1-3
Site Survey Guide for Deploying Cisco 7920 IP Phones
OL-6315-01
Chapter1 Overview
Recommendations for Successful VoIP Surveys
Figure1-1 ACU Site Survey Window
Noise is defined as a 2.4-GHz signal that is not in an 802.11 DSSS format but is in the frequency range
of the access point’s configured channel. The noise can originate from an 802.11 2.4-GHz
frequency-hopping radio, a 2.4-GHz wireless phone, a 2.4-GHz HAM radio, a Microwave oven, or a
Bluetooth radio. Signals from a distant out-of-network 802.11b o r 802.11g radio may also be seen as
noise. Any 2.4-GHz signal that the access point cannot decode is considered noise. However, valid data
packets from 802.11b or 802.11g radios that are not associated to the access point are considered data
traffic. Those packets are decoded by the access point and client devices but are discarded. However,
they increase the channel utilization on the access point, thus limiting the number of voice clients that
can associate. Same-channel interference must be minimized.
If the signal strength on a valid packet is higher than the receiver threshold of the acc ess point radio or
the client device radio, the data packet is decoded. Most 802.11 radios have a receiver sensitivity value
of –94 dBm to –85 dBm at a data rate of 1 Mbps (the lower the dBm value, the better the radio’s receiver
sensitivity). Radio receiver sensitivity changes with data rates; for example, an access poin t radio might
have a receiver sensitivity of –94 dBm at 1 Mbps, but the radio sensitivity might be –84 dBm at 11 Mbps.
The access point discards random data traffic--valid packets that can be decoded but which are not from
clients associated to the access point. Random data traffic can originate from a shared media or from a
client device that is transmitting at a data rate that the access point does not support.
In Figure 1-1, the noise level reported by the ACU is –92 dBm and the PCM350 receiver sensitivity at
11 Mbps is –84 dBm, which provides a margin of 8 dB at the rece iver. A signal strength value of –48
dBm less the noise value of –92 dBm equals an SNR of 44 dB as report ed by the ACU (see Figure 1-2).