Power mystery solved

A little while ago I posted about some issues seen with some new Aruba APs apparently putting out less power than the older model they replaced. The mystery has now been solved, and it isn’t all that mysterious.

A number of AP125 units were replaced with AP314 and users started complaining of problems. We had two unconnected issues. The main problem was an Aruba OS bug, fixed with a software update, but the power output was also lower than the replaced units.

At the Aruba Atmosphere conference I had an opportunity to chat with a few Aruba folks about what I’d found and there’s a good, if unexpected reason for it.

When a WiFi device has a single radio and antenna, therefore one spatial stream, it’s easy to calculate power levels as being transmitter output, minus cable loss, plus antenna gain. When there are multiple spatial streams you effectively have multiple transmitters in the same AP. With 802.11ac all spatial streams are transmitted through all the antennas at the same time, so you can see that by increasing the number of simultaneous spatial streams on the same antenna, you are potentially increasing the amount of RF energy being radiated.

It’s therefore possible that with everything remaining as it was, an AP configured with a transmit power level at the limit of the regulatory domain could in fact exceed the maximum allowable transmit power as the number of spatial streams increased.

The new APs are indeed putting out a lower power level because the calculation used to determine power output has been changed in order to ensure the regulatory domain power limit is not exceeded.

There are implications to this. Mixing 802.11n and 802.11ac APs in the same ap group will result in a larger cell size for the 11n models, so that’s something to be avoided. Design software such as Ekahau Site Survey will be unaware of this change as currently such software assumes a simple power calculation that doesn’t take into account 802.11ac multiple spatial streams. This could lead to folk designing for one power level but operating on 3dB or so less.

It’s possible some manufacturers might turn a blind eye to this issue and therefore their APs could appear to offer superior coverage… Time will tell if that becomes an issue.

What’s interesting about all this to me is we trust manufacturers to be doing this stuff properly, not least because it’s not really possible for most of us to measure output power levels.

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