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Designing a Defense for Mobile Applications

Column It's always been a bit of a mystery to the average corporate PC user: why, if Wi-Fi runs at nearly 60 megabits per second, can you still plug several Wi-Fi access points into a 100 megabit Ethernet socket and drive it from your 8 megabit broadband ADSL router?

Shouldn't you need several 100 megabit lines - one for each access point? And why does it matter, anyway, since most websites run at about half a megabit?

So this week's announcement of a "pre-n" wireless aimed at the corporate market by Meru Networks may take a bit of explaining to the chief financial officer, or whoever is meant to sign the cheque to upgrade the firm's wireless networking - because, for the first time, that poor CFO is going to be faced with a request for upgrading the entire office Ethernet to GigE.

And that poor CFO is going to say that the sums suddenly don't add up. Meru will be followed by a cascade of other 11n copycats, probably starting with Cisco/Airespace this summer, as corporate suppliers switched from their previous pose of "802.11n can wait till the standard is ratified" to "Oh, crap, our rivals are shipping - get something out the door! FAST!" - but even then, an awful lot of people are doomed to disappointment.

And the main reason they're doomed to disappointment is the proliferation of Wi-Fi mobile phones. It's all very well saying how fast Wi-Fi can run. The question is how fast it will run in reality - once pulled off the enthusiastic pages of Aruba and Trapeze and Meru's websites, and stuck in offices with concrete walls and multiple floors and neighbours on either side, all with their own wireless. And, with an increasing number of things like the Nokia E61.

OK, get your calculator out, and watch me get lost in hundreds, tens, and units, because this is dead hard sums. First question: how fast does a Wi-Fi phone run?

The official answer is that Wi-Fi phones run at 11 megabits per second, and that's a lie for a start.

Wi-Fi phones use the oldest Wi-Fi standard still on the market: 802.11b and "b" has a theoretical maximum speed of 11 megabits. Or, if you actually plug it in and turn it on, roughly one megabit per second, if you're the only user, and right next to the access point. That's because the actual throughput in the standard is the radio data modulation speed divided by a complex figure - a figure which gets bigger if there is more than one access point in range, there is more than one client device attached to the access point, or the client device is further than 10 feet away from the point.

The thing about Wi-Fi today is that most computers don't use 11b. They have 802.11g radios in them, and most access point wirelesses are also 802.11g. Theoretically, this combination will run twice as fast, at 22 megabits, as 11b. That is also a lie: again, because in both cases that's the raw modulation rate, while in reality you're looking at everybody sharing a cargo-carrying rate of around 10 megabits. It's a lot quicker than 11g, but it really isn't 22 megabits.

That is, it would be a lot faster - until some idiot walks into the room with a b device. At which point, the AP has to slow down. It has to build in a delay protocol before sending a packet, and that delay was a trivial one in the first 11b spec, which ran at two megabits. But it's an awfully long delay if you're running at 22 megabits modulation.

Several Wi-Fi suppliers have found ways to dodge this problem, and under ideal conditions you can appear to show 11g devices running at full speed even with 11b wireless in the room.

But in a busy environment, the dodges quickly run out of air. I've played with some wireless systems which claim 90 megabits per second for their 11g radios. This isn't a comparative review, so I'll summarise: if the shop won't take them back, use the recycle bin. Even under ideal conditions they tend not to run at more than 40 megabits, they tend not to work with rival hardware, they tend to blot out wireless reception for everybody else in the neighbourhood, and they tend to simply ignore the presence of 11b devices anyway. Also, they tend to be unreviewable crap.

All this changes, more or less, with the 11n standard - using a new MIMO (multi input, multi output) wireless system; and when the IEEE finally ratifies 802.11n in 2009, it will include MIMO in the standard. That's the basic system now appearing in the open market, with the possibility of minor changes in the IEEE committees, all probably fixable with a software update.

And, for the first time this technology - coupled with channel bonding - means it is really, not just theoretically, possible to get more traffic through a Wi-Fi access point than through a 100 megabit Ethernet switch. It will really be the case that an 11n access point will handle more users at a higher rate than an Ethernet switch rated at 100 megabits.

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