Chapter 14: Drm - Dynamic Radio Management; Factors Influencing The Radio Environment; Signal Strength; Co-Channel Interference - Extreme Networks Summit WM Technical Reference Manual

14 DRM – Dynamic Radio Management
The performance and reliability of a WLAN network is becoming increasingly important as more and
more applications are switching from wired to wireless, which places more users into the same wireless
space. In addition, WLAN applications have expanded to include VoIP, real-time, and mission-critical
services that place extra demands on network performance and reliability. Under normal circumstances,
i.e. proper wired infrastructure with proper QoS settings, the quality of the RF environment actually
determines the performance of the WLAN network.

Factors influencing the radio environment

There are many factors that need to be considered when analyzing the RF environment. The following
subsections discuss these factors.

Signal strength

The most important factor to consider when deploying Wireless APs and adjusting their RF
configuration is signal strength. Typically, a certain minimum signal strength must be obtained
throughout the coverage area. This is important in order to guarantee uninterrupted service for roaming
clients, but also to guarantee capacity. A signal that it is too weak will cause a lot of packets to be lost,
therefore reducing, for example the voice quality. A weaker signal will result in lower PHY rates,
therefore consuming more medium time for the same payload.
As described below, interference in WLAN does not cause severe service degradation, so minimizing
interference is not as high a priority as is guaranteeing the minimum required signal strength
throughout the desired coverage area.
The signal strength is dependent on two factors: the Wireless AP transmit power and the propagation
characteristics of the medium. The Wireless AP transmit power can be controlled, so it will never
change without the Wireless AP being instructed to do so. However, the characteristics of the medium
may change as furniture or people move around.

Co-channel interference

It is essential to understand that the radio medium is a shared medium that is divided in time,
frequency (i.e. channels), and space. Only one transmitter can operate at the same time, on the same
frequency, and in the same space. Two transmitters with overlapping coverage must either use a
different non-overlapping channel or time-share the same channel in order to provide coverage in the
overlapped area.
NOTE
802.11b/g channels are spaced at only 5MHz apart, while non-overlapping channels should be at least 20MHz apart
for OFDM PHY rates (6, 9, 12, 18, 24, 36, 48 and 54 Mbps) and 25MHz apart for CCK PHY rates (1, 2, 5.5 and
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