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Site Survey Guide for Deploying Cisco 7920 IP Phones
OL-6315-01
Chapter 1 Overview
Recommendations for Successful VoIP Surveys
The characteristics of VoIP require a meticulous WIPT site survey. To avoid a call with jitter, the delay
variation between packets should not exceed 30 ms. The one-way delay should not exceed 150 ms and
packet loss should not exceed one percent. Surveying for a one percent packet loss is important because
VoIP uses the Real-Time Transport Protocol (RTP), which does not retransmit for lost packets. Voice
packets are small but are sent at consistent intervals. The 7920 client assumes that it has lost its
connection to an access point if it misses 3 consecutive beacons. The access point sends a beacon every
100 ms, which means that the Cisco 7920 looks for another access point if it does not see beacons every
300 ms from the access point to which it is associated.
Multi-floor survey
At many sites it is important to consider the effect of above-floor and below-floor coverage. An antenna
placed near a ceiling in a multi-floor facility can easily provide coverage to the floor above. There will
be considerable loss in dBm signal strength, but the signal can still have enough quality to be usable.
Depending on the capacity, throughput, and coverage requirement for the site, the power of the access
point may be turned down to minimize the signal coverage between floors. An Omni antenna propagates
directly above it into the floor above. A patch antenna will also propagate signal into the floor above,
but not directly above as the Omni antenna will. Although directional antennas help focus the signal
energy in a particular direction, which can help to overcome fading and multipath signals, multipath
signals reduce the focused power of a directional antenna and the amount of multipath seen by a user at
a long distance from the access point can be much greater. Directional antennas used indoors typically
are low gain (5 dBi to 10 dBi), and therefore have lower front-to-back and front-to-side lobe ratios. This
reduces the radio’s ability to reject or reduce the interference signals received from directions outside
the primary lobe area. A low-gain directional antenna, such as a 6.5-dBi patch, will receive signal from
the sides and the back, but the primary coverage area will be forward. The coverage in Figure 1-5
represents an Omni antenna coverage. The cell coverage overlap and same-channel separation should be
maintained at a multi-floor site.
Figure 1-5 Survey in a Building with Multiple Floors
Different types of facilities, such as hospitals and schools, have different signal propagation patterns,
multipath levels, and attenuation levels.
