Wi-Fi 6 (802.11ax) Technical Guide

Wi-Fi 6 or 802.11ax?

With the coming of 802.11ax, the Wi-Fi Alliance has introduced a Wi-Fi generation numbering scheme to simplify how the industry differentiates Wi-Fi technologies and increase user awareness of the features available with different standards. Rather than memorizing each IEEE 802.11 amendment, each standard may be referred to as "Wi-Fi," followed by a generation number. 

Wi-Fi 6 and 802.11ax represent the newest wireless standard, however the terms are not interchangeable. Not every feature available in the IEEE 802.11.ax standard is required for the Wi-Fi Alliance's Wi-Fi 6 certification.

 

Wi-Fi Generation	IEEE Standard	Year Ratified
Wi-Fi 4	802.11n	2009
Wi-Fi 5	802.11ac	2014
Wi-Fi 6	802.11ax	2021

802.11ax Feature Overview

The following table compares 802.11ax to the previous two standards. These features and their abbreviations will be further explored later in this document.

 

Capabilities	802.11n	802.11ac	802.11ax
Physical Layer (PHY)	High Throughput (HT)	Very High Throughput (VHT)	High-Efficiency Wireless (HEW)
Operating Bands	2.4 and 5 GHz	5 GHz only	2.4 and 5 GHz
MU-MIMO	N/A	DL MU-MIMO only	DL and UL MU-MIMO
Channel Width	20, 40, 80 MHz	20, 40, 80, 80+80, 160 MHz	20, 40, 80, 80+80, 160 MHz
Guard Interval	800/400 ns	800/400 ns	800/1600/3200 ns
Spread Spectrum Technology	OFDM	OFDM	OFDM, OFDMA
Frequency Modulation	64 QAM	256 QAM	1024 QAM with MCS 10, 11
Power Save	STBC, U-APSD	STBC, U-APSD	STBC, U-APSD, TWT
Spectral Efficiency	N/A	N/A	BSS Coloring

 

Cisco Meraki 802.11ax Access Points

Orthogonal frequency division multiple access (OFDMA)

Wi-Fi 6 uses a combination of two modulation methods: orthogonal frequency division multiplexing (OFDM) and orthogonal frequency division multiple access (OFDMA). 

OFDMA is introduced in the 802.11ax standard for data transmissions. While OFDM is used in previous standards, it continues to be used in 802.11ax for management and control frames in order to maintain backward compatibility with legacy devices. 

The sections ahead will examine what makes OFDMA unique from OFDM. As they make comparisons, bear in mind the following equation for calculating the maximum PHY throughput. 

 

Multiple Input, Multiple Output (MIMO)

MIMO technology, introduced in 802.11n, uses multiple antennas to take advantage of multipath, the way a signal takes different paths to reach the antenna. MIMO allows devices to identify the different paths the signals take to the receiver and sends unique streams of data, known as spatial streams, along different paths. Transmission across multiple spatial streams to a single client is known as single-user MIMO (SU-MIMO). Doubling the number of spatial streams effectively doubles the available throughput.

802.11ac expanded MIMO to allow the AP to use different spatial streams to transmit to multiple clients (up to four) simultaneously. This is known as downlink multiuser MIMO (DL-MU-MIMO). 

 

 

MU-MIMO in 802.11ax allows up to eight clients across eight spatial streams and allows for clients to transmit to the AP simultaneously across different spatial streams (uplink, UL-MU-MIMO). 

 

The number of spatial streams a device supports varies by manufacturer and model. Antenna capabilities are typically written in the form transmitters x receivers: spatial streams. For example, the MR55 5 GHz antenna capabilities are 8x8:8. This means it has 8 transmit antennas, 8 receive antennas, and 8 spatial streams.

Client devices do not have to be 802.11ax-capable to take advantage of APs with 8x8:8 antennas. Through techniques like maximal ratio combining (MRC), the AP can improve the strength of the signal it receives from its clients. Stronger signal strength means higher data rates and longer range for APs with 8x8:8 antennas over those with 4x4:4.

Both MU-OFDMA and MU-MIMO allow APs to communicate with multiple clients simultaneously. The difference is that MU-OFDMA uses different frequencies for each client and MU-MIMO reuses the same frequency in different spatial streams. 

For more information on MIMO technologies, check out our article on Meraki MR SU-MIMO, MU-MIMO, and Beamforming.  

Note that early 802.11ax APs, such as the MR45 and 55, do not support UL-MU-MIMO. They support DL-MU-MIMO with up to four clients per band. For more information about specific Meraki access point models, check out our website.

Basic Service Set (BSS) Coloring

Wi-Fi’s collision-avoidance mechanism, carrier-sense multiple access with collision avoidance (CSMA/CA), allows only one device to transmit on a given frequency at a time. Before transmitting, STAs check if there are any other transmissions on the channel. This check is known as a clear channel assessment (CCA). If any signal is heard, the STA backs off and tries again later. 

Consider the following 2.4 GHz channel plan. To provide sufficient coverage for the site, four APs are required. However, there are only three channels available. In this case, channel 1 is reused on APs 1 and 4. 

Note that BSS coloring is not specific to 2.4 GHz. 802.11ax also allows this feature for 5 GHz. 2.4 GHz is only used as an example.

While the APs are positioned to minimize overlap on the same channel (co-channel interference), some signals from AP 4 may still reach clients on AP 1, and vice versa. When this happens, clients in between APs 1 and 4 using channel 1 can hear traffic being transmitted in both coverage cells. Each time they need to transmit data, they must wait for both cells to be clear before transmitting. 

A wireless network broadcast by a particular AP is known as a basic service set (BSS). To help mitigate the effects of co-channel interference, 802.11ax allows for a 6-bit BSS color field in the SIG-A field at the physical layer, as well as within management frames. This allows for up to 63 different BSS color values. If an STA hears traffic being transmitted, it checks the BSS color value. If the color value matches its own, the STA backs off to let the transmission complete. If the color value is different, it must be an overlapping cell; to avoid co-channel interference, the STA may disregard the ongoing transmission and transmit anyway. 

Using BSS coloring in the previous example, AP 4 can use a different color value than AP 1. A client device physically located in between APs 1 and 4, associated to AP 1, disregards transmission it hears from AP 4 so it only needs to wait for AP 1’s BSS to be clear. 

 

 

Target Wake Time (TWT)

TWT is a power-saving technology that allows an AP and client to negotiate when and how long a client can put its wireless radio into power-save mode. The goal is to minimize wireless medium contention and maximize the amount of time client wireless radios spend in power-save (PS) mode based on the client’s traffic needs. This allows the AP to manage wireless contention and potentially coordinate simultaneous transmissions with multiuser technologies as discussed in earlier sections of this document. 

For more information on similar protocols, check out our document on Power-Saving Technologies. 

Modulation and Coding Set (MCS) 10 and 11

MCSs define the possible data rates based on a variety of factors.

1024 QAM is used in MCS 10 and 11. To learn more about MCS values, check out the product overview and specifications.

For more information on modulation, refer to our 802.11 Fundamentals: Modulation document.