Clearing Up Optional and Required Features of IEEE 802.11ac

ieee440SQUARE176788532Let’s start by making something very clear. Wave 1 and wave 2 terminology is more from the Wi-Fi Alliance than from the IEEE. This seems to have caused confusion on what is an optional feature for 802.11ac and what is required.

So far as the IEEE standard goes, it is done.

802.11ac is simply a standard and is complete within itself. Wave 1 and wave 2 came from the Wi-Fi Alliance as they began certifying equipment to specifications based on what older chipsets support and will later certify wireless equipment on what those chipsets will support.

Let it be stated clearly: There is not a wave 1 or wave 2 in the standard.
The standard defines optional and required features for 802.11ac, and the Wi-Fi Alliance defines requirements for their certifications.

Most vendors want to have Wi-Fi Alliance -certified hardware, so the Wi-Fi Alliance specifications are important, but in this post, I want to cover that which is optional and required in the standard. This is what we will test as exams incorporate 802.11ac (July 1st, the new CWNA exam will be the first to include questions on the new 802.11ac VHT PHY).

NOTE: The Wi-Fi Alliance influences us from the perspective of practicality when writing exam questions. Their certifications impact what we commonly see in real-world implementations. For this reason, we tend to test on knowledge related to the features included in the Wi-Fi Alliance certifications simply because those features are more likely to be seen in the field.

Section 4.3.10a Very High Throughput (VHT) STA of the 802.11ac amendment defines the new optional and required features of 802.11 MAC and VHT PHY.

Mandatory (required) PHY features:

  • Support for 40 and 80 MHz channels
  • Support for VHT single-user (SU) PPDUs (or, more simply, support for single user MIMO using VHT)

Mandatory (required) MAC features:

  • Support for A-MPDU padding with VHT PPDUs
  • Support for VHT single MPDU
  • Support for responding to bandwidth indications in non-HT and non-HT duplicate RTS frames

From these two lists, we see that some features that were rumored in 802.11ac are simply not required. Examples include 150 MHz channels and multi-user (MU)-MIMO.

The following features are all considered optional:

  • Support for 160 MHz channels and 80+80 channels
  • Support for beamforming
  • Support for MU-MIMO
  • Support for VHT-MCSs of 8 and 9
  • Support for MPDUs of up to 11,454 octets
  • Support for A-MPDU pre-EOF padding
  • Support for VHT link adaptation

Now just because a feature is optional does not mean that it will not be implemented. In fact some are already looking at beamforming and support for 160 MHz channels; however, we must act as if many organizations will not use the optional features, particularly 160 MHz channels. There is, after all, only so much frequency space.

Beamforming, MU-MIMO and 160 MHz channels are all optional in the standard. Even as wave 2 (remember, this is a Wi-Fi Alliance and chipset vendor term) chips arrive, it’s likely to see wireless networks implemented that do not support these options through configuration even though the chips may support it. Long term, this is more likely to be the case in the consumer device market.

I highly suggest that you skim the standard, even if you don’t read all of it. A cursory view of the 802.11ac standard will reveal important facts like those discussed here.

What have you seen in the 802.11ac standard that has surprised you? Comment below!

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