Channel Bonding

"Legacy" 802.11 clients use 20MHz channels. 802.11n specifies the use of 40MHz wide channels to achieve higher data rates. Channel bonding is supported in 5GHz and commonly uses two contiguous 20MHz channels to create a single 40MHz channel. While channel bonding can provide higher data rates, it will also result in fewer 5GHz channels. 

In some high-density client environments with high wireless saturation, having more available 20MHz channels to spread client usage provides better performance than having fewer 40MHz channels. In addition, not all regulatory domains support using a 40-MHz channel.  

A wide 5GHz channel comprises "primary" and "secondary" channels.  The "primary" channel is used for 802.11n clients that only support 20MHz channel bandwidth (e.g., “legacy: clients). Both primary and secondary channels can be used for clients that support wide channel (40MHz) capabilities. 

The location of the primary channel is specified when you manually configure channel selection in 5GHz. The primary channel is either the upper or lower half of the 40MHz channel. For example, if you are using a 40MHz-wide channel in 5GHz and choose channel 36, then channel 36 (lower half) would be the primary channel, and the secondary channel would be channel 40 (upper half). Conversely, if you specified that channel 40 would be selected as the primary (upper half) and the secondary would be channel 36 (lower half).