Fun Fact 1:

Signals don’t actually interfere with each other

If you have 2 signals on the same frequency and they hit each other they don’t actually interfere with each other. The “Interference” that is referred to is actually the result of a receiving radio hearing multiple signals at the same time which of course can confuse it, but is not the result of the signals effecting each other. A great example is point to point links where you can have 2 links crossing each other, but not causing problems as although they cross paths, the receiving radios will not receive both signals.

Fun Fact 2:

If 2 signals are received at once, they cause a collision

This is not always the case. If there is enough of a Signal Strength difference than the lower strength signal can be viewed as noise by some vendors so that it doesn’t effect the hight strength signal. This is no different to designing a network and inspecting the background noise and allowing for a reasonable SNR (Signal to noise ratio).

If you’ve been keeping up to date with our blogs on wi-fi interference, you could be forgiven for thinking we are making a bit of a fuss and racket about not a lot, and that simply banishing the office microwave and ditching Bluetooth headsets would solve the problem. You would be wrong – there are many wireless products that operate within the unlicensed band and are likely to be within your network. These include watches, shoes, MP3 payers and countless other minute consumer devices and products. Your security solution, whilst working very well at keeping out burglers, could well be causing network jamming levels of interference.

The level of interference caused by some of these devices can be estimated by assessing the following:

1. The output power – the greater this is, the larger zone of interference the device will create.

2. The time signals are emitted for – some devices constantly output a signal (older cordless telephones), others send intermittent signals (new digital cordless telephones). Unsurprisingly, the more time a device emits a signal, the greater the interference.

3. The signal frequency – some devices operate on one frequency and will affect specific channels, others hop frequency and will impact all channels a little bit, and finally some sweep across the frequency spectrum and will cause serious interruptions on many frequencies.

In a recent study, and analogue telephone and a video camera, placed 25 feet away from the AP caused the network to collapse completely… definitely a problem!

It is tempting to ignore interference, and many network users begin accept a sluggish network. But this will have a long term impact on the productivity of your business, so it is worth assessing how often wifi interference occurs, and its impact. Several reports from technicians working for a major wifi infrastructure companies showed that interference from around 20 devices in a business were causing 50% of the problems their network was experiencing. In a residential environment, it is reported that 67% of problems were caused by interference. So it can be concluded that interference is an issue that needs addressing.

Understanding the radio frequencies that wifi networks run on is key to managing interference, and this may take a little patience – but help is at hand, if you need it. Tools are available that will classify the sources in interference on your network and make RF easier to understand. Once the sources have been identified, they can be removed from the network.

How to remove (or move) the offending source of the interference is key – you could replace a microwave or cordless headset with different models that operate outside the radio frequency band. If the source of the interference is an integral to the business as the wifi network, you could try moving the affected access point or changing its operating channel so that it is different to the interfering device and therefore unaffected. Finally, you can try to shield the problem, so that the equipment that causes the RF interference is isolated to a particular room, well away from access points, or shielded with foils or insulating foams in the walls.

So what are the best ways to analyse and protect your wireless network from inference – the aim being to keep it in tip top shape, and performing to the best of its abilities.

Some businesses, keen to limit interference on their network, have brought in wireless policies to tackle interference caused by employees or visitors bringing wireless devices into the building. This is a good idea, and will have a positive impact on the level of interference suffered, but does not tackle problems caused by devices found in the office already – some of which might be vital to the business operation (i.e. those cordless telephones and the lunchtime microwave!).

So what about packet sniffers? You can find free software with a quick search online, which will interface with an 802.11 client card and passively capture (‘sniff’) 802.11 transmissions within your wireless WLAN. They will assess and feedback data on secondary indicators of interference, such as increased retransmission and lower data rates, but they won’t help you determine the cause of the inference or tell you where it is located. Given that in order to get rid of interference you would hope to identify the source of the problem and then decide if you want to remove it, this is a major problem and renders packet sniffers slightly impotent.

So what next for your limping, interference addled network? Could you move it to run at 5GHz, as most devices run on 2.4GHz, thus removing most of the problem without having to actually remove the devices causing it? This might bring short-term gain and some improvement, but more and more devices, including some of those omnipresent cordless telephones, are operating at 5GHz, and many more devices will follow. Then you will be back to square one, still hamstrung by interference, and still trying to address the problem.

Last time we covered how your network might be affect by interference even if it appears to be working well, and what can cause this interference – this time around we are starting to look at how you can assess your problem, and how to deal with it.

It is prudent to sweep your network for wi-fi interference, and one can be forgiven for thinking a walk around your business taking measurements of your network’s capacity would be sufficient. We would recommend a little more care is taken, and that an initial sweep should be carried out for an extended time, and repeated at different times of the day on different days. This is because the interference will be intermittent, due to the varying causes already discussed.  Even periodic sweeping will not guarantee an interference free network (but should help you to assess how much of a problem you may have), so what else can be done?

Some newer to market switch-based WLAN infrastructure products have interference management technology that detects the presence of non-802.11 signals, and changes the channel of the access points in the area. This is helpful, but will not solve problems with broadband based devices such as cordless phones and Bluetooth devices. So the problem will still need addressing…

There is a school of thought that suggests if you stick an access point in each room of your network, you can overcome interference simply because the network is spread out. Sadly this is not the case, as the access points may begin to cause interference to each other… somewhat negating the point of having lots of them!

Next time we will look at how a wireless policy in your business might work, packet sniffers and other wi-fi interference geekery – don’t miss it. At some point we promise to tell you what will work to stop interference in your network!