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CeBIT over-does the WiFi - it falls over!

If anyone ever doubted that wireless data had arrived, they only need to visit the CeBIT exhibition at Hanover. Not only have the show organisers provided WiFi access in each of the thirty plus halls, but many stands have also installed their own WiFi access points. It's probably the highest concentration of wireless networks that have ever been deployed.

Unfortunately it's demonstrating a problem that has often been discussed by wireless theorists, but never properly tested - what happens when too many WiFi networks try to exist in the same area.

I've been working in Hall 2 - one of the IT halls and trying to connect to any of the multiple access points. Instead of finding a good high speed connection the data is barely dribbling through. To try and understand why I wandered round the hall with a Network Analyser - a standard piece of WiFi detection software and discovered 76 different WiFi access points. The effect of this concentration is that they're all interfering, reducing the effective range of most to a few metres at best and giving data rates that barely get into the tens of Kbps.

The problem is that WiFi uses a fixed chunk of spectrum within the 2.4GHz ISM band. Once an access point is set up it has no ability to search for a clearer part of the spectrum. If something else is using that part of the band it just has to keep on trying in the hope that eventually data will get through. And at Cebit "eventually" is becoming the operative word. There's always been a concern that if more than half a dozen different WiFi networks tried to coexist there would be a fall back in data rate. Cebit is demonstrating just how dire that fallback can be. I've been trying all of the wireless connection options I have to compare them and the throughput with WiFi is frequently even worse than the GSM networks.

In contrast Bluetooth has been performing much better. Rather than using a fixed part of the spectrum Bluetooth constantly hops to a different frequency, giving it a good chance of avoiding interference. We've detected around ten Bluetooth access points in the hall, along with several hundred Bluetooth PDAs, PCs, phones and headsets, but they're standing up pretty well.

It is all very variable. The speed of different methods of wireless access are changing throughout the day and depend heavily on where you are. Moving a few metres can have a major effect. I did a number of measurements during the first day and got the following averages in hall 2:

One of the companies exhibiting in Hall 2 is Ekahau (www.ekahau.com). They are a small Finnish company that has developed some neat software to map out WiFi coverage and illustrate the signal to noise level. It's designed for companies planning to install WiFi who want to audit the radio spectrum so that they can plan where to position access points. It is also providing a fascinating insight into exactly how bad the problem is at Cebit.

Amongst other parameters they can plot the levels of signal to noise and interference. Their scans show the following results:

To get a decent connection you really want to see interference below the -80 dBm level. That's represented by the dark blue shaded regions, which don't exist in Hall 2. The bulk of the hall is experiencing interference levels well above -60dBm and that means that the networks crawl. Wherever the map goes pink it's likely to bring you to a complete standstill.

There's an irony here. Two years ago some of the Bluetooth community tried to install a Bluetooth network in advance of the final standard and proven interoperability. The result was a tranche of bad press for Bluetooth when it failed to work. This year WiFi is considered to be mature and mainstream, but its over-enthusiastic deployment is causing an identical user perception. The Cebit organisers are selling wireless access to visitors at €7.50 per hour - they thought it would be a sign of their technical competence - the queue of frustrated customers at the WLAN service centre shows that they didn't anticipate the scale of the interference problem.

A tip for any would be frustrated WLAN users - there's one area which seems to have good WiFi with no interference - the showground church. And you don't even have to pray for a signal.