Greenfield mode is an operational mode of an 802.11n network that can maximize the speed of data transfers. The performance boost of Greenfield mode comes with some significant costs in any environment that includes pre-802.11n client radios.
As an overview, the 802.11n standard provides three operational modes:
- With Legacy mode, every 802.11n access point (AP) acts like a pre-802.11n (legacy) AP, with no 802.11n enhancements enabled and no associated performance increase. The only benefit of Legacy mode is that it allows all legacy devices to communicate with the AP.
- Mixed mode enables the entire 802.11 spectrum to function in harmony. Legacy devices can successfully communicate in Mixed mode, and most performance benefits of 802.11n are implemented for N-enabled devices in the same network. Because Mixed mode allows backward compatibility, the small performance decrease it experiences is well worth the trade-off.
- With Greenfield mode, an 802.11n AP can communicate only with 802.11n devices. All pre-802.11n communications are perceived as noise. While Greenfield mode does optimize the performance of 802.11n devices, its inability to play nicely with legacy devices is a major downfall to this mode.
When operating on the same channel as 802.11n radios in Greenfield Mode, a pre-802.11n radio cannot decode the 802.11n transmissions and receives no specific indication that they are 802.11 transmissions. As a result, the pre-802.11n (or legacy) radio treats the 802.11n transmissions as noise. When the legacy radio transmits, it effectively interrupts and “shouts over” the 802.11n traffic on that channel. 802.11n devices in Greenfield Mode can have the same effect on nearby legacy networks.
Potentially chaotic results include un-received transmissions, continuing retries, and increasing interference. This is especially true for large or established networks that have accrued many devices that require backwards compatibility. Another disadvantage of Greenfield mode is the toll it takes on transmit power and throughput, effectively shortening radio waves and the overall distance in which a transmission can travel.
According to Matthew Gast in this book, 802.11n: A Survival Guide, “I have yet to come across a network that requires the marginally higher speed that Greenfield mode provides but can nevertheless be restricted only to 802.11n devices. Furthermore, a Greenfield-mode network has the potential to destabilize overlapping networks that provide service to 802.11a/b/g devices.”
Gast also notes a security issue with Greenfield mode. Because networks in Greenfield mode are uncommon, many network analysis tools (like sniffers) and rogue AP analysis systems don’t detect these networks.
Due to the significant issues of Greenfield mode, it is not supported by Summit radios. Instead, Laird Technologies recommends a Multiple-In, Multiple-Out (MIMO) antenna solution to achieve the additional throughput, as well as implementation of Mixed mode to reduce transmission collisions from legacy devices.
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