • The four APs are using only two channels (1 and 6).
    This isn't necessarily a bad thing as we'll see shortly.

  • One of the APs is not broadcasting an SSID (third one down).
    See why blocking SSID broadcast doesn't really hide you from the bad guys?

  • All of the APs are 802.11g
    This is indicated by the 54 Mbps speed. 11b APs would show 11 Mbps.

  • The closest (strongest) AP is "Net"
    "Net" has a -35 dBm signal level, the highest value recorded. (Smaller numeric values represent higher signal levels since the values are negative.) The farthest AP is 2WIRE534, since it has the lowest signal level.

Although NetStumbler sometimes displays Noise and SNR (Signal-to-Noise) values, you're better off using the Signal value in most cases to determine AP signal strength. The reason is that Wi-Fi adapters can't really measure non-802.11 signal levels. So we really don't know what is being reported here. For an accurate assessment of signal levels from microwave ovens, cordless phones, Bluetooth headsets and other devices that use the 2.4 GHz band and can interfere with your wireless network, you need to use a spectrum analyzer, as noted earlier.

TipTip: If the adapter that you are using does report "noise", you'll see it as a varying red bar overlaid on the green signal bar as shown in Figure 5.

Example of graph view of AP signal and noise levels.

Figure 5: Example of graph view of AP signal and noise levels.

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