CHAPTER
3-1
Site Survey Guide for Deploying Cisco 7920 IP Phones
OL-6315-01
3
Surveying for Cell Capacity and Channel Re-Use
This chapter describes how to perform a site survey to design a wireless LAN for cell capacity and
channel re-use.

Survey Design for Cell Capacity

Cisco recommends that you install enough access points to provide all users with quality calls.
Maintaining high call quality might require small coverage cells that limit the number of calls per access
point to 7 to meet the QBSS level on access points below 35. With a recommended RSSI level of 20 dB
throughout the RF network, the access point transmit powers could be as low as 20mW, with low-gain
antennas on the access points. You should consider these design factors to ensure adequate capacity and
coverage:
•removing interfering devices
•configuring older 802.11 devices to lower transmit power
•configuring older 802.11 devices to the highest possible data rate
When you configure the access points for higher data rates and lower transmit power, the cell coverage
from the access point becomes smaller. However, if you adjust the access points for high data rates and
low transmit power but you do not make similar adjustments on the clients, the cell might be unbalanced,
as in Figure 3-1, except that the client signal exceeds the access point signal.
Figure3-1 Unbalanced Cell Due to Mismatched Transmit Power and Data Rate Settings
Depending on the configurable and non-configurable parameters of the client, it might continue to
transmit packets at a 1 Mbps data rate. The transmit coverage of a client could be 400 feet when the
transmit coverage of the access point is 100 feet. This mismatch could affect the call capacity and QBSS
of an access point several cells away from the access point to which the client is associated.
100mW
5mW
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