CTS 159: Wi-fi 6 (802.11ax) Overview

I decided to finally get myself a little familiar with 802.11ax. I’m not sure why but I’ve pretty much ignored it until now. In this episode, I’m going to provide my overview of 802.11ax, or Wi-Fi 6. This episode will be the start of a mini series diving into detail of the components of 802.11ax.

802.11ax = High Efficiency (HE) and the marketing term for it is Wi-Fi 6.

Currently in draft, there are no devices yet to support 802.11ax. Laptop and Samsung phone coming this year to support 802.11ax draft.

Wi-Fi Alliance has their certifications coming later in 2019 for 802.11ax, Aerohive is shipping 802.11ax APs, and I predict we will see ratification in early 2020.

Main PHY features in 802.11ax (HE) not in 802.11ac (VHT) and 802.11n (HT)

  • Mandatory support for DL & UL OFDMA
  • Mandatory support for DL MU-MIMO
  • Optional support for HE sounding protocol for beam forming
  • Optional support for UL MU-MIMO

Main MAC features in HE not in previous protocols

  • AP has optional support for two NAV operation
  • Client has mandatory support for two NAV operation
  • Mandatory AP support for TWT
  • Optional client support for TWT
  • Optional support for UL OFDMA-based random access
  • Optional support for spatial reuse operation

What are the general topics I’ll talk about in this episode? Here they are in no special order:

Channel access for 802.11ax.
An HE BSS can use RTS and CTS for transmit opportunity. Clients use RTS and CTS to initiate transmit opportunity.

The Multi-user RTS and CTS lets an AP initiate transmit opportunity. The MU-RTS Trigger frame is used to solicit simultaneous CTS responses from multiple 11ax clients.

MU-RTS and CTS from 802.11ax Draft 3.0

MU Operation
HE allows simultaneous downlink transmissions from AP to client in both DL-OFDMA and DL MU-MIMO.

OFDMA is the biggest enhancement in 802.11ax which creates a multi-user version of OFDM.

It may seem like the same definition as MU-MIMO but it isn’t. OFDMA is multiple access for OFDM. In OFDMA, the channel is subdivided into small channels called resource units or RUs. On each channel can be a different transmission hence multiple access.

11ax allows UL MU operation by letting the AP solicit simultaneous responses from one or more 11ax clients. For an AP to use UL MU operation it must follow EDCA HCF procedure.

OFDMA is not new. It is implemented in LTE technologies. We’re simply using it here for Wi-Fi 🙂

In OFDM, the channel was divided into multiple subcarriers. Specifically it was 64 subcarriers in which 52 carried data, 4 subcarriers for pilot, and 8 subcarriers for guard bands. The width of the subcarriers is 312.5 KHz.

When it comes to OFDMA, the subcarriers are now much smaller, 78.125 KHz! That equates to 256 subcarriers for OFDMA. It will maintain the different types of subcarriers for data, pilot, and guard.

Resource Units
Prior to 802.11ax, AP will transmit or receive across the whole OFDM channel, the entire frequency, for a single client.

In OFDMA, the 256 subcarriers are further divided into resource units (RUs). An 802.11ax AP can determine the allocation of RUs used for a client or multiple clients. Yes, the AP can service multiple clients simultaneously using resource units and various resource unit combinations.

BSS Frame Determination
802.11ax introduces BSS colors to determine if a frame is destined for the same BSS or not. The color itself is really a digit for identification. A client receiving a frame will determine if it is part of the BSS if the BSS Color is the same as the BSS the client is joined to. If the BSS Color is not the same as the client, it is not the same BSS.

BSS Coloring
802.11ax introduces a new way of handling co-channel interference, called BSS Color. We know that if an AP operating on channel 149 hears another AP transmitting on the same channel it must defer. Likewise, if a client transmitting on channel 157, any other client or AP operating on that channel and hears that client’s transmission they must defer.

What BSS Color is identifies a BSS with a number. The BSS Color is in the 802.11ax preamble. The color information can be seen in the HE information element subfield for BSS coloring.

How does it work? If a client detects a frame that is the same BSS color as its own, t is part of the same BSS. If the frame is a different BSS color than the client then it is from another BSS. If it is from a different BSS then the frame is ignored and the client or AP can transmit at the same time.

Target Wake Time
802.11ax is to introduce a new power-saving mechanisms by scheduling target wake times for clients in power save mode. The goal of TWT is to optimize how often a client needs to wake up to determine if it has data and keeps the client asleep longer.

The TWT capability is broadcasted in the HE Capabilities element. An HE client will inherit the TWT values from the TWT element advertised by the BSSID and will follow the TWT schedule.

The AP can control when clients contend for air time by scheduling when clients can wake up for transmission. The TWT can be negotiated per client. When the AP sends a scheduled TWT, clients go into a doze state until the next scheduled wake up time.

Two NAVs
A client will need to maintain two NAVs. An HE AP has the option of maintaining two NAVs. The NAVs are: the intra-BSS NAV and a basic NAV.

The basic NAV is updated by an inter-BSS that is not classified as an intra-BSS or inter-BSS.

Benefits of two NAVs may be useful for dense scenarios for protection of clients from other frames transmitted by clients within its BSS and to avoid interference from other clients in neighboring BSS (the inter-BSS).

Links & Resources

CTS 106: 802.11ax with Broadcom

Vijay Nagarajan of Broadcom joins CTS to discuss what’s new with 802.11ax and it’s future with Broadcom.

This episode is sponsored by Metageek

Sponsored by Metageek

Broadcom Delivers 802.11ax Solutions

On August 15, 2017 Broadcom announced their 802.11ax ecosystem of products labeled Max WiFi. In this episode, Vijay Nagarajan joins us on the show to discuss the future standard – 802.11ax. Vijay is head of marketing for all connectivity at Broadcom. He has also represented Broadcom at the Wi-Fi Alliance for 802.11ac.

802.11ax touts increased efficiency of Wi-Fi communications. Whereas previous standards had focused more on throughput, 802.11ax will get higher throughput by using the wireless medium more efficiently.

In our discussion with Vijay, he mentions how there is substantially more video consumption on wireless devices. And with 802.11ac we saw throughput increase and standardized beamforming. More use cases are being added on such as video uploads from places such as concerts and sporting events.

Upload traffic has risen because of social media and sync technologies. Everyone wants to stream the latest game or concert. And with the number of devices increasing per person we will see even more traffic.

The biggest news about 802.11ax is OFDMA or Orthogonal Frequency-Division Multiple Access. An analogy Vijay uses – picture a highway. You slice up that highway into multiple lanes. In RF, splice the frequency spectrum into bandwidths that are proportional for the devices. Devices are scheduled for when they can transmit or receive using target wake time (TWT). I thought of this as like giving devices tokens – access to transmit or receive.

OFDMA explainer

Broadom’s Max WiFi OFDMA. From http://maxwifi.org/how-max-works/

In OFDM, we had one device transmit or receive data while other devices waited for their chance to use the shared medium. And this caused draining batteries while waiting to access the medium.

François posed the question, why haven’t we not used OFDMA before? Vijay’s opinion was because Wi-Fi was built initially with simplicity. There was availability of spectrum. Now we need efficiency.

What else is new with 802.11ax? Coloring with spacial channel reuse. There is a signature associated with each BSS. Coloring will help networks use available spectrum more efficiently.

And then theres MU-MIMO. 802.11ac introduced downlink MU-MIMO only. Now, 802.11ax introduces both uplink and downlink MU-MIMO. While I haven’t seen this used in the wild yet (MU-MIMO), I am still skeptical if this will be used in the future but only time and device drivers will tell.

In conclusion, Broadcom has taken strides to become bleeding edge. They’ve released 802.11ax chips and with CES 2018 we saw announcement of the first 802.11ax routers. Even Aerohive has made the announcement of their own 802.11ax access points. Downside, we don’t have any device clients, that I know of, in the wild. It will be interesting to see if the adoption of 802.11ax will be stronger than 802.11ac or just as strong as 802.11n was.

Links and Resources