What does one consider to be good wireless network
performance (baseline
SNR?)
and how does one measure it?
Power output can be expressed in absolute terms (milliWatts, mW) or
relative terms (decibels, dB). For example, +3 dB means a two-fold
increase in power (mW). Signal strength is typically given as a percentage
or expressed in decibel-milliWatts (dBm), a relative measurement that
indicates signal gain or loss on a scale that starts at 1 mW. Noise is
measured in a similar fashion.
Signal-to-noise ratio (SNR) is just what it sounds like - a comparison
between the strengths of a radio's transmission signal and noise. For
example, when signal strength is -37 dBm and noise is -100 dBm, SNR is 63.
According to the CWNA Study Guide, an SNR of at least 22 is considered
viable if absolute signal strength is larger than the recipient's receive
threshold and conditions are relatively stable.
Many 802.11 cards come with client utilities that let you view a nearby
AP's signal, noise, and SNR. Some can write these numbers to a log at
regular intervals. Freeware discovery tools like
NetStumbler usually display and log these values for all discovered
APs. WLAN analyzers like
AiroPeek and
AirMagnet can also monitor signal and noise, plotting these values on
charts over time and analyzing results to help you spot problems or
changes in comparison to baseline measurements. In some cases, a noise
source can be identified rather easily - for example, co-channel
interference from another AP operating nearby. More difficult cases may
require true RF spectrum analysis tools like
Yellowjacket.
SNR = Signal to Noise Ratio |
Understanding Decibels and Their Use in Radio Systems
This article discusses the basic unit of measurement used in radio
signals: the decibel or dB. It is named after Alexander Graham Bell - that
is why the "B" is capitalized. There are several variations of the dB used
in radio. This paper contains some definitions and explanations of dBs,
plus other related terms and concepts that you might find useful in
implementing your wireless systems.
dB (Decibel)
The difference (or ratio) between two signal levels. Used to describe the
effect of system devices on signal strength. For example, a cable has 6 dB
signal loss or an amplifier has 15 dB of gain. This is useful since signal
strengths vary logarithmically, not linearly. Since the dB scale is a
logarithmic measure, it produces in simple numbers for large-scale
variations in signals. It is very useful because system gains and losses
can be calculated by adding and subtracting whole numbers.
Every time you double (or halve) the power level, you add (or subtract) 3
dB to the power level. This corresponds to a 50% gain or reduction. 10 dB
gain/loss corresponds to a ten-fold increase/decrease in signal level. A
20 dB gain/loss corresponds to a hundred-fold increase/decrease in signal
level. In other words, a device (like a cable) that has 20 dB loss through
it will lose 99% of its signal by the time it gets to the other side.
Thus, big variations in signal levels are easily handled with simple
digits.
dBm (dB milliWatt)
A signal strength or power level. 0 dBm is defined as 1 mW (milliWatt) of
power into a terminating load such as an antenna or power meter. Small
signals are negative numbers (e.g. -83 dBm).
For example, typical 802.11b WLAN cards have +15 dBm (32mW) of output
power. They also spec a -83 dBm RX sensitivity (minimum RX signal level
required for 11Mbps reception).
For example, 125 mW is 21 dBm and 250 mW is 24 dBm.
dBd (dB dipole) The gain an antenna has over a dipole antenna at
the same frequency. A dipole antenna is the smallest, least gain practical
antenna that can be made. The term dBd (or sometimes just called dB)
generally is used to describe antenna gain for antennas that operate under
1GHz (1000Mhz). The reason why the gain of many antennas, especially
VHF/UHF antennas, is measured in dBd is because antenna manufacturers
calibrate their equipment using a simple dipole antenna as the standard.
Then they replace it with the antenna they are testing. The difference in
gain (in dB) is reference to the signal from the dipole.
dBi (dB isotropic)
The gain a given antenna has over a theoretical isotropic (point source)
antenna. Unfortunately, an isotropic antenna cannot be made in the real
world, but it is useful for calculating theoretical fade and System
Operating Margins. The gain of Microwave antennas (above 1 GHz) is
generally given in dBi. A dipole antenna has 2.14 dB gain over a 0 dBi
isotropic antenna. So if an antenna gain is given in dBd, not dBi, add
2.15 to it to get the dBi rating, For example, it an omni antenna has 5
dBd gain, it would have 5 + 2.15 = 7.15 dBi gain.
NOTE: |
If an antenna gain is just specified in dB from a manufacturer, be
sure to ask if it is dBi or dBd. If they cannot tell you or do not
know the difference, then you should consider buying from another
vendor! |
EIRP (Effective Isotopic Radiated Power)
Effective Isotropic Radiated Power is defined as the effective power found
in the main lobe of a transmitter antenna relative to an Isotropic
radiator which has 0 dB of gain. It is equal to the sum of the antenna
gain (in dBi) plus the power (in dBm) into that antenna. For example, if a
12 dBi gain antenna is fed with 15 dBm of power has an Effective Radiated
Power (ERP) of:
12 dBi + 15dBm = 27 dBm (500 mW).
With an amp that has 24 dBm (250mW) output - max allowed by the FCC into a
12 dBi omni:
12 dBi + 24dBm = 36 dBm (4 Watts)
- which, BTW, is the same as 1W (+30 dBm) into a 6 dBi omni:
6 dBi + 30 dBm = 36 dBm (4 Watts)
But it is much better to have a higher gain omni antenna since, while the
ERP is the same, a higher gain antenna has the gain on receive as well.
This is where you really need it since most of your clients will not be
equipped with amplifiers.
NOTE: |
The ERP is found in the main lobe only. If you are using a high-gain
omni-directional antenna, the radiation pattern is very flat and
narrow (like a pancake). If the antenna is too high, the main lobe
will actually shoot over the heads of your customers. But oftentimes
need great height to clear obstacle from the WIPOP antenna to your
customers! A solution is to use down-tilt sector antennas. They have
more gain than omni-antennas and the main lobe can be focused into the
desired coverage area. Doing this also defines a "cell" that will
prevent radio coverage all the way to the horizon. This has the
benefit of not only minimizing interference at the WIPOP from distant
signals, but also will enable you to re-use the frequency at another
cell several miles away. |
FSL (Free Space Loss)
Free Space Loss is defined as the loss that a radio signal experiences
when traveling through free space. The formula at 2.4 GHz is:
FSL = 104.2 + 20 log D (Where: D = Distance in miles)
Example: At 5 miles FSL is 118 dB
NOTE: |
Every time you double (or halve) the distance from the transmitter to
the receiver, the signal level is lowered (or increased) by 6dB. |
System Operating Margin (SOM)
System Operating Margin (also referred to as Fade Margin) is defined as
the difference between the received signal level (in dBm) and the receiver
sensitivity (in dBm) needed for error free reception. For example, if the
received signal level is -71 dBm and the receiver sensitivity is -83dBm
(typical for a 11Mbps WLAN), then the SOM is:
-71dBm - (-83 dBm) = 12 dB SOM
This should work if there is not bad interference. recommended 10 dB SOM
or more. 20 dB is excellent.
NOTE: |
If your WIPOP (Wireless Internet Point of Presence) is amplified and
your customer's WLAN card or AP is not, then the SOM needs to be
calculated from the remote site back to the WIPOP. This is because the
remote site has the weakest TX signal in the system. |
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