Most important, least capable

An important principle when working on wireless network design is the most important but least capable device. The best example I’ve come across is a handheld barcode scanner used in a supermarket that has particularly poor RF performance and works only on the 2.4GHz band.

For this device to work well there needs to be what looks like a bad RF design with too many APs on the same channel, likely all running at reasonably high power levels. In fact a network that works well for this scanner might perform poorly for other uses, especially anything requiring a lot of throughput.

But it’s all about the requirements; in this case the requirements of a device that’s operationally important to the site and that is, in the parlance of ham radio, a bit deaf.

Another example I encountered on our campus was a student’s laptop. In this case a modern machine with an 802.11ac (Wi-Fi5) network interface. The student had recently moved into the room and they were experiencing issues with the network.

Checking the RF using a Fluke Netscout Netally Aircheck showed the 5GHz signal strength from the nearest AP was -67dB, right on the design spec money. However this user’s laptop reported a received signal strength (RSSI) of -77dB. My iPhone 6 “Real-World-Test-O-Meter” reported -70dB, so who’s right? The answer is… all three.

As a general rule tablets outperform phones and laptops tend to be the best – it’s all about how much room there is for the antenna – after that it’s about how much money is thrown at clever design. Normally my iPhone 6 is a good real-world, worst-case scenario test because it’s fairly old now and the antenna designs have got better. However in this case we can see the laptop is really quite poor. From the infrastructure side it’s possible to see the strength of the signals received by the AP from the client – APs really ought to have the best RF performance of all – but this can still be a useful indicator. A reasonably safe working assumption is if the AP is reporting a low RSSI from the client, the client is probably picking up even less from the AP.

Because Wi-Fi equipment is not calibrated (nominally identical devices will report different signal strength) whilst it’s fine to say the design is for -67dBm minimum across the service area, the question has to be asked: “As measured by what?

The general rule I’ve come up with for the campus environment I support is to assume the least capable device likely to be seen on our network will ‘hear’ the signal at 10dB lower than the measuring equipment we use, either the Aircheck G2 or Ekahau Sidekick. This isn’t an exact science – I can’t ask all the students’ personal devices to report back their RSSI – more’s the pity.

It just so happens, in this case, it works out perfectly. There was a 10dB difference and resolving that fixed the student’s problems.

Sometimes it will be obvious which your least capable devices are, and which of those are the most important. Sometimes it isn’t and you’ll just discover them along the way. The most important thing to remember is it’s perfectly possible to design a network that performs brilliantly for device A and really badly for device B so if device B matters to you, make sure you calibrate your design parameters accordingly.

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