Wi-Fi: The next generation

• Tags:
• Wi-Fi,
• 802.11n

Andrew Donoghue ZDNet.co.uk

Published: 24 Aug 2007 16:59 BST

• 
• 
• 
• 
• 

Despite interminable delays to the forthcoming 802.11n wireless networking standard, there's a consensus it will finally materialise in March 2009. Meantime, it's being touted as a fix-all for Wi-Fi's various shortcomings.

The standard promises to improve range and boost traffic throughput, from the existing 54Mbps (megabits per second) offered by 802.11g-based technology, to between 100Mbps and 300Mbps, depending on a range of factors.

The performance improvement takes place because, while existing 802.11b- and g-based products are only able to use a single 20MHz channel in the unlicensed 2.4GHz radio spectrum, 802.11n products can use either one or two channels; plus, says Mark Main, a senior analyst at Ovum, there is "a bit of space between the channels to increase the available radio spectrum".

"In the 2.4GHz spectrum, there's a maximum of 14 overlapping individual channels in most countries. Currently, any particular radio system can only use one. This has been one of the fundamental limitations to speed in wireless Lan systems, but 802.11n allows the channels to be bonded together. The idea is that, like water, you can get more through two pipes than just one and so this makes throughput quicker," Main explains.

Another advantage of the new standard relates to improvements in traffic handling, as it incorporates new scheduling mechanisms harvested from another 802.11 standard: namely e, which was ratified by the IEEE in late 2005. These mechanisms include the Power Save Multi-poll, which helps improve quality of service for such isochronous (meaning simultaneous) transmissions as voice.

Including support for multiple-input multiple-output (MIMO) technology also provides a benefit. "11n products use MIMO technology to improve reception, which helps both with the ability of systems to overcome interference and improves speed by using multiple antennae, which is like using two ears instead of one," says Main. MIMO effectively creates as many links as there are combinations of receive and transmit antennas, so with three transmission and three reception antennas there are nine separate transmission paths, each capable of reusing the same frequencies as its fellows.

Speeding ahead
But, as ever with these things, the situation is not quite as straightforward as it seems. While the idea is that 802.11n-based products will be able to operate within two radio spectrums — 2.4GHz and 5GHz, the latter of which is currently also used by 802.11a-based offerings, although adoption here is low — most vendors, including Aruba Networks, Broadcom and Intel, have to date focused on designing products for 2.4GHz.

5GHz is a lot cleaner; it has only recently had its licensing restrictions relaxed, so is free of the cacophony of unlicensed equipment such as Bluetooth devices and microwave ovens that have infested 2.4GHz over the years. The problem is that 5GHz has a much shorter range. This means that performance traditionally has not been as good, because its higher-frequency radio waves tend to be absorbed by walls, people and the like more easily.

But in the new 3G world, there is likely to be an issue with the more widely used 2.4GHz spectrum. "Because 802.11n uses multiple radio channels, if it detects other radio systems, it's supposed to back off from being greedy because of the quest to be backwards-compatible. So if you have n systems in the presence of b or g systems, the new one will detect the presence of the old and back off from being a bad neighbour," explains Main.

This means that unless organisations either deploy n-based equipment in a greenfield site or are prepared to replace all of their existing access points with new ones, "it's very probable that 802.11n won't perform up to headline speeds" in the 2.4GHz spectrum.

Real world throughput for existing 802.11 products is currently about half theoretical speeds, with 11g-based technology, for example, generally providing 20 to 22Mbps rather than the promised 54Mbps. But a mix of such kit with 11n-based products is likely to result in performance being hit by as much as a factor of four, as the latter defaults to the lowest common denominator offering in the vicinity.

"All the headline stuff about speed has to be taken with a pinch of salt. If you use b and g now, there'll be little improvement. Just because technology is theoretically capable of going faster doesn't mean to say that the user experience will match that, although it's most likely to happen in an environment where the situation is controlled," Main warns. As a result, he recommends operating equipment in the 5GHz spectrum.

Another area of concern, however, relates to the fact that, although 802.11n provides wider and more robust coverage than previous generations of wireless standards, this coverage is more variable due to...

• 
• 
• 
• 

• Share this article:
• Digg
• Slashdot
• Del.ici.ous
• Stumble
• Reddit