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.
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Figure 1.
In this figure the horizontal white lines show the |
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
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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. |
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Figure 3. A microwave oven is running. Here you can see that it emits RF energy across the entire 2.4 GHz band. |
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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. |
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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. |