However, look at the spectrum of the 40MHz channel (as shown in Figure 16 below). The 40MHz signal
formed by the combination of CH1 and CH5 raises the noise floor of CH11 by 20dB!
As mentioned in the previous section of SNR, the signal quality is not to depended on the absolute signal
strength, but on the difference between the signal strength and the noise floor.
Figure 16: Spectrum of the 40MHz channel
According to the 802.11n specification, the AP and the client must work in the channel bandwidth mode of
20 MHz for the 2.4 GHz band by default. They can switch to the 40 MHz channel bandwidth mode by fulfill
multiple rules. On the other hand, it also regulates the scenario for AP to return back to the 20 MHz.
Following are the rules:
• If an AP1 detects that there is another AP2 in the 40 MHz channel range, regardless of whether the
wireless access point has client connection or transmission activity, it has to fall back to 20MHz
mode;
• If the AP detects a client device that does not support the 40MHz channel (identified by the
management frame flag sent by the client), it will fall back to 20MHz mode.
That is to say, the 40MHz channel bandwidth usage conditions are very demanding, so it is not
recommended for use in enterprise deployments.
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