Polycom H340 - Manual (Page 5)

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Deploying SpectraLink e340, h340 and i640 Wireless Telephones BEST PRACTICES GUIDE
October 2010
2 Wireless LAN Layout Considerations
SpectraLink handsets utilize a Wi-Fi network consisting of WLAN access points (APs) distributed throughout a
building or campus. The required number and placement of APs in a given environment is driven by multiple factors,
including intended coverage area, system capacity, access point type, power output, physical environment, and radio
types.
2.1 Coverage
One of the most critical considerations in deployment of SpectraLink handsets is to ensure sufficient wireless
signaling coverage. Enterprise Wi-Fi networks are often initially laid out for data applications and may not provide
adequate coverage for voice users. Such networks may be designed to only cover areas where data devices are
commonly used, and may not include coverage in other areas such as stairwells, break rooms or building entrances
– all places where telephone conversations are likely to occur.
The overall quality of coverage is more important for telephony applications. Coverage that may be suitable for data
applications may not be seamless enough to support the requirements of VoWLAN. Most data communication
protocols provide a mechanism for retransmission of lost or corrupted packets. Delays caused by retransmissions
are not harmful, or even discernable, for most data applications. Likewise, data applications will typically tolerate
more retries than voice applications, as the useful life of a voice packet is short. However, the real-time nature of a
full-duplex telephone conversation requires that voice packets be received correctly within tens of milliseconds of
their transmission. There is little time for retransmission, and lost or corrupted packets must be discarded after
limited retries. In areas of poor wireless coverage, the performance of data applications may be acceptable due to
retransmission of data packets, but for real-time voice, audio quality will likely suffer.
Another factor to consider when determining the coverage area is the device usage. Wireless telephones are used
differently than wireless data devices. Handset users tend to walk as they talk, while data users are usually
stationary or periodically nomadic. Wireless voice requires full mobility while data generally requires simple
portability. Wireless handsets are typically held close to the user’s body, introducing additional radio signal
attenuation. Data devices are usually set on a surface or held away from the body. The usage factor may result in
reduced range for a wireless telephone as compared with a data device. Therefore, the WLAN layout should account
for some reduction of radio signal propagation.
2.1.1 Overlapping Coverage
Wi-Fi cell overlap must be considered when planning your VoWLAN deployment. Handsets make a determination to
roam in less than half the overlapping coverage area. Therefore, the coverage area must be adequate enough so
that when a voice user is moving, the handset has time to discover the next AP before signal on the existing AP
becomes too weak.
A properly designed Wi-Fi network will position APs with sufficient overlapping coverage to ensure there are no
coverage gaps, or “dead spots”, between them. The result is seamless handoff between APs and excellent voice
quality throughout the facility. Sufficient overlapping coverage is usually considered 15% to 20% signal overlap
between AP cells in a deployment utilizing maximum transmit power for both handsets and APs. Smaller cells will
need larger overlaps due to the potential for much smaller cell size which causes a decrease in overall overlap from
a maximum transmit power deployment. The 15% to 20% of signal overlap between AP cells generally works well
with a typical walking speed of the user (the average walking speed of an individual is 3 mph). If the speed of the
moving user is greater (such as a golf cart, fork lift or running/jogging) then a different overlap strategy may be
necessary for successful handoff between APs.
The WLAN layout must factor in the transmission settings that are configured within the APs. The transmission of
voice requires relatively low data rates and a small amount of bandwidth compared to other applications. The 802.11
standard includes automatic rate switching capabilities so that as a user moves away from the AP, the radio adapts
and uses a less complex and slower transmission scheme to send the data. The result is increased range when
operating at reduced transmission data rates. When voice is an application on the WLAN, APs should be configured