Superfast Wi-Fi Standard Solidifies

Being a Good Neighbor

The other major development in Draft 2 has to do with the controversial issue of implementing good-neighbor technology. Products based on 802.11n that are backwards compatible with 802.11b and g operate in the 2.4GHz frequency, which only has three non-overlapping 20-mhz channels. But to achieve top speeds, 802.11n products use two 20-mhz channels (a practice known as channel-bonding), which essentially means only one other neighboring network can co-exist (and that network can only use the remaining 20mhz channel). Other neighboring networks face being knocked off the airwaves.

This problem first surfaced as vendors began using proprietary channel-bonding technology to speed up 802.11g products, and concern in the Wi-Fi community about the potential of the technology to slow down or shut down neighboring networks was so great that the Wi-Fi Alliance eventually decided to withhold certification from 802.11g products with channel bonding that did not by default revert to 20mhz channels in the presence of neighboring networks.

Since channel-bonding is part of the 802.11n spec, a policy for coexistence with legacy networks was imperativea??and yet the initial 802.11n draft provided no specifics, saying only that provision must be made to deal with them. Not surprisingly, chip vendors each went their own way on this so-called good-neighbor technology issue.

The 802.11n Way

Draft 2, fortunately, is quite specific on how 802.11n networks must deal with neighbors on other 2.4GHz networks (e.g. 802.11b, g, or other n networks). Basically, they have to revert to 20mhz mode if they detect activity (meaning data transfers as opposed to signals that merely show a network is available) on a neighboring 2.4GHz network.

This means that if your neighbor goes on vacation but leaves his Wi-Fi equipment on in his empty home, your network can double its theoretical speed by using a 40mhz channel. But when he return and starts browsing the Web, your network must immediately step down to 20 20mhz when it detects the activity--and it cannot try to resume 40mhz operations for at least 30 minutes.

Older Wi-Fi networks aren't the only show-stoppers for 40mhz operations by 2.4Ghz 802.11n networks. Bluetooth also operates in the 2.4Ghz band, and can suffer in the presence of bandwidth-hogging channel-bonding. So the new version of the 802.11n standard provides for recognition of a "Bluetooth intolerant" bit that developers of Bluetooth devices can put in their products. When 802.11n devices detect that bit, they must also cease 40mhz operations and step down to 20mhz channels.

These strict new rules will place severe restrictions on a 2.4GHz 802.11n network to use channel bonding in cities or suburbs where lots of people now have 802.11b or g networks in homes and offices.

"Generally speaking, they're not going to be able to use 40mhz on the 2.4GHz band," said Bill McFarland, a member of the IEEE 802.11n task force and chief technical officer at Atheros, a major Wi-Fi chip developer.

Eric Deming, an executive at Cisco subsidiary Linksys--an Atheros customer and a major vendor of Wi-Fi equipment--agrees with McFarland's assessment, but says he's not unhappy with the new rules.

"We're extremely happy because there could have been outcomes that would have been worse, like banning 40mhz channels altogether," said Deming, who is senior product marketing manager for Linksys' consumer business organization.