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WiFi Spectrum Analyzers – Optimally Configure and Troubleshoot 802.11 Wireless Networks

Wi-Fi wireless networks have grown in popularity
in recent years and are found ubiquitously in business, industry and
the home. WiFi networks operate in the 2.4 GHz Industry,
Scientific, Medical (ISM) band — a public band that is unlicensed
by the FCC. Many types of wireless devices compete for air space in
the 2.4 GHz ISM band — and these devices transmit RF energy that
introduces interference that negatively impacts the performance of a
wireless network. A 2.4 GHz spectrum analyzer is the tool of choice
for detecting and identifying sources of interference and providing
information that allows optimal configuration of a WiFi network.

1. Overview

2. Diagnostic Tools

3. Examples

1. Overview

With wireless systems it is very difficult to predict the
propagation of radio waves and detect the presence of interfering
signals without the use of test equipment. Radio waves don't travel
the same distance in all directions — instead walls, doors,
elevator shafts, people, and other obstacles offer varying degrees
of attenuation, which cause the Radio Frequency (RF) radiation
pattern to be irregular and unpredictable. 
In order to achieve optimal reliability and throughput for
your WiFi( 802.11) wireless network it is necessary to detect and
identify sources of interference that impact negatively on its
performance.

There are a multitude of electronic devices that transmit RF energy
into the airspace.  WiFi(802.11 b/g) operates in the 2.4 GHz Industrial, Scientific, and
Medical (ISM) band.  This
particular range of frequencies (2.401 GHz through 2.473 GHz) is
public and its use does not require licensing by the FCC. 
As a result, this band tends to get rather crowded – not only
with 802.11 devices but also microwaves, bluetooth devices, cordless
phones, baby monitors, audio/video senders, wireless cameras, etc.


AirHORN transmitting on channel 6 - as displayed by AirSleuth 2.4 GHz spectrum analyzer
Figure 1. 

In this figure the horizontal white lines show the
boundaries of the 13, 22 MHz channels in the 2.4 GHz band
(1-11 in North America, 1-13 in Europe).
Notice how adjacent channels overlap with one another. 


Professional installers of wireless computer networks attempt to
optimize their client’s WiFi network by strategically choosing a
subset of the 2.4 GHz band for use by their wireless network
devices. 
The 2.4 GHz range of 2.401 – 2.473 GHz is separated into 11, 22 MHz
channels.  Note that 2.473GHz
– 2.401GHz = 72 MHz, yet 11 times 22 MHz is 242 MHz. 
So, how does 242 MHz fit into 72 MHz? 
Answer – the channels overlap. 
An installer can configure the wireless router or access
point (AP) to use channel 6, and then all data communication will
occur over the range of frequencies associated with channel 6 (i.e.
2.426 GHz – 2.448 GHz).  But
if other wireless devices (802.11 or non-802.11) are also
transmitting over this range of frequencies then your wireless
network will suffer.  So,
when installing a wireless network or troubleshooting a poorly
performing one, it is important to choose a channel that is not
subject to interference from other devices – i.e. you don’t want
your wireless network to compete with other devices for the same
range of frequencies.

2. Diagnostic Tools

There are two categories or types of tools that are available for
helping you choose the best WiFi channel for your wireless network
— WiFi Scanners (i.e. 802.11 discovery tools) and 2.4 GHz Spectrum Analyzers:

WiFi Scanner or 802.11 (WLAN) discovery utility:

The best known in this category is NetStumbler
— we also recommend
NetSurveyor. The discovery tool will
report the Service Set Identifier (SSID) for each access point (AP)
it detects, along with the channel used by the AP. 
The way this works is that roughly every 100 mSec an AP sends
an “I’m here” beacon – and the discovery tool (running on your
laptop and using its 802.11 wireless adapter) picks up that beacon
and adds the SSID to its list. 
In addition, the discovery utility may report the SNR
(signal-to-noise) ratio for each AP, which is approximately an
indication of how close the AP is to your current location. 
Though this is useful information, it doesn’t tell you
anything about non-802.11 devices or even how busy the APs are. 
That is, your laptop could be sitting next to a microwave
oven, sending all kinds of RF energy into the room, and the
discovery tool would be clueless to its existence. 
The discovery tool only knows about 802.11 devices – that is,
devices that transmit RF energy according to the 802.11 protocol —
and can not see non-802.11 transmissions.
 Here’s an analogy – you’re
in a large auditorium and you hear someone across the room
occasionally yell out “Hello, I’m Joe!”. 
Joe is an access point and he occasionally sends a beacon to
let whoever is out there know that he’s there. 
It might be that Joe is really busy and talking with many
people or he might be by himself. 
Or there might be a lot of noise in the auditorium that would
make it difficult to carry on a conversation with Joe from a
distance.  The discovery tool
can’t help you with that – it only hears Joe’s occasional beacon
“Hello, I’m Joe!”.

2.4 GHz Spectrum Analyzer:

This is the instrument of choice for detecting and identifying
sources of RF interference. 

Spectrum analyzers are a basic tool used for observing radio
frequency (RF) signals – they give you a better picture of the RF
environment to help identify and find devices interfering with your
Wireless LAN (WLAN). 
Typically they’ll employ a 2-dimensional display where the vertical
axis (Y-axis) represents the magnitude of a signal and the
horizontal axis (X-axis) represents the frequency of a signal. 
Dedicated hardware spectrum analyzers can run into the tens
of thousands of dollars (they employ many arrays of
analog-to-digital converters). 
Recently, PC-based analyzers have appeared on the market. 
Yet they are also fairly expensive – in the neighborhood of
$4000. 

AirSleuth
is a low-cost 2.4 GHz spectrum analyzer that displays RF data in a variety of diagnostic views that help you
detect the presence of interfering devices and then choose the best
channel for your wireless network.

3. Examples

Below are shown 4 examples of measurements taken with the
AirSleuth

2.4 GHz spectrum analyzer.  
In each figure there are 3 items to take note of:

a.
The maximum value on the Y-axis
b.
The horizontal white lines that denote the boundaries of the 11,
overlapping 802.11 channels

c.
The location (i.e. frequency) and height (relative signal strength)
of the major RF peaks


Background noise - as displayed by AirSleuth 2.4 GHz spectrum analyzer
 
Figure 2. 
The air space is relatively quiet and there are no wireless
devices transmitting in the 2.4 GHz band. 
Notice the maximum value on the Y-axis has a signal
strength of -90 dBm, which is very weak. 
The reason you see peaks (and the output isn’t
totally flat) is this display has enabled automatic scaling, such
that the largest peak (regardless of how small it really is)
will fill-up the screen. 
We know this is what background noise looks like
because the maximum value on the Y-axis is -90 dBm. 

Microwave RF energy - as displayed by AirSleuth 2.4 GHz spectrum analyzer
 
Figure 3. 
A microwave oven is running. 
Here you can see that it emits RF energy across the
entire 2.4 GHz band. 

2.4 GHz cordless phone - as displayed by AirSleuth 2.4 GHz spectrum analyzer
 
Figure 4. 
A 2.4 GHz cordless phone has been
powered on.  Notice
the height of the peak in the vicinity of channel 1 is
around -60 dBm. 
This is strong enough to knock-out wireless networks
configured to use WiFi channel 1. 
In addition, because of the way that channels overlap
the first peak would probably also interfere with a wireless
network using channel 2. 

Wireless network transmitting on channel 6 - as displayed by AirSleuth 2.4 GHz spectrum analyzer
 
Figure 5. 
Shows the pattern of activity from a wireless network
(configured to use WiFi channel 6) as it is actively transmitting
a large stream of data. 

Troubleshooting Tips – AirSleuth Spectrum Analyzer


Question:

Which products support an external antenna?

Answer:

The PC Cards that come with the PCMCIA versions of AirSleuth-Pro have an MMCX connector that can be used with an external antenna.  We recommend replacing the snap-on antenna with the external, omni-directional antenna provided in the package.  We have found that both laptop and desktop machines may generate RF signals in the 2.4 GHz range and if the antenna is too close to the machine (e.g. the PC Card's snap-on antenna) then it becomes more difficult to interpret the frequency data (since you'll need to take into account that the source of some signals may be the machine's internal components).  By locating the external antenna several inches from the machine you'll achieve more accurate results.


Question:

During the software installation process the Setup wizard complains about not being able to create files or access the Registry.

Answer:

If this occurs it probably indicates you’ll need to repeat the installation from an account that has Administrative privileges.


Question:

Does AirSleuth interfere with my existing wireless network?

Answer:

No.  When the AirSleuth application is running the wireless device that it communicates with is operating in a “passive” mode – hence, it will not interfere with your existing wireless network.


Question:

When using an external antenna, does it's location or orientation make a difference?

Answer:

Whether using the PCMCIA or USB wireless device, the orientation of the antenna may influence the strength of RF signals measured by the device. From the ‘Traces’ view, experiment by changing the orientation of the antenna to see what affect that has on the frequency spectrum displayed by the application.


Question:

How can I verify that the analyzer is working?

Answer:

As a quick check you could turn on a microwave oven. This will generate radio waves across the entire 2.4 GHz spectrum. An even better test would be a 2.4 GHz cordless phone – most seem to transmit a continuous signal in the vicinity of 2.41 GHz (i.e. approximately in the middle of 802.11 channel 1).


Question:

What is the relation between frequency and 802.11 or WiFi channels?

Answer:

The 802.11 wireless protocol divides the 2.4 GHz frequency spectrum into 14 channels.  Moreover, there is a lot of overlap between channels – that is, a particular frequency will belong to more than one channel.  An access point (AP) is configured to use one of the WiFi channels. When an AP communicates with its client stations it uses a range of frequencies within the channel – not just the ones in the middle of the channel.


Question:

At my office (or home) I have an AP configured to use channel 6. I expect to see a big peak of activity around channel 6 on the analyzer – but I don’t. Why is that?

Answer:

802.11 is a robust, wireless communications protocol that was designed to accommodate multiple 802.11 devices within close proximity. You could install multiple APs within a building and they would all work fine (for the most part) — even if they were configured to use the same channel. That is, 802.11 devices and networks know how to play nicely in a crowded backyard. (NOTE: problems do arise, though, when large files are transferred or someone streams video images).

For a spectrum analyzer to detect a wireless device, that device has to (a) transmit continuously, and (b) the transmitted signal has to be strong enough and within range of the receiving antenna. 802.11 APs satisfy criteria (b), but unless the network is actively transmitting data between devices then the only transmissions are occasional beacon signals from the access point. The beacon signals alone are not sufficient for the analyzer to pick up — they are short in duration and occur approximately once every 100 milliseconds. As a result of their "bursty", intermittent pattern of beacon transmission then 802.11 access points don't generate a lot of signal activity that could interfere with other types of wireless devices that operate in the 2.4 GHz band.

When a 2.4 GHz spectrum analyzer is combined with an 802.11 discovery utility — such as NetStumbler or
NetSurveyor — the two complement one another to provide a powerful diagnostic tool for setting up and trouble-shooting WiFi networks. Keep in mind, however, the 802.11 discovery tool picks-up the intermittent beacon broadcasts from an AP and reports the signal strength of that broadcast transmission. This does not necessarily mean you will get good 802.11 performance when connecting with that AP — since after connecting to the AP there could be interference from other devices in the vicinity or there could already be many clients already associated with that AP.


Question:

What kinds of devices could interfere with a WiFi(i.e. 802.11) network?

Answer:

When setting up a wireless network or trouble-shooting a poorly performing one –usually its the non-802.11 devices that you have to worry about most.  Things like wireless video or security cameras, bluetooth devices, cordless phones, wireless (baby) monitors, security systems, RF Excited lighting (Fusion lighting), wireless broadband (i.e. WiMAX), etc. Also, large file transfers and video streaming between 802.11 devices could degrade the performance of other wireless networks in the vicinity.


Question:

In general, what kinds of things affect how a signal from a wireless device is displayed by the analyzer?

Answer:

Using a 2.4 GHz cordless phone as an example, 3 important parameters that will affect how a device appears to the analyzer include the following — the strength of the phone’s transmitted signal, the phone’s distance from the analyzer’s receiving antenna, and the orientation of the analyzer’s antenna relative to the radio waves transmitted by the phone. Changing the orientation of the analyzer’s antenna by rotating the device could change the results that are displayed. Another subtle effect, which is difficult to quantify but which you should keep in mind, is that when you move the transmitting device to different locations this will change the way the RF waves bounce off of objects and walls — which could affect the power of the waves on the receiving antenna and how the signal will be displayed.


Question:

How do I install the external antenna that came with the PCMCIA version of the product?

Answer:

The PCMCIA card comes with a snap-on antenna. To remove it grab the card with one hand and the flat antenna between your thumb and index finger of your other hand. And then gently pull them apart. As you pull them apart you can jiggle (or rock) the antenna a little bit from side to side, but keep the card and antenna within the same 2-dimensional plane (i.e. don't twist). Once you remove the snap-on antenna it will reveal the card's female MMCX connector — which is where you'll connect the external antenna.


Question:

When using AirSleuth-Pro to determine the ˜Best" channel to use for my access point (AP) what preliminary precautions should be taken?

Answer:

While the measurement is being performed temporarily power-off your known access point(s) – otherwise, legitimate network activity occurring over that channel will be interpreted as interference and could hurt that channel’s chances of being selected as the ‘Best’ channel.

Let’s get started on your wireless project…

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