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This visualization shows the
signal-to-interference ratio (SIR) measured in dB. SIR is a measure
to quantify by how much the signal level of an AP (interfered AP)
exceeds the interference level. The interfering signal is the
signal being transmitted by other APs (interfering APs) that may or
may not belong to your WLAN and that use the same or one of the
adjacent 802.11 channels. In low SIR zones, client devices may
experience low throughput. SIR is shown for the AP that experiences
the worst interference in the given map area among the APs selected
for analysis. You can deselect one or several of the selected APs
to see SIR values for the APs that experience less
interference.
SIR is best illustrated with an example.
Consider an area where the AP signal strength is -50 dBm, and the
AP works on channel 1. In the same area, a -70 dBm signal from
another AP that works on the same channel can be seen. If WLAN
utilization is 100% (i.e., if the
APs send radio waves all the time), the SIR
value would be 20 dB. However, real-world WLAN utilization is
almost never that high, which decreases the interference and
increases SIR. If the interfered and interfering APs have the same
signal strength, the SIR value would be 0 dB. In classical,
non-digital radios, a SIR value of 0 dB makes signal reception
impossible, but 802.11 devices use a technology that allows them to
operate despite a zero or even negative SIR value, which sounds
counterintuitive.
Simply put, if an AP is not
heavily loaded, it transmits just a few hundred packets per second.
If a nearby AP working on the same channel also transmits a few
hundred packets per second, the transmissions “collide” very
infrequently, thus resulting in virtually zero interference. The
average network utilization that is used for SIR computations is a
configurable option (see below).
Interference is highest when
the interfered and interfering APs work on the same channel. In the
2.4 GHz band, where channel frequencies overlap, adjacent channel
interference is still substantial when the interfered and
interfering APs are one and two channels apart and becomes
virtually non-existent when they are five channels apart. In the 5
GHz band, there is no adjacent channel interference. Because many
802.11n devices use channel bonding (i.e., two 20-Mhz channels at
the same time), for example, channel 11 as the primary channel and
channel 6 as the secondary one, TamoGraph also factors in
interference on the secondary 802.11n channel, if any. It should
also be noted that when visualizing SIR, the application takes into
consideration interfering signals from all APs, regardless of
whether they are selected in the AP
list. The
application also detects multi-SSID APs and does not consider
separate different SSIDs with different MAC addresses of the same
AP as sources of interference with each other.
The following options on
the Visualization
Settings panel
(located on the Options tab of the right-side panel) affect the way
SIR is analyzed:
§Area
is considered covered if signal strength is at least
– This setting defines the AP
coverage area based on the minimum signal strength. If the signal
strength is below the specified level, the area is considered to be
not covered, and no SIR values will be computed for the area (such
areas will appear as white spots). This improves the SIR
visualization clarity: In low signal areas, SIR is almost always
very low, but such areas should not distract your attention, as
they cannot ensure good connectivity or throughput
anyway.
§Average network
utilization – This
setting defines how heavy the interference is from the interfering
APs. If the interfering signal strength is high, but the network
utilization is low, the interfering AP does not create much
interference. A typical office WLAN has a network utilization of
between 10% and 25%. Adjust this setting to match the actual value
for your WLAN.
Double-clicking on the SIR
legend on the status bar allows you to configure the color scheme
and change its value range.
Suggested Solutions
Low SIR areas are not uncommon in WLANs.
The presence of such areas does not necessarily mean that the WLAN
will suffer from low throughput. However, if such zones cover most
of your site and are located close to APs, corrective actions
should be taken. When low SIR areas are discovered, the following
solutions are suggested:
·Change the channel
selection. APs working in close proximity should never use
overlapping channels. Consider the classical “honeycomb” AP
placement, if possible. Note that in some 802.11n equipment, the
position of the secondary channel (below or above the primary one)
is a user-configurable option, which gives you an additional degree
of freedom.
·If you experience low
SIR values in the 2.4 GHz band, consider switching your APs to the
5 GHz band, where there are more non-overlapping channels from
which to choose. If you use an 802.11n AP with 40 MHz bandwidth in
the 2.4 GHz band, you have virtually no way of avoiding
interference. For example, if the primary channel is set to 1, the
secondary channel is set to 5. In the United States, where there
are eleven channels in the 2.4 GHz band, all you can do is
configure the next AP to work on the primary channel 11, the
secondary being 6. As a result, the secondary channels would be
only one channel apart, which may result in high interference. If
channel bonding is not used (i.e., a single 20 MHz channel), you
have three non-overlapping channels from which to choose: 1, 6, and
11. This is illustrated in the image below.
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