Can surveys be accurate?

A little while ago I read some fairly barbed comments from someone about the pointlessness and futility of using an Ekahau Sidekick for wireless surveys.

The argument went something along the lines of: because the Sidekick’s Wi-Fi interfaces and antennas are not the same as the devices actually using the network, the reported results are meaningless. The only way to survey realistically is to use the device the network is designed for.

These ideas weren’t really presented as part of a discussion, more a proclamation that anyone carrying out surveys using a Sidekick is producing misleading results. It’s quite the claim but at first glance the logic is hard to argue with, so does this position have any merit?

My immediate reaction, based mainly on my own experience, was “not really”.

It’s true a network that looks good to the Sidekick can be problematic for a client like an iPhone 5 and this is entirely down to the high quality of the Sidekick antennas, especially relative to the design compromised antenna found in a smartphone.

When analysing survey results in Ekahau an offset can be added to compensate for this. Working on a university campus I’ve always used -10dB as this fits with the previously mentioned iPhone – the most common client.

What’s more, because Wi-Fi chipsets are not calibrated there can be significant variation between devices of the same type. Three iPhone 6 handsets will likely give you three different received signal levels.

So how do you know whether the client you’re using to carry out a representative test of the network is good, average or a poorly performing example? You can take multiple devices and take an average, or take the worst performing example and use that… but you still don’t know whether there’s another one that’s even worse.

In other words how do you apply any rigor to your surveying if nothing is accurate and devices vary? But it gets worse.

Take a look at this post by the brilliant WifiNigel. Nigel has demonstrated (with a nice little rig and measurements and everything) just how much the orientation of a device changes the received signal strength.

What Nigel’s work demonstrates is just how important it is to get your device offset right. If the network is design for a voip client, it’s important to test that device while it’s just off vertical, gripped in a hand and held against a, presumably, human ear… not sitting horizontally on a desk at hip level…

Whilst the Sidekick is not calibrated with accuracy any RF lab would find acceptable, they are tuned to a reference point so ought to be more reliable than the network clients.

It is key to know what is a realistic device offset to use and as far as possible that needs to be based on devices in use, not sitting on a desk in a different orientation.

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