802.11n introduced channel bonding. An 802.11n station/AP will indicate the support for 40 MHz operation in the HT capabilities Information element. The information element and the particular bit that indicates 40 MHz support is shown below
Fig Courtesy: 802.11 standard
The Wireless Capture below shows the Supported channel width setting
The Access point in the HT operation element indicates the primary channel and the secondary channel offset that is selected. The HT operation element and the relevant information is shown below
FIG COURTESY: 802.11-2012TM Standard
As can be seen the AP advertises the primary channel and whether the secondary channel is above (SCA), Secondary channel is below (SCB) and whether there is no secondary channel (SCN).
The Wireless Capture below shows the primary channel and secondary channel offset settings broadcast in the HT capabilities IE
