* 2.4GHz channel-bonding limits will inhibit performance enhancements
Earlier this month, the IEEE 802.11 Working Group approved Draft 2.0 of the 802.11n specification. This means that the spec is not likely to change substantially from now until the standard is ratified, probably in the October 2008 time frame, so compliant products should get under construction and out the door, fairly soon after finalization.
802.11n represents the next generation of Wi-Fi networking that will deliver significantly higher throughput – up to six times in the near term – through the use of multiple-input multiple-output (MIMO) spatial multiplexing technology and signal processing. How much throughput you get depends largely on how many transmitting and receiving antennas are on each end of the communications link. Early systems using two spatial streams and channel bonding techniques, which I’ll discuss in a minute, will likely get a theoretical maximum throughput of 270Mbps to 300Mbps.
You’ve likely heard that 802.11n is required to be backward compatible with existing 802.11a/b/g networks. This means that it will run in both the 2.4GHz band of 802.11b/g and the 5GHz band of 802.11a. The real promise of 802.11n, though, is in the 5GHz band.
5GHz has lots of non-overlapping channels (12 in the U.S.; up to 23 worldwide). Because of this flexibility, channel bonding will be most practical in the 5GHz band. Channel bonding is part of the 802.11n standard, and it allows you to bond two 20MHz channels together for double the aggregate throughput – sort of like the concept of N-by-T1 inverse multiplexing in the WAN discipline.
In the 2.4GHz band, you only have three non-overlapping channels. Generally, you overlap cells of coverage using the three different channels. Each non-overlapping channel in a checkerboard-style layout is far enough from the same channel in the next cell so as not to cause interference. When you bond channels in the 2.4GHz band, you reduce the number of non-overlapping channels from three to two. This means you have to repeat use of the same channel a third more often. This significantly limits your cell design flexibility, which could create interference. So channel bonding won’t be used much in the 2.4GHz band, which inhibits 802.11n’s potential for speed in that band.
