H3C Technologies H3C WX3000E Series Wireless Switches - Manual (Page 363)

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Radio configuration
Radio overview
Radio frequency (RF) refers to electrical signals that can be transferred over the space to a long distance.
802.11b/g in the IEEE 802.11 standards operates at the 2.4 GHz band, 802.11a operates at the 5 GHz
band, and 802.11n operates at both the 2.4 GHz and 5 GHz bands. Radio frequency is allocated in
bands, each of which corresponds to a range of frequencies.
WLAN RRM overview
Radio signals are susceptible to surrounding interference. The causes of radio signal attenuation in
different directions are very complex, so you need to make careful plans before deploying a WLAN
network. After WLAN deployment, the running parameters must still 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 ensures the optimal communication quality.
Dynamic frequency selection
A WLAN has limited working channels. Channel overlapping can easily 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 calculates the channel quality. The AP does not use the new
channel until the channel quality difference between the new and old channels exceeds the tolerance
level.