Edging closer to superfast Wi-Fi


Voting among the engineers charged with developing the IEEE’s 802.11n Wi-Fi standard ended on Tuesday, with the draft proposal for this standard receiving a unanimous 100-0 vote).

For those who don’t follow the ins and outs of wireless networking, the 802.11n standard promises theoretical maximum speeds of up to 300 megabits per second, compared to 54 mbps for its predecessor, 802.11g, and 11 mbps for the original 802.11b standard.

Of course, real-world throughput is invariably a lot slower, especially if great distances and data encryption are involved. In our tests last fall of the first Wi-Fi gear based on the initial draft of the 802.11n standard, though, we saw speeds approaching those of ethernet connections at close range.

If, as widely expected, the IEEE 802.11n committee does approve the latest version (submitted as draft 1.10) as draft 2, it will be good news for people who had either already invested in Wi-fi products based on the initial draft of 802.11n or have been holding off on buying draft-n products in anticipation of more stable and compatible products.

While final publication of the completed standard is still a long way off (thank complicated IEEE procedures for that), vendors say the new draft finally settles issues that have made for iffy interoperability and significant performance differences between products based on the original draft-n chips from different vendors.

More importantly, major Wi-Fi chip vendors say the first draft-n products, which began shipping last spring, will be firmware-upgradable to compliance with draft 2. This means owners of existing Wi-Fi equipment should begin checking the vendor’s Web site for free draft 2 firmware updates, which should begin appearing by early spring.

Last, but not least, agreement on draft 2 puts the Wi-Fi Alliance on track to start compatibility testing for products based on draft 2 in late spring or early summer. The Alliance, the trade group that for years has performed similar testing for products based on various 802.11 specs (including 802.11b, g, and a), is working on a logo program to help Wi-Fi buyers identify which draft-2 802.11n products have been certified interoperable with other draft-2 products.

What’s new in draft 2?

So what’s new with the latest draft, which follows a rocky start for products based on the initial draft? (In our October review of the first 802.11n gear, PC World recommended holding off on purchases because of uncertainty over the upgrade path and unstable performance.)

The ability to upgrade existing draft-n products to draft 2 is obviously huge for those who took a chance on these early routers and PC cards (most vendors did not guarantee such firmware upgradability). Compatibility improvements should also be significant: the new spec settles once and for all something called “Information Elements,” which allows devices based on draft-n chips from different vendors to recognize each other as birds of a feather.

In our initial tests, products from different vendors could only interoperate at 802.11g speeds for which they had been Wi-Fi Alliance certified. Major chip vendors Atheros and Broadcom say they’d been working together on this problem and had achieved 802.11n-speed interoperability for their products subsequent to our testing last summer, but all products (regardless of vendor) that the Wi-Fi Alliance certifies for Draft 2 of 802.11n should work together at top speed.

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.

More room on 5Ghz

McFarland notes that the so-called politeness requirements are not nearly as strict for 802.11n networks that use the 5GHz band (which will be backwards compatible with today’s 802.11a products). Before sending a 40mhz packet on a 5GHz network, the equipment just has to check that neither 20mhz channel is in use by a nearby network. But channel overcrowding is not nearly as big an issue for 5GHz networks as it is for 2.4GHZ ones since the 5GHz band has some 20 non-overlapping 20mhz channels. And legacy 802.11a n

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