What is the difference between HT20 and HT40

The channel bandwidth in HT20 mode is 20 MHz, and the channel bandwidth in HT40 mode is 40 MHz. Two neighboring 20 MHz channels are bundled to form a 40 MHz channel. One channel functions as the primary channel, and the other as the secondary channel. The primary channel sends Beacon packets and data packets, and the secondary channel sends other packets. When the HT40 mode is used in the 2.4 GHz frequency band, there is only one non-overlapping channel. Therefore, you are not advised to use the HT40 mode in the 2.4 GHz frequency band.
If two neighboring 20 MHz channels are bundled and the center frequency of the auxiliary 20MHz bandwidth is lower than that of the main channel, the value is minus; otherwise, it is a plus. For example, 149plus indicates 40MHz channel is composed of the 20MHz channel with the central frequency of 149 and the 20MHz channel with the center frequency of 153.

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