Frequently Asked Questions
10 Things You Should Know About 80211.n
1. Dual-Band
802.11n can operate in the 2.4GHz or 5GHz band. IT buyers should be aware that almost all 802.11n clients based on CardBus or ExpressCard operate only in 2.4Ghz band at present, but several embedded or mini-PCI adapters currently work on either band.
2. 40MHz Channels
802.11n can operate using the standard 20MHz channels or with wide 40MHz channels that confer higher top-end throughput. Draft 2.0 of the specification recommends that 40MHz channels be used only in the 5GHz band, but many implementations will allow customers to manually configure 40MHz channels in the 2.4GHZ band as well.
3. MIMO
MIMO, or multiple input, multiple output) is a key component of 802.11n that uses spatial multiplexing to simultaneously transmit different chains of information at the same time on the same channel, leveraging multipath reflection to make sure everything gets to its destination without interference or dropouts. These simultaneous transmissions are the key ingredient of 802.11n's higher throughput.
4. Antennas
The most common antenna configurations are three-by- three or two-by-three transmit/receive chains, but 802.11n will have many more options over time. Vendors will be able to explore both the low end and high end for the budget conscious and those in need of a lot of bandwidth. Buyers should be aware that fewer transmit chains equals less bandwidth.
5. Power Over Ethernet
The current POE standard, 802.3af, may not supply enough power to access points with three-by-three or greater antenna configurations. A new POE standard 802.3at, is in the making, but in the meantime administrators may need to get creative with the number of antennas, the number of devices per POE switch or some kind of midline, nonstandard injection.
6. Network Drop
With possible top-end wireless throughput in excess of 100M bps, wireless implementers need to look at upgrading the access point's wired network connection to Gigabit Ethernet.
7. Network Aggregation
With each access point's wireless link capable of pushing over 100M bps and a new Gigabit Ethernet infrastructure, the traffic bottleneck may move to the wireless controller. Wireless switches that perform traffic decryption or are otherwise in the middle of the data plane may not be able to handle as many access points as before.
8. Backware-Compatibility
802.11n is designed to be backward-compatible with bot 802.11b/g in the 2.4GHz band and 802.11a in the 5GHz band. The majority of wireless clients on an 802.11n network will be legacy devices, so administrators must continue to plan WLAN development in order to support the devices.
9. WiFi Cell Shape
While the traditional WLAN cell shape is spherical, MIMO and spatial multiplexing will make 802.11n "cells" spiky and less predictable depending on environmental factors.
10. WiFi Alliance
The WiFi Alliance started certifying devices for compliance with Version 2.0 of the 802.11n draft specification during the summer of 2007. To ensure the best levels of compatibility among 802.11n APs and clients, upgrade to firmware and drivers that are WiFi certified for 802.11 Draft 2.0 if available.
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