2-2
Site Survey Guide for Deploying Cisco 7920 IP Phones
OL-6315-01
Chapter 2 Survey Tools
Figure 2-2 AirMagnet Packet Decode Tool
The 7920 survey tool reports the signal strength of the signal from the access point as an RSSI value.
Most client card utilities report signal strength as dBm or percent of signal strength. In this chapter, each
of the tools tested will be reviewed showing how they measure signal strength. Table 2-1 provides a cross
reference of RSSI, dBm, and percent of signal strength as reported by different cards and utilities. The
numbers were captured in a live over-the-air test with each device in the same location for comparison.
These are not exact numbers. A plus or minus of five percent in radio-to-radio performance is considered
within tolerance. All surveys regardless of the tool used must be measured to the signal strength
requirements of the 7920. Cisco recommends that cell size for the 7920 be an RSSI value of 35 for the
7920 survey utility; this equals a PCM350 ACU value of -67 dBm and a Cisco a/b/g client Aironet
Desktop Utility (ADU) value of -66dBm.
Table 2-1 RSSI, dBm, and Signal Strength Cross-Referenced for 7920 Phones
1200 Series Access Point @ 11 Mbps @ 20 mW with 2.2 dBm Antennas
Feet
SniffPro
Signal %
access
point dBm 7920 RSSI
350 ACU
dBm
AirMagnet
350 (PDA)
AiroPeek
CB21 dBm
ADU CB21
dBm
10 78 –55 52 > –45 –47 –51 –51
20 72 –60 46 –52 –49 –52 –53
30 68 –67 46 –55 –54 –58 –60
40 63 –72 43 –59 –54 –68 –68
50 68 –74 37 –59 –58 –68 –70
60 72 –72 38 –63 –64 –68 –67
70 30 –78 35 –67 –67 –68 –66
