The 802.11n standard
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[b]Overview of the 802.11n Standard[/b]
802.11n is the next generation of Wi-Fi technology. While the final specification is expected to be ratified in the second half of 2009, the Wi-Fi Alliance has started certifying interoperability for 802.11n Draft 2.0 standard products this summer. Draft 2.0 laptops are shipping in volume now and are expected to be software upgradeable to the standard when it is finally ratified.
The IEEE 802.11n standard was conceived with the goal of increasing wireless local area network (WLAN) data throughput to a theoretical level of 540Mbps, or 10 times the speed of 802.11g. Built on OFDM technology used in the 802.11g standard, 802.11n achieves its higher throughput, in part, by increasing channel spectrum from 20MHz to 40MHz.
In addition to increasing channel utilization through MAC aggregation techniques, 802.11n also incorporates multiple antennas using a scheme called multiple-input multiple-output antenna technology, or MIMO. MIMO provides spatial diversity and spatial multiplexing for increased range and throughput, respectively. MIMO is achieved by using multiple “antenna chains”. These are discrete antennas and radios. The way they are used is expressed as 2x2, 2x3 and 3x3. A 3x3 system uses 3 antennas on both sides. At the moment, all systems use 2 spatial streams. In the future, this might go up to 3 or 4. The current draft v2 specifies the use of 2 streams. Using another system will result in incompatibilities or in the best case, just the use of two streams.
The main strength of MIMO comes form multiple streams and multiple physical paths. 802.11n benefits thus from reflections where reflections always have been bad in WLAN networks. Unfortunately, the reflections result in very unpredictable coverage. With 11n, an AP may cover a certain area around it followed with an uncovered area to find a perfect covered spot even further. This coverage depends on the actual situation. Closing a door can change the coverage area!
Today, 802.11n offers today speeds up to 300Mbps theoretical bandwidth. The next release might go for 3 or 4 spatial streams and 4x4 which will result in up to 600Mbps. Proxim currently holds the speed record with up to 320Mbps per AP!
The major speed advantage versus the legacy standards is attained by the 40MHz channel. This means that backwards compatibility with legacy 802.11abg clients will kill most of the speed advantage. The coexistence and backwards compatibility that is build into the standard again results in throughput trade offs. The Meru solution handles this best as it is able to separate the clients.
Most solutions require 802.3at power supplies for their 3x3 operation as the AP needs more than the 15.4W specified in the 802.3af clause 33 standard. Proxim is the only solution offering 3x3 operation at af power.
