H3C Technologies H3C WX3000E Series Wireless Switches - Manual (Page 110)

98
Configuring WLAN RRM
Overview
Radio signals are susceptible to surrounding interference. The causes of radio signal attenuation in
different directions are very complex. Therefore, we need to make careful plans before deploying a
WLAN network. After WLAN deployment, the running parameters still need to be adjusted because the
radio environment is always varying due to interference from mobile obstacles, micro-wave ovens and so
on. To adapt to environment changes, radio resources such as working channels and transmit power
should be dynamically adjusted. Such adjustments are complex and require experienced personnel to
implement regularly, which brings high maintenance costs.
WLAN radio resource management (RRM) is a scalable radio resource management solution. Through
information collection (APs collect radio environment information in real time), information analysis (The
AC analyzes the collected information), decision-making (The AC makes radio resource adjustment
configuration according to analysis results), and implementation (APs implement the configuration made
by the AC for radio resource optimization), WLAN RRM delivers a real-time, intelligent, integrated radio
resource management solution, which enables a WLAN network to quickly adapt to radio environment
changes and keep staying in a healthy state.
Dynamic frequency selection
A WLAN has limited working channels. Channel overlapping is very easy to occur. In addition, other
radio sources such as radar and micro-wave ovens may interfere with the operation of APs. Dynamic
frequency selection (DFS) can solve these problems.
With DFS, the AC selects an optimal channel for each AP in real time to avoid co-channel interference
and interference from other radio sources.
The following conditions determine DFS:
• Error code rate—Physical layer error code and CRC errors.
• Interference—Influence of 802.11 and non-802.11 wireless signals on wireless services.
• Retransmission—APs retransmit data if they do not receive ACK messages from the AC.
• Radar signal detected on a working channel—The AC immediately notifies the AP to change its
working channel.
If the first three conditions are met, the AC selects a new channel. The AP does not use the new channel
until the channel quality difference between the new and old channels exceeds the tolerance level.
Transmit power control
Traditionally, an AP uses the maximum power to cover an area as large as possible. This method,
however, affects the operation of surrounding wireless devices. Transmit power control (TPC) is used to
select a proper transmission power for each AP to satisfy both coverage and usage requirements.
Whether the transmission power of an AP is increased or decreased is determined by these factors: the
maximum number of neighbors (detected neighbors that are managed by the same AC), the neighbor
AP that performs power detection, and the power adjustment threshold.