Polycom H340 - Manual (Page 10)

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Deploying SpectraLink e340, h340 and i640 Wireless Telephones BEST PRACTICES GUIDE
October 2010
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.
2.2.4 Multipath and Signal Distortion
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.
Multipath can cause severe network throughput degradation because of high error rates and packet retries. This in
turn can lead to severe voice quality impairment with SpectraLink Wireless Telephones. Correctly locating antennas
and choosing the right type of antenna can help reduce the effects of multipath interference.
AP diversity antennas should always be used to help improve performance in a multipath environment. A diversity
solution uses two antennas for each radio, and will send and receive signals on the antenna which is receiving the
best signal from the wireless client. Diversity in an AP with two antennas, which provide signaling to the same
geographic area, provides a unique signal path from each antenna to the handset. This greatly increases the
probability that both the AP and the handset will receive a better signal quality in multipath environments. Most
access points support receive diversity in that they accept the received transmission on the antenna that is getting
the best signal. Some also support full transmit diversity where the transmission is made on the same antenna that
was last used to receive a signal from that specific client. In order to provide optimal voice quality, Polycom
recommends the use of APs supporting both receive and full transmit diversity in environments where multipath is an
issue. This will help optimize the WLAN for all wireless clients. External antennas provide additional flexibility in type
(omni or directional), mounting options and gain. External antennas can be separated from 4.5 inches to 5 feet at
each AP radio.
Access point antennas should not be placed near a metal roof, wall, beam or other metal obstruction in any
environment, as this will amplify the reflection effects. Additionally, antennas should be positioned so that they have
line of sight (LoS) to most of the clients that they service. Additional instructions from the wireless network
infrastructure vendor should be followed with regard to antenna selection and placement to provide correct diversity
operation.
2.2.5 Site Surveys
A wireless RF site survey is highly recommended for any wireless network deployment. However, it is especially
critical for VoWLAN and is essential for large or complex facilities. An RF site survey can ensure that the wireless
network is optimally designed and configured to support voice by confirming RF placement, cell overlap, channel
allocation/reuse, packet transmission quality, packet retry rates, and other deployment considerations. While many
tools exist that allow customers to perform their own assessment, Polycom recommends a professional site survey to
ensure optimum coverage and minimum interference. Polycom offers a full suite of site-survey services that will
ensure a WLAN is properly configured to support wireless voice.
To verify coverage of an installed Wi-Fi network, Polycom handsets offer a site-survey mode that can be used to
validate the AP locations and configurations are both correct and adequate. This mode detects the four strongest AP