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).

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