Which channel does Vigor Router use in 40MHz bandwidth mode?

IEEE 802.11n supports 40MHz channel bandwidth, which means it uses two 20MHz channels simultaneously to increase throughput. The main channel and the extension channel are always four-channel apart. The main channels is the one Administrators select on the Wireless LAN(2.4GHz) >> General Setup page, and the extension channels used by the wireless Vigor Router are listed below: