Contents

Introduction

While theoretical circular coverage areas give a crude estimate on wireless
access point (AP) coverage, the only way to get a true picture of how an AP
will propagate its signals throughout a building is to do a wireless site
survey. In this article, we’ll cover some of the tools used for wireless
LAN (WLAN) surveys using popular free and commercial software.

NetStumbler

NetStumbler is an excellent free tool for performing WLAN site surveys (note that
NetStumbler is not useful for penetration or security testing, but that’s not in the scope
of this article). It won’t give you all the polished reports, and it takes a
little extra work to come up with the results. However, with a little help from
this tutorial and our downloadable Excel Macro tool, this is an excellent free solution.

First, you simply walk around the building
with a laptop running NetStumbler on a predetermined path with sampling points
where you want to measure the RF signal levels.You will need a simple
floor plan with selected sampling points along the path that you’re going to
walk but before you start walking, you must start NetStumbler and make sure you
see live Access Points to make sure NetStumbler is working correctly (see Figure
A below).

Figure A

As you hit each
sampling point along the path, you simply enter a time stamp in our Excel
macro download,
using the shortcut keys CTRL-SHIFT-A to stamp the location and time for
you. The first time you press the keys, it puts down a timestamp, along
with a sample number 1. The second time you press it, it puts down a new
timestamp along with sample number 2 and so on (See Figure B below).

Figure B

While this is happening, NetStumbler will collect signal-level values once a second and
allow you to export a raw text file with signal levels of each AP that
it can detect. Once you’ve walked the entire path entering timestamps
along the way, you simply output the NetStumbler data to a raw text file using
the File | Export | Text menu command. With the raw NetStumbler data, you
can hit the “Run Report” button shown in Figure B. It will prompt you to
find the raw NetStumbler data, and you’ll navigate to that file and select it.
The “Run Report” macro will now ask you to enter in the time offset from GMT for
your location. If you’re on Pacific Standard Time with daylight-savings,
then you would enter the value -7 (-8 during the Winter months). For
Eastern Standard Time, you would enter -4 during the summer daylight-saving
months. Once you’ve entered in all the data, it might take up to a minute
or more to complete the report. Figure C shows the output from “Run
Report” macro.

Figure C

Site surveying with commercial software

With commercial software like
AirMagnet Surveyor,
you would actually import your site floor plans into the software. Once
imported, you walk around and tap on the floor plan with a mouse (or stylus if
you have a Tablet PC) on the location you’re standing. Once you’ve
collected enough samples, AirMagnet will overlay a color-coded scheme over your
floor plan as shown in Figure D.

Figure D

Radio frequency Spectrum Analyzers

While a wireless site survey using the tools described above are great,
they’re not great at factoring in Radio Frequency (RF) interference from non-802.11 devices that use the 2.4 or 5.8 GHz range. For that, you need a
true Spectrum Analyzer to measure RF activity from any radio-emitting device
such as microwave ovens, wireless cordless phones, or wireless video surveillance
cameras. Dedicated hardware analyzers can cost several thousand dollars,
but laptop owners have a new, cheaper way of measuring RF activity using
commodity laptop computers. AirMagnet offers a new line of
Spectrum
Analyzers (see Figure E) that come in the form of software and a PCMCIA
card that can map RF activity in both the 2.4 and 5 GHz range.

Figure E

Strategies for avoiding interference

To avoid these interference issues with WLANs, here are some best
practices to consider:

  • Measure for excessively noisy 2.4 and 5 GHz using a spectrum analyzer
    and get rid of them. If you can’t afford one, you can still do the
    following to minimize interference issues.
  • Stick with newer 5.8 GHz digital spread spectrum cordless phones.
  • Stick with wired video cameras. For any fixed devices that don’t
    need to move around like a laptop or PDA, CAT-5 cabling is the best way to
    go because not only does it transmit data, but also electricity to power
    the device, so that you don’t need to use an AC adapter and a thick power
    cord.
  • If you must use a wireless camera because it’s not feasible to run CAT-5
    cabling, use an 802.11 camera (preferably 802.11a if possible) because they
    use less radio spectrum.
  • Analog 2.4 GHz cordless phones are also notorious for being spectrum
    hogs, and cordless 2.4 GHz analog video cameras are also in the same
    category.
  • In most cases, you’ll find that newer, well-made microwave ovens will
    produce less noise than an older model that can easily overwhelm a 802.11
    b/g WLAN with sheer power.
  • Bluetooth devices can also cause interference in the 2.4 GHz range, but
    their power levels are usually limited especially with the newer Bluetooth
    1.2 standard that’s designed to play nicer with 802.11.

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