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How do I evaluate the quality of my wireless connection?
A number of factors can affect the quality of wireless connections at UCB. These include signal strength, possible
problems that can degrade even a strong signal, and factors that can affect the data transmission rate (throughput).
Step
Procedure
1
Determining Signal Strength
The wireless standard 802.11g operates in the 2.4-2.485GHz (gigahertz) radio frequency (RF) band; RF is
measured in decibels (dB). Wireless cards often come with client software that displays signal strength in dB or
dBm (a variant of dB that provides an exact correlation to the power of the radio signal in watts).
In the samples below we are looking at the Intel Wireless and the Windows Connection managers to check
information about the wireless connection. These are but two examples of what wireless managers may look like
and the information they provide. You may use a different manager with your card, but all of these wireless
connection managers should provide similar information.
Intel Wireless Software (open "Intel PROset Wireless" software | click "Advanced"
"Advanced Statistics")
2
1
3
1. Signal Strength as a Graph
2. Speed & Signal Quality (54.0 Mbps & "Very Good" is maximum)
3. Signal Strength in dBm
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How Do I Evaluate the Quality of my Wireless Connection? (Continued)
Windows Wireless Software (double-click "Wireless Network Connection..." in "Control Panel"
"Network Connections")
Speed &
Signal
Strength
Step
Procedure
2
Calculating & Interpreting Signal Strength
Note: The minimum power sensitivity on most 802.11g clients is -96dBm (very low). If your software displays
"Signal/Noise" or "SNR" in dBm, you can convert this to dB by subtracting the minimum power sensitivity, -96dBm,
from the number displayed as "SNR" or "Signal/Noise". For example, if the Signal/Noise is -47dBm, you would
convert this to dB as follows:
-47dBm (-96dBm) = 49dB
If your wireless card software is indicating that the signal-to-noise ratio (SNR) is greater than 80dB, you are getting
the maximum available bandwidth, or 54 Mbps (megabits per second) for 802.11g. An SNR higher than 80dB
won't increase the amount of bandwidth beyond this maximum (in the example above, with an SNR of 49dB, the
bandwidth is still 54 Mbps, the maximum available rate). When the SNR drops below 80dB, however, the maximum
data rate drops:
SNR
Maximum Raw
SNR
Maximum Raw
SNR
Maximum Raw
(dB)
Data Rate (Mbps)
(dB)
Data Rate (Mbps)
(dB)
Data Rate (Mbps)
--
48
--
18
--
6
--
36
--
12
10
11
60
24
--
9
8
5.5
Even though the maximum data rate goes down, the connection will still be maintained as long as you have an SNR
of 4dB or greater
.
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Last Updated: 7/4/2008
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Created by Mike H.
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How Do I Evaluate the Quality of my Wireless Connection? (Continued)
Step
Procedure
3
Possible Problems (Diagnosis)
Other factors can affect the quality of your wireless connection. The list below is not complete, but it may offer
some explanation for poor performance that occurs even when the signal strength is good:
MULTIPATH: In general, an RF signal grows wider as it is transmitted further. As it spreads, the RF signal will meet
objects in its path that will interfere with the signal in various ways (e.g. by reflecting it). When the signal is reflected
by an object (e.g. a metal object) while moving toward a receiver, multiple wave fronts are created, one for each
reflection point. This can result in a large number of waves, depending on how many reflecting surfaces the original
signal encounters. Many of these reflected waves are still moving toward the receiver, creating a condition known
as multipath. As the number of reflective surfaces increases, the signal deteriorates.
SOLUTION: Move closer to the wireless access point.
Wireless
Your Laptop
Access Point
NEAR/FAR: Near/far is a problem that can happen when multiple wireless users have devices that are very near an
access point, much closer than a user who's on the radio signal boundary. The device farthest away simply cannot
be heard over the traffic from the devices closer to the access point. SOLUTION: Move the more distant device
closer to the access point.
Wireless
Access Point
Your Laptop
NEAR
FAR
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How Do I Evaluate the Quality of my Wireless Connection? (Continued)
HIDDEN NODE: When you turn on an 802.11g capable laptop or handheld device, it immediately scans the
airwaves for access points. It quickly evaluates the signal strength of the available access points, and the number
of users per access point. Based on this, the device will choose the access point with the strongest RF signal and
the fewest users. In hidden node situations, at least one client (node) is unable to "hear" one or more of the other
clients connected to the same access point. Usually this is because of some physical obstruction between it and
other users. As a result, there can be problems in the way the clients share the available bandwidth, causing data
"collisions" or bit errors. When a bit error occurs, the clients need to re-transmit the data. These collisions can
result in significantly degraded data transmission rates in the wireless network.
DYNAMIC RATE SHIFTING (DRS): Adaptive (or Automatic) Rate Selection (ARS) & Dynamic Rate Shifting (DRS)
are terms used to describe how bandwidth is adjusted dynamically by wireless clients. This adjustment in speed
occurs as distance increases between the client and the access point (or possibly if interference increases). As the
distance grows greater, the signal strength will decrease to a point where the current data rate cannot be
maintained. As the signal strength drops, the client will drop its data rate to the next lower specified data rate, for
example, from 11 Mbps to 5.5 Mbps, or from 2 Mbps to 1 Mbps.
INAPPROPRIATE AP SELECTION: Some wireless clients will attempt to connect to a distant high bandwidth
(802.11g/n) access point with a weak signal, rather than connecting to a nearby low bandwidth (802.11b/g) access
point with a strong signal. The result can be dropped connection with reduced throughput.
Step
Procedure
4
Throughput (Possible Explanations)
Throughput is a measure of the speed of your wireless connection. Defined as the amount of data transmitted
in a given time period, throughput is based on many factors. Five important factors for your consideration...
SHARED BANDWIDTH: All wireless clients connected to an access point must share the bandwidth available. As
more people connect, the contention for the available bandwidth increases, causing interference and increased
transmission error rates.
POSSIBLE SOL'N - connect to another access point which does not have as many people using it.
INTERFERENCE FROM ANOTHER RADIO FREQUENCY SOURCE: 802.11b/g operates in the radio frequency
(RF) band between 2.4 and 2.485 GHz. This is an "unlicensed" range of frequencies, meaning the Federal
Communications Commission (FCC) allows anyone to use it. Unfortunately for 802.11b/g users, microwave ovens
and 2.4 GHz cordless phones operate in the same frequency band. The radio signal emanating from such devices
can severely degrade an 802.11b/g signal.
POSSIBLE SOL'N - move away from sources that operate in the 2.4 GHz frequency range.
SECURITY: 802.11b/g networks are susceptible to hacking and data theft by unauthorized users. EVERY wireless
user should use some form of security, most often SSL. This adds more overhead to the wireless data packets,
absolutely necessary but using valuable bandwidth. Although 802.11g allows for 54 Mbps maximum throughput, a
user will typically only get about 36-48 Mbps of data.
DISTANCE: Greater distances between the transmitter (access point) and receiver (client, you) will cause the
throughput to decrease because of an increase in the number of errors. 802.11 can recognize these errors and
retransmits data. Also, 802.11g is configured to make discrete jumps to specified data rates (54, 48, 36, 24, 18, 12,
9, and 6 Mbps). If 54 Mbps cannot be maintained because of errors & degrading signal strength, then the device
will drop to 48 Mbps, then down to 36 Mbps, all the way down to 6 Mbps, with the eventual loss of the connection.
Remember that, because of overhead, you will only receive about half of the available bandwidth.
POSSIBLE SOL'N - move closer to a wireless access point or stop moving away from one.
B/G INTEROPERABILITY: The presence of an 802.11b participant significantly reduces the speed of an 802.11g
network. When both 802.11b and 802.11g clients are connected to an 802.11g access point, the performance of
the 802.11g clients can suffer.
RESOLVED - Airbears currently only allows for 802.11g clients and so does not experience this problem.
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How Do I Evaluate the Quality of my Wireless Connection? (Continued)
Step
Procedure
5
Recommendations
Avoid congested wireless networks and move closer to access points when in and around Haas.
Use wired connections in any of the classrooms, around Haas, or in the main lab (you are able to check out an
Ethernet cable from Haas Media Services in S300).
Make sure your wireless card is capable of using 802.11a or 802.11g - you will be unable to connect to Airbears
otherwise.
If you cannot get past the Airbears Wireless Vulnerability Scan, please do one of the following:
Shut down and completely power off your computer for 20 to 30 minutes and then try again.
o
Continue to use your computer with the wireless card disabled for 20 to 30 minutes and then try again.
o
While doing one of the above, check-out an Ethernet cable from Haas Media Services and continue to use
o
the internet from a wired connection.
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Created by Mike H.