The 40 MHz channels used by HT and VHT radios are essentially two 20 MHz OFDM channels that are bonded together. Each 40 MHz channel consists of a primary and secondary 20 MHz channel. The primary and secondary 20 MHz channels must be adjacent 20 MHz channels in the frequency in which they operate. The primary field indicates the number of the primary channel. With 802.11n 40 MHz channels, a positive or negative offset indicates whether the secondary channel is above or below the primary channel. 802.11ac (VHT) does not reference any channel offsets but instead references the center frequency of the 40 MHz channel. However, WLAN vendors do not specify a center frequency when configuring a 40 MHz channel on an 802.11ac access point. Instead, a 20 MHz channel number is selected and that 20 MHz channel functions as the primary channel. Primary and secondary channels are used together only for data frame transmissions between an 802.11n/ac AP and an 802.11n/ac client. For backward compatibility, all 802.11 management and control frames are transmitted only on the primary channel. Additionally, only the primary channel is used for data transmissions between an 802.11n/ac AP and legacy 802.11a/g clients.
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