Interference; Multipath And Signal Distortion - Polycom SpectraLink 8020 Deployment Manual

Deploying SpectraLink 8020/8030
Wireless Telephones
July 2009
matches that of the AP. However the handset will never exceed the power limit statically set in its handset
administration menus or HAT.
For SpectraLink handsets running pre-R3.0 code, the transmission power setting must be the same for all
APs in a facility, and match the handset‟s statically configured transmit power setting. The transmit power
setting on the phone should be based on the AP‟s configured transmit power setting, not the EIRP
(Effective Isotropic Radiated Power) of the AP. Any AP antenna gain will increase signal gain in both
directions.
When running pre-R3.0 code, regardless of the selected power level settings, all APs and handsets must
be configured with the same transmit settings to avoid channel conflicts or unwanted cross-channel
interference. For access points that support automatic transmission power adjustments, Polycom
recommends using only static power settings to ensure optimal performance of SpectraLink Wireless
Telephones pre-R3.0.
In mixed 802.11b/g environments, Polycom recommends configuring the transmit power of the 802.11b
and 802.11g radios to the same setting, if they are separately configurable. For example, set both radios
to 30mW to ensure identical coverage on both radios. For mixed 802.11a/b/g environments, where the
AP utilizes all three radios types, AP placement should first be determined by modeling for the
characteristics of 802.11a, since this environment will typically have the shortest range. Then, the
transmit power of the 802.11b and 802.11g radios should be adjusted to provide the required coverage
levels and cell overlap for those networks, within the already established AP locations.
2.4.3

Interference

Interference on a wireless network may originate from many sources. Microwave ovens, Bluetooth
devices, cordless phones, wireless video cameras, wireless motion detectors, and rogue APs are among
the many potential interfering RF (radio frequency) sources. In general, devices that employ or emit radio
frequency signals within a given radio coverage area will have the potential to cause unwanted signal
interference.
Radio frequency spectrum analyzers can be used to help identify the sources of such interference. Once
identified, interference is best mitigated by removing the interfering device(s) from the network area.
Otherwise, it may be possible to change the channel setting of the interfering device to avoid conflict with
the surrounding APs. If this is also not possible, then it may be possible to change the channel of the
surrounding APs to avoid as much radio frequency overlap with the interfering device.
A documented facility-wide radio frequency usage policy will help control sources of RF energy. Ideally,
any RF generating device should have prior approval before introduction onto the property or installation
in any building or structures.
2.4.4

Multipath and Signal Distortion

For 802.11a/b/g environments, multipath distortion is a form of RF interference that occurs when a radio
signal has more than one path between the transmitter and the receiver causing multiple signals to be
detected by the receiver. This is typically caused by the radio signal reflecting off physical barriers such
as metal walls, ceilings and other structures and is a very common problem in factories and storage
environments. Multiple converging wave fronts may be received as either an attenuated or amplified
signal by the receiver. In some instances, if the signals arrive exactly out of phase, the result is a
complete cancellation of any RF signal. In 802.11n networks multipath is an exploited feature, rather than
a potential interference problem. The multiple radios used for 802.11n (up to three in an AP) provide
increased throughput. The resulting multipath effects of the multiple radios are used to obtain increased
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Best Practices Guide