Sharing is good...

But it helps if you understand a bit about the technology so you can be careful just how much you are sharing.

This article looks at the technical side of WiFi which might help you recognise possible technical problems and then move on to looks at some security issues.

Sharing the spectrum

Part of the frequency spectrum has been allocated by international treaty to the industrial, science and medical community. However one important device from the industrial/domestic community had already occupied a significant part of the spectrum required by the newcomer, WiFi.

Microwave ovens are forced to use the 2.45 GHz frequency. They do not have any choice as there is just one, narrow frequency where the energy is absorbed by the water molecules inside the food just as our skin is a good detector and absorber of infra-red energy from another part of the spectrum. 

And it is the capture of this microwave energy which causes food (or rather the water in the food) to heat up. In an ideal world all micro energy would stay within the oven but in the supersensitive world of WiFi where you're dealing with power levels that would not warm a cold nat, the possibility of interference with microwave ovens cannot be ruled out.

How it works

Information is added to the very high frequency signal using frequency modulation. This is best explained by thinking about the ubiquitous barcode. In a barcode you see that the lines of greater or lesser thickness been printed and these can be scanned and converted back to the numbers. So it is the spacing or thickness of the lines that encodes the numbers. Most barcodes convey only 13 digits.

If you expand the concept and look at a barcode with 20 million lines, you begin to get some idea of how, by altering the spacing of the lines, a prodigious quantity of information can be included within that 'electromagnetic barcode'. The amount by which the frequency can be altered, or modulated, is known as the bandwidth and this equates in our model to the number of lines on our electronic barcode.

WiFi works by sending out the little packets of information - If these are received and understood they are acknowledged. If they are not understood, or get lost in the noise, the packet is sent again. Each packet is repeated until it is acknowledged. In a quiet, electronic environment, nice long packets can be sent and received but if the spectrum is busy and noisy the packets are kept short. This leads to a slowing down of a network but it does mean that even if a large number of people by sharing the same channel the system can nevertheless keep the data flowing to everybody, even if the speed is temorarily reduced.

With WiFi needing a bandwidth of 20 MHz to transmit data effectively, WiFi can offer 13 theoretical ‘channels’, each 5 MHz wide, starting at 2.4 MHz, (the microwave oven frequency).

Some simple maths will show that there is a big problem: 13 channels are only 5 MHz apart but the channels on WiFi equipment need 20 MHz to carry enough data. So there is clearly going to be overlap between the various channels. Overlap is not a disaster – it is rather like being in a noisy room – it is still possible to hear those nearest to you if you tune into, and concentrate on, their voice.

WiFi setup

In practice only three channels are commonly used: Channel 1, Channel 6 and Channel 11 since this allows reasonable spacing and reduces the chance of interference. In the US Channel 12 and Channel 13 can also be used and Japan has Channel 4. These are not available outside those areas so when you come to set up your WiFi, the country you are using it in will be one of the first questions asked. But if you find yourself in one of those rare places where you are in conflict with other signals then tweaking the channels might work for you.

Be thankful that this is handled for you.

The streaming problem

With the latest version of WiFi (known as WMM), people who are streaming multi-media files, such as films, have priority and your 'ordinary' data will have to wait for any gaps left in the entertainment train of data. This is designed to avoid the freeze-frame that occurs if there is no new image to display.

The streaming system is designed to degrade slowly so that as more people use the channels there is not a sudden and dramatic drop off. If you have the bandwidth all to yourself, the throughput you can achieve is impressive. So it is worth thinking about the time when you plan to do your data file shifting if you are working on WiFi. If your system is shared by those who like to watch movies then be aware that they will increasingly be given priority over your data.

WiFi Security

Sharing, as we said, is good provided you know who you are sharing with. One of the joys of WiFi is that you can access your e-mail while you're sitting in a cafe or on a journey, miles away from home.

Modern operating systems are keen to allow you to access available resources so WiFi detection is built in. It allows you to notice any nearby WiFi systems. There is no problem with this provided all that people can see is that your network exists - You can walk through a crowd with your bag and your wallet tightly closed. The problem comes if you make it possible for people to dip into your handbag or take your wallet.

Most WiFi systems provide access control and it is very unwise, indeed foolhardy, to provide your WiFi as an open system that anybody can use. WEP remains the baseline security but the WiFi Alliance does not consider WEP to be a secure solution. Products running in WPA2 [WiFi Protected Access 2™] mode but it cannot support WEP devices at the same time.

The encryption keys used for each client on the network is unique and specific to that connected client. Every packet sent over the air is encrypted with a unique key. The ability to avoid key reuse, and provide unique, fresh encryption keys is a basic design feature of any good security practice and both WPA (and WPA2) offer such good security.

If you are asked to select a password be warned: One security agency recently revealed that the hackers have 100 GB password file which, if they are allowed enough time to launch an attack, they can work their way through the list in an attempt to gain access. People must think a lot more seriously about passwords and making these as difficult as possible to crack. The answer is to use an utterly random, long and strong password.

Link to the WiFi organisation.

Hz is an abbreviation of Hertz and is itself the unit in which frequency is measured. Electromagnetic energy is a wave and the frequency determines the properties of the wave. So visible light is stopped by solids (apart from a strange material we know as glass) while most can pass through walls.

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