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Draft 2.0 802.11n moves forward
|16 марта 2007|
The major new elements in draft 2.0 are the ability to upgrade existing draft-n products to draft 2.0, and, as important, compatibility improvements (the new spec thus settles what is called the "information elements" of the standard), as well as the good-neighbor technology. The good-neighbor technology addresses the following problem: 802.11n products which are backward-compatible with 802.11b and g, operate in the 2.4 GHz frequency, which has only three non-overlapping 20 MHz channels. To achieve top speeds, 802.11n products use two 20 MHz channels (what is called channel bonding), meaning that only one other neighboring network can co-exist, and that this network can use only the one remaining 20 MHz channel. Other neighboring networks face being knocked off the airwaves.
Draft 2 is specific on how 802.11n networks must deal with neighbors on other 2.4 GHz networks (that is, 802.11b, g, or other n networks): 802.11n networks must deal with neighbors on other 2.4 GHz networks by reverting to 20 MHz mode if they detect activity on a neighboring 2.4 GHz network (note that this applies to data transfers rather than signals that merely show
that a network is available). Draft 2.0 is respectful not only of older 802.11 systems, but also of Bluetooth, which also operates in the 2.4 GHz range.
Coexistence is good, but it means that there are serious restrictions on a 2.4 GHz 802.11n network: "Generally speaking, they're not going to be able to use 40 MHz on the 2.4 GHz band," says Bill McFarland, a member of the IEEE 802.11n task force and chief technical officer at Atheros.
The good-neighbor policy will not be as onerous for 802.11n networks operating in the 5.0 GHz: the 5 GHz band is not nearly as crowded as the 2.4 GHZ one, and the 5 GHz band has some 20 non-overlapping 20 MHz channels.
After draft 2.0 is announced, it will be submitted to all 600 or so members of the 802.11 working group and group members will have 30 days to vote, with 75 percent approval required. There will be several more rounds of voting, with October 2008 as the target date for final publication of the 802.11n standard.
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