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Showcase WLAN humbles HP, Proxim and MS at TechEd

Things happen when you build a really, really big wireless LAN. Especially when you have 70 access points in one small area, used by 7,000 delegates, with nearly 4,000 of them online at any one time. And what happened, this week in Barcelona, is that the showpiece wireless network fell over, and died.

As of this moment, the problem is still not fixed - despite an all-night blitz on the suspected issues by the team. And they've discovered several things which they didn't know before, said Andrew Cheeseman, Microsoft's network systems boss for the UK.

For example, it turns out that there is a packet - nobody will reveal what - which when transmitted by a notebook PC over WiFi, will reliably crash a Proxim access point. "Naturally, there was one rogue device, among the 4,000 odd being used here, which kept generating this packet, and then the local access point would close down, and re-start, dumping all the local users off the network."

Another example of something they discovered, is: it turns out that there is a critical number of client devices, beyond which it becomes absurd to expect them to hear each other over the background wireless noise. And, it turns out, the solutions can be really hard to find.

The network at TechEd, Microsoft's big training seminar for software developers, is a whopper. Cheeseman set up a huge server farm in London, a 75 megabit fat pipe from London to Catalonia, a 30 megabit wired LAN around the conference halls, and 70-odd access points to the wireless network, with another 30 megabits of capacity..

He also innovated by having not just standard WiFi at 11 megabits over 802.11b - he installed the new 54 megabit standard, 11g - and the 5 GHz band 11a, also at 54 megabits.

Those delegates who have used the wired network have had no trouble. Acres of desk space has been allocated for rows and rows of Compaq-brand PCs and tablets, and they have all been fine.

But delegates have been posting rude messages on the seminar bulletin board about wireless performance - and it seems that the full solution still eludes the network manager. This morning, although the theory given last night was that "all we have to do is increase the number of access points" and despite a massive all-night re-install session by Microsoft and Proxim and HP staff, the WLAN seems still to be playing up.

Theories which have been thrown out include: co-existence problems between 11b and 11g (sharing the same 2.4 GHz frequency, with different protocols) or alternatively, that the wireless cell sizes were too large; or even that the problem was caused by background wireless noise from the sheer number of devices. Also exonerated: the power supplies of the Proxim access points.

One prime candidate for the role of scapegoat was the exhibition area, where Cheeseman's staff found dozens of pirate access points being run, unauthorised, by exhibitors. But this morning, with most of the exhibitor APs being 11b standard, it was discovered that even 11a based clients were still experiencing the same problem - long delays of up to half a minute, when connecting to the internet.

Blocking the rogue device's network card MAC address has stopped the access points rebooting. "Now we're working on the performance problem," said Cheeseman. "This has been a really important project," he added, "and we're getting loads and loads of new data about high density networks which simply wasn't available before, and there will be important dissertations done at the end of this."

If the problem does turn out to be related to the background noise generated by so many wireless clients, then future technologies may be able to mitigate it. A technology called transmit power control, which will be available on the next iteration of 802.11a, reduces transmit power when talking to close wirelesses. "We would expect the problem to go away, if we had that technology," said Cheeseman. But there are no plans in the IEEE to revise the 11b standard to include this feature.