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Since testing began on June 25 2007, the Wi-Fi Alliance has been certifying 802.11n draft 2.0 devices for interoperability and backwards compatibility. As of this writing, the number of certified products is 14 and counting1.
Will these devices disrupt legacy networks? Will 802.11n-based video streaming applications operating in the double-width 40 MHz channels squeeze out low data rate applications such as VoIP and remote control?
802.11n coexistence issues and protection mode
The emerging 802.11n specification is more than just a new physical layer (PHY) for 802.11. It is a whole new standard with efficiency improvements at the MAC (medium access control) and PHY.
These improvements bring impressive range and throughput performance to new generation Wi-Fi networks promising streaming video in the home and support for mission critical applications in the office with throughput approaching that of Ethernet.
However, without proper protection protocols, 802.11n devices have the potential of hampering the performance of legacy networks.
To achieve backwards compatibility and coexistence with legacy networks, 802.11n has to pay special attention to the 802.11 collision avoidance protocol. Collision avoidance is central to all 802.11 networks and lets multiple stations share the airlink efficiently.
Without collision avoidance, stations are unable to get on and off the air in an orderly way and may disrupt each other's access to the airlink, drastically reducing aggregate throughput of the network.
For collision avoidance to work, all stations must be able to interpret each other's packets and exchange information on how long each station intends to occupy the medium.
With major changes to the 802.11 PHY and MAC layers, native 802.11n transmissions cannot be interpreted by the legacy 802.11 a/b/g stations and have the potential of disrupting the collision avoidance protocol for these networks. 802.11n transmissions may appear to legacy stations as strong and pervasive interference—packets that legacy stations are unable to interpret.
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Footnotes:
1 801.11n draft 2.0 certified products are listed at http://certifications.wi-fi.org/wbcs_certified_products.php?.
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