Strategies and Tools for Troubleshooting WiFi
Networks
"Using the (802.11) infrastructure to
troubleshoot the infrastructure..."
Here is a brief
introduction to 4 troubleshooting techniques
(network discovery, RF spectrum analysis, WiFi
channel analysis, and WiFi connection analysis),
along with short descriptions of how RF signal
generation and 802.11 packet injection can also aid
in finding and solving wireless problems.
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Network Discovery
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An 802.11 network
discovery tool will report the Service Set
Identifier (SSID) for each access point (AP) it
detects, along with the channel used by the AP.
Approximately every 100 mSec an AP transmits a small
beacon packet and a discovery tool (running on your
laptop and using its 802.11 wireless adapter)
detects the beacon and adds the packet information
(including the AP's SSID) to its list of known
access points. In addition, the discovery utility
may report signal strength (in dBm units) of the
beacon as detected by the client adapter. The
beacon's signal strength is a reflection of how
close the AP is to your current location. Though
this is useful information, it does not tell you
anything about non-802.11 devices or even how busy
the access points are. That is, your laptop could
be sitting next to a microwave oven and the
discovery tool would be clueless as to its
existence. The discovery tool only knows about
beacon packets transmitted by 802.11 devices and can
not see non-802.11 transmissions.
A network discovery tool reports all APs -- both
those that belong to you and your neighbors.
For troubleshooting purposes, one is primarily
concerned with your APs -- in
particular, the signal strengths of their beacons
(often reported as RSSI). When this value is
above a certain threshold, e.g. -75 dBm, then the AP
and client adapter are close enough to communicate.
Other than reporting that the AP and client adapter
are within proximity of one another for
communication purposes, there is not much more the
network discovery tool can provide in
terms of helping you troubleshoot RF-related
problems. Though network discovery tools may
also report which channels are used by neighboring
APs, this is not nearly as important an issue as RF
interference caused by non-802.11 devices.
Since network discovery tools can only see
neighboring 802.11 APs, then they are of no use in
detecting non-802.11 wireless devices.
Network discovery tools use the 802.11 adapter built
into your laptop or an external USB 802.11 adapter.
Since they do not require additional proprietary
hardware, then they are relatively inexpensive (even
free).
AP Beacon Strength Is
Not A Measure Of Performance
The signal strength reported by a network discovery
tool is the signal strength of a beacon as measured
by the 802.11 wireless adapter installed on your
laptop or desktop machine. Each access point (AP)
sends out a short pulse or beacon of information
approximately every 100 mSec. It's equivalent to an
'I'm over here!' shout. It does not expect a
response from the 802.11 client adapters that may
hear it -- it's just a one-way shout. The signal
strength that the network discovery tool reports is
the signal strength of a beacon, and the signal
strength of a beacon is a reflection of how close
that AP is located to you. It is not
a reflection of the performance or throughput you
can expect by associating with that AP -- rather, it
is an indication of the AP's physical location
relative to you. If the AP with the strongest beacon
has 24 client adapters associated with it that are
actively transmitting and receiving information, and
if you connect with that AP then you will be client
number 25 and your network connection will seem
slow. On the other hand, if you instead choose to
associate with an AP whose beacon strength is weaker
but which is not associated with other client
adapters, then you will likely experience better
performance. Furthermore, the AP with the strongest
beacon signal may be using a channel that is subject
to RF interference -- again, degrading its
performance. When it comes to networking (both wired
and wireless) what we care most about is
performance. And the key to performance is
'throughput' (i.e. bytes-per-second). Though a
beacon's signal strength can affect it's
performance, what's more important is the number of
client stations that are competing for the same AP
and whether the channel currently used by the AP is
subject to RF interference from other wireless
devices in the vicinity. |
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An RF spectrum
analyzer 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. Since they detect
all RF
transmissions (both 802.11 and non-802.11) then they provide a much better
picture of the RF environment, which then helps you
identify and, perhaps, locate devices that could be
interfering with your Wi-Fi network. Typically an RF
spectrum analyzer will employ a 2-dimensional
display where the vertical axis (Y-axis) represents
the strength of a signal and the horizontal axis
(X-axis) represents the frequency of a signal.
If the spectral trace of the interfering RF
transmissions have previously been documented, then
it might be possible to determine which type of
device is causing the disturbance. As for
tracking-down and attempting to locate an
interferer, in practice this is more difficult than
it might seem on the surface. Not only does it
require the use of a directional antenna, but in an
indoor environment with waves bouncing all over the
place (off of objects and walls) then how do you
discern from which direction the wave originated.
In other words, when your directional antenna
measures a signal from a wave you don't know whether
that's the original wave or the result of a wave
that has bounced off of an object or wall in the
room. |
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Today, one of the
hottest topics discussed by Wi-Fi infrastructure
manufacturers is "using the infrastructure to
troubleshoot the infrastructure". That is --
using 802.11 devices to troubleshoot an 802.11
network. Channel analysis is a new technique
we have championed and pioneered. This type of tool
uses 802.11 hardware to perform data acquisition --
hence, the results truly reflect how RF interference
in the local environment affects throughput
performance of 802.11 channels. This is not
possible using an RF spectrum analyzer. By virtue of
the fact an 802.11 channel analyzer views the RF
world through the eyes of an 802.11 device, then the
diagnostic information it provides more closely
mirrors the performance you can expect from your own
802.11 client adapters. This makes it easier to
troubleshoot and fix problems and allows you to make
better-informed decisions regarding how best to
configure your wireless network for optimal
throughput performance. |
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Ultimately, the
bottom line for any network (wired or wireless)
comes down to throughput performance -- that is, how
many bytes-per-sec can be transferred from one node
on the network to another. The dBm and RSSI values
that are often referred to in the context of
wireless networks don't mean much if you can't
somehow relate them to a performance metric. Before
we can really begin to troubleshoot a wireless
network we need a way to benchmark its performance,
so as modifications are made we can determine
whether or not they really make a difference in the
network's performance. A connection analysis tool
allows you to directly compare the performance and
reliability of different combinations of 802.11
adapters and access points. |
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An 802.11 channel
/ signal generator transmits RF signals and can be
used in testing Wi-Fi antennas, RF shields and
wireless networks. A signal generation system is a
key to any test system. Combined with an RF spectrum
analyzer or WiFi channel analyzer, you now have the
ability to locate "dead spots" in any locale where a
WiFi network is installed. Also, the channel /
signal generator can be used to stress-test an
existing Wi-Fi network and for aligning directional
antennas. |
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When
troubleshooting a wireless network, being able to
measure packet capture rates may be more informative
than RF spectrum analysis -- because now you are
measuring the real thing. Though poor packet capture
rates can't distinguish between interference or
obstacles that create 'dead spots' or poor antenna
location or positioning, because you are measuring
packet rates then whatever changes you make to the
system to improve packet rates should directly be
reflected in improved performance. That is, if you
reconfigure a wireless system (i.e. channel
selection, antenna location and position) such that
packet rate is doubled, then you should see a
similar improvement in the overall performance of
the wireless network. You can't make these sorts of
measurements using an RF spectrum analyzer, since it
doesn't measure performance. A packet
injection tool creates wireless traffic on a
particular channel -- at a known level (i.e.
packets / sec) and size (i.e. bytes / packet)
without the need of the client adapter associating
with an access point. This makes it much
easier to measure the quality of the transmission
and reception performance on a particular 802.11
channel. |
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