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The Many Faces Of 802.11n
Wireless local networking has been a massive hit with consumers and businesses, but the promise of 802.11n, the latest version of the Wi-Fi standard, is clouded with confusion. There are literally dozens of different combinations of the core technologies that make up "wireless n", and many offer performance no better than existing 802.11g networks. Sorting these out is difficult for even the most experienced wireless networking engineer, let alone the average consumer.
802.11n is the fifth major revision to the industry-standard 802.11 wireless Ethernet specification. Like its predecessors, 802.11n maintains backward-compatibility while still moving forward with new features and greater performance.
The core components of 802.11n include wider radio channels, MIMO multi-stream architecture, and tuning of the data frames sent over the network. But most of these new components are optional or scalable, so not all 802.11n connections are equal. In fact, many casual users will see no difference when moving from 802.11g to "wireless n"!
- A wireless client or access point may support a new frame aggregation scheme for MAC Protocol Data Units (MPDUs) to reduce the overhead of data transmission. Some also support aggregation of MAC Service Data Units (MSDUs) and a shorter inter-symbol "guard interval".
- 802.11n devices must be compatible with multi-antenna "MIMO" radio signals, though they do not need to implement multiple antennas or radios. Devices with more than one radio transmitter and receiver may implement multiple "spatial streams" or independent data transmissions. Multiple radios and streams are necessary to deliver maximum throughput, but no current system reaches the 600 MB/s promised in marketing literature.
- 802.11n radios can operate in either the 2.4 GHz channels also used by 802.11b and 802.11g (not to mention Bluetooth and a host of other wireless systems) or the 5 GHz channel previously only used by 802.11a. The use of these high-frequency channels is optional, however, and many 802.11n clients do not support them. Not all "dual-band" devices support both 2.4 and 5 GHz operation simultaneously, though.
- Finally, 802.11n can optionally combine two 20 MHz channels into a single 40 MHz channel with twice the performance. This functions much better in 5 GHz, but is possible in 2.4 as long as a coexistence mechanism is in place for older devices. An optional "green-field mode" eliminates legacy compatibility for the packets transmitted as well.
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