IEEE 802.11ac – Optional and Required Features

IEEE 802.11ac – Optional and Required Features

By CWNP On 03/07/2014 - 15 Comments

A summary of the mandatory and optional features of the 802.11ac standard, versus the implementation of the standard in the Wifi Alliance certification of hardware .

Written By: Tom Carpenter, CTO, CWNP

Confusion abounds over the difference between optional and required features in 802.11ac. It appears, this is born out of the wave 1 and wave 2 terminology, which, itself, is more from the Wi-Fi Alliance than the IEEE.

From the perspective of the standard, it is done.

802.11ac simply is what it is as a standard; however, the Wi-Fi Alliance first began certifying equipment to specifications based on what early chipsets would support and they will later certify equipment based on what those chipsets will support. The end result: wave 1 and wave 2.

Let me state it clearly: there is not a wave 1 or a wave 2 in the standard.

The standard defines optional and required features and then the Wi-Fi Alliance defines requirements for their certifications.

Of course, most vendors want the Wi-Fi Alliance to certify their hardware and so their 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 again as exams incorporate 802.11ac in the future (later this year, 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. This is because their certifications impact what we commonly see in real-world implementations. For this reason, we are more likely to test on knowledge related to the features included in Wi-Fi certifications simply because those features are more likely to be seen in the field.

The section of the 802.11ac amendment that defines the optional and required new features of the 802.11 MAC and VHT PHY is 4.3.10a Very high throughput (VHT) STA. It lists the following 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)

It further lists the following mandatory 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 we've heard about in 802.11ac are simply not required. Examples include 150 MHz channels and multi-user (MU)-MIMO.

The following list of features are all 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 these features are optional, we cannot assume they will not be implemented. In fact, we will see beamforming and support for 160 MHz channels in the future; however, we must act as if many organizations will use the optional features, particularly 160 MHz channels. There is, after all, only so much frequency space.

The glaring reality is that beamforming, MU-MIMO and 160 MHz channels are all optional in the standard. Even as wave 2 (remember, this is more of a Wi-Fi Alliance and chipset vendor term than a standard term) chips arrive, we are likely to see some devices implemented that do not support these options through configuration even though the chips may support it. This is more likely to be the case in the consumer device market.

This is why I suggest skimming the standard even if you don't have the patience to read it all. A cursory viewing of the document will reveal important facts like those discussed here.

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

Tagged with: 802.11ac, Wave 1, Wave 2, Wireless, Wifi Alliance

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