In a single channel architecture (SCA) WLAN system, 802.

Client stations would see two different sets of AP capabilities as they roam between APs with the same BSSID.

The added range of 802.11n APs would cause additional co-channel interference.

The additional beacon material in 802.11n beacons would confuse 802.11b/g client stations.

Beacons from 802.11n APs use 40 MHz channels, while beacons from 802.11g APs use 20 MHz channels.