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How 802.11n wireless APs in Greenfield mode affect nearby networks

What 802.11 access points (APs) operate in Greenfield mode? What affect does Greenfield mode have on neighboring 802.11a/b/g networks?

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Greenfield mode -- the common term for 802.11n high throughput (HT) mode operation -- refers to prefacing each 802.11 transmission with an optimized HT format preamble and nothing more.

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.

Diagnosing wireless conflicts
To learn more about how to diagnose potential wireless conflicts, see Lisa Phifer's WLAN analysis primer and her review of AirMagnet's OQO Analyzer/Survey bundle.
802.11n defines a new HT format preamble that occupies less air time to reduce per-transmission overhead. However, older devices don't understand the HT format preamble -- they only understand the longer legacy format preamble used by 802.11a/g. When those old and new devices operate in close proximity, transmissions sent by new devices operating in HT mode would collide with those sent by old devices listening for legacy preambles. Collisions would generate errors and retransmissions, resulting in degraded throughput for everyone operating on the same shared medium (channel).

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.


Click image to enlarge.

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

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