At the physical (PHY) layer, 802.11 devices precede each transmission with a sequence of training fields (called a preamble) to let other devices lock onto the signal. Not only is this preamble necessary to receive the data that follows -- it lets other devices defer the medium to avoid causing collisions.
You can use a wireless LAN (WLAN) analyzer to determine whether any 802.11n AP is capable of (and actually operating in) Greenfield (HT) mode. For example, the following figure shows AirMagnet's 802.11n Efficiency analysis of a Cisco/Linksys WRT150N. Here, we can see that Greenfield operation is supported, but that no traffic has been observed using it.
In fact, this particular AP is operating in mixed mode. In mixed mode, new 802.11n devices precede transmissions with the legacy preamble, followed by the HT preamble. Mixed mode enables more peaceful coexistence because nearby 802.11a/g devices can lock onto the old legacy preamble to defer the medium. Although they won't be able to understand the HT preamble and data that follows, they won't transmit over it.
AirMagnet's 802.11n Efficiency/Greenfield Operation help text shown above includes a few examples of throughput degradation caused by operating mixed mode, compared to Greenfield mode. Ultimately, the extent to which old and new WLANs impact each other depends upon their proximity, but Greenfield mode should only be used in locations (and on channels) where no legacy devices are present.
This was first published in April 2009