The 802.11n standard is backwardly compatible to previous standards (IEEE 802.11a/b/g). However, some of the advantages of the new technology are only available when, in addition to the access points, the wireless LAN clients are also compatible with 802.11n.
In order to allow the co-existence of wireless LAN clients based on 802.11a/b/g (called "legacy clients") 802.11n access points offer special mechanisms for mixed operation, where performance increases over 802.11a/b/g are not as high. Only in all-802.11n environments is the "greenfield mode" used, which can exploit all the advantages of the new technology. In greenfield mode both access points and wireless LAN clients support the 802.11n Draft, and access points reject connections with legacy clients.