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Part 1: What Nobody's Telling You About 256-QAM

In part 1 of this 3-part blog series, we strip away the hype and expose hidden details that WLAN vendors gloss over – or perhaps leave out. Our aim is to tell you what to expect from 802.11ac in the real world and how best to migrate to it as your device population shifts to 802.11ac.

Part 2: What Nobody is Telling You About 80MHz and 160MHz Channel Bonding

Part 3: What Nobody is Telling You about Wave 2

First, let’s talk about what 256-QAM (quadrature amplitude modulation) is and why it’s important. 802.11n’s top encoding scheme (modulation) i.e. how data is represented by a waveform going through the air, is called 64-QAM. 802.11ac employs a new scheme, 256-QAM. It packs 33% more data into the signal than 64-QAM.

The significance of 256-QAM is the availability of two new data rates, MCS 8 and MCS 9. These achieve higher throughput than the previously highest data rate of MCS 7, in the same amount of airtime. Needless to say, all big number claims for 802.11ac presume 256-QAM is being used.



Because 256-QAM packs more information into a waveform, you might naturally expect it requires more sensitive equipment to detect the different phases and amplitudes. It does. Therefore...

  • 256-QAM is more sensitive to disruption by noise in the environment.
  • This means you need a cleaner signal between the client and the AP.
  • In practice, you need line of sight access between AP and client from a max range of about 20 feet to get the benefits of 256-QAM.

In real-world settings, rate control will choose to use 256-QAM only in close proximity of the AP, like 10-20 feet, otherwise it will drop down to the next lower data rate namely MCS 7 using 64-QAM, no different from 802.11n. 

What about beamforming? Some vendors claim you can go much further when beamforming is employed. This isn't definite, though, so be leary. Beamforming can only extend 256-QAM range in open spaces with clean air and with direct line of sight. This implies larger cells, which you don’t want anyway. As well, there are other issues with beamforming and 802.11ac, which will get covered in Part 3 of this series.

At high data rates 11ac does not penetrate walls any better than 802.11n, so you are not gaining anything in improved coverage, except in open spaces. In fact, in traditional office environments, distance using 256-QAM is shorter than it is for 802.11n’s top data rate at 5GHz and significantly less than 802.11n at 2.4 GHz. Add 80 MHz channel bonding, and it only gets worse (more on this in Part 2). AP power level drops, which means coverage area shrinks dramatically, and receive sensitivity is also reduced.

In buildings with heavy concrete walls or a lot of iron and steel, range is even worse and 256-QAM is virtually unusable beyond 15 feet! This is all due to higher signal-to-noise ratios required by these new data rates. When 256-QAM is not viable, 802.11ac radios automatically downshift to 64-QAM, matching what you already have with 802.11n. In this case, the bottom line is - no benefit.

Aruba Recieve Sensitivity by Encoding Scheme

The technical explanation for this relates to receiving sensitivity. While the top data rate for a single stream of 802.11n on a 20 MHz channel only requires a receive sensitivity of -64 dBm, 802.11ac’s top data rate using 256-QAM requires a receive sensitivity of -59 dBm.  This means clients must use more Tx power to be heard by the 802.11ac radio in order to get that top data rate, all other things being equal. In practice, since clients don’t adjust their power, the client must therefore be closer to the AP. Hence 256-QAM is only attainable at relatively short range.

So what does all this mean for 802.11ac AP placement? To get the benefits of 256-QAM, APs should be placed in open areas, where they can literally be seen by users. Cubicle office areas, hotel lobby areas and outdoors are the best use cases. While the worst use cases are warehouses and buildings with heavy concrete or steel construction.

Of course, only 802.11ac compatible clients, such as the iPhone 6, can utilize the 256-QAM encoding scheme when it is viable. It is of no value to 802.11n and other legacy clients except indirectly by relieving the contention for airtime.

We’re done quite yet, so keep reading for more permitent information. In part 2 of this series, we’ll discuss the value of 80 MHz and 160 MHz bonded channels and where you can practically use them.

802.11ac real world best practices aren’t always easy to understand, but we’re here to help. If you want to jump ahead and read the entire 802.11ac Migration: Real World Best Practices guide now, then click below. You’re one click away from industry leading expert advice!

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