802.11ax Frequently Asked Questions

This article covers what the 802.11ax standard is and some common questions regarding this new standard and its accompanying features.

Answers Compiled by:

• David Coleman, CWNE #4
• Perry Correll, Extreme Network’s IEEE and Wi-Fi Alliance representative
• Gregor Vucajnk, CWNE #96

1. What is 802.11ax?

802.11ax is an IEEE draft amendment that defines modifications to the 802.11physical layer (PHY) and the medium access control (MAC) sublayer for high-efficiency operation in frequency bands between 1 GHz and 6 GHz. The technical term for an 802.11ax is High Efficiency (HE).

2. Why is 802.11ax needed?

Past amendments defined 802.11 higher data rates and wider channels but did not address efficiency. The bulk of 802.11 data frames (75-80%) are small and under 256 bytes. The result is excessive overhead at the MAC sublayer and medium contention overhead for each small frame. Higher data rates and wider channels is not the goal of 802.11ax. The goal is better and more efficient 802.11 traffic management. Another goal is to increase the average throughput 4X per user in high-density WLAN environments.

3. When will 802.11ax be an official standard?

The IEEE is currently scheduled to ratify the 802.11ax amendment in Q1 2020. The Wi-Fi Alliance has launched it’s 802.11ax certification program as Wi-Fi 6. As was done with 802.11n and 802.11ac, WLAN vendors have released 802.11ax products prior to the ratification of the amendment. As a matter of fact, Extreme Networks already has an entire family of eight 802.11ax APs with more on the way in 2020.

4. What is the Wi-Fi 6?

Recently the Wi-Fi Alliance adopted a new generational naming convention for Wi-Fi technologies. The goal is that the new naming convention will be easier to understand for the average consumer as opposed to the alphabet-soup naming used by the IEEE. Because 802.11ax technology is such a major paradigm shift from previous versions of 802.11 technology, it has been bestowed with the generational name of Wi-Fi 6. 802.11ax and Wi-Fi 6 mean the same thing, but the term Wi-Fi 6 will be more prevalent with the general population. The Wi-Fi Alliance certification for the technology is also called Wi-Fi 6.

5. Will the existing Extreme 802.11ax APs be certified for Wi-Fi 6?

Yes

6. In which frequency bands will 802.11ax radios operate?

Unlike 802.11ac, which is technology for 5 GHz only, 802.11ax radios can transmit and receive on either the 2.4 GHz or 5 GHz frequency bands. In the future, 802.11ax technology will also be available in the 6 GHz band.

7. Is 802.11ax backward compatible with older Wi-Fi radios?

Yes, 802.11ax radios will be able to communicate will legacy 802.11a/b/g/n/ac radios. 802.11ax radios will communicate with other 802.11ax radios using OFDMA and/or OFDMA. 802.11ax radios will communicate with legacy radios using OFDM or HR-DSSS. When 802.11ax-only OFDMA conversations are occurring, RTS/CTS mechanisms will be used to defer legacy transmissions.

8. Will a customer need to upgrade their Wi-Fi clients to take advantage of 802.11ax capabilities?

802.11ax APs will not improve the performance or range of any legacy Wi-Fi clients (802.11a/b/g/n/ac). 802.11ax clients will be needed to take full advantage of 802.11ax high-efficiency capabilities such as multi-user OFDMA. While there will be no PHY improvements with legacy clients, there will be performance improvements as a result of newer hardware capabilities of the new 802.11ax APs, such as stronger CPUs, better memory handling, and other normal hardware advancements. However, as we see more 802.11ax clients mixed into the client population, the efficiency improvements gained by 802.11ax client devices will free valuable airtime for those older clients, therefore improving the overall efficiency of the system.

9. When will we see 802.11ax clients?

70% of client radios are manufactured by Broadcom. Wi-Fi 6 clients have already entered the marketplace and a Wi-Fi 6 client population explosion is coming soon. All the major chipset vendors such as Broadcom, Qualcomm, and Intel are manufacturing 2x2:2 Wi-Fi 6 radios that will find their way into smartphones, tablets, and laptops. Samsung released the Galaxy S10, the first Wi-Fi 6 smartphone, into the market in February of 2019. The Apple iPhones introduced in September 2019 also utilize Wi-Fi 6 radios. Intel has announced 100s of new Wi-Fi products by end of year. Industry analysts all agree that the Wi-Fi 6 technology growth will be fast and furious. Several analysts already are predicting 1 billion Wi-Fi 6 chipsets will ship annually by 2022

10. Does a customer need to upgrade from 802.11ac APs to 802.11ax APs? Why should a customer buy 802.11ax APs if there are not a lot of 802.11ax clients?

11ax APs will have faster processors and provide future-proofing as 802.11ax clients find their way into the marketplace. If you are choosing between buying a new 802.11ax or 802.11ac, we would recommend going with the latest technology for the long-term return on investment.

11. What does Multi-User (MU) mean?

The term multi-user (MU) simply means that transmissions between an AP and multiple clients can occur at the same time dependent on the supported technology. However, the MU terminology can be very confusing when discussing 802.11ax. MU capabilities exist for both OFDMA and MU-MIMO. Please understand the differences as explained further in this field note.

12. What is OFDMA?

Orthogonal Frequency Division Multiple Access (OFDMA) a multi-user version of the OFDM digital modulation technology. 802.11a/g/n/ac radios currently OFDM for single-user transmissions on an 802.11 frequency. OFDMA subdivides a channel into smaller frequency allocations called resource units (RUs). By subdividing the channel, parallel transmissions of small frames to multiple users happens simultaneously. Think of OFDMA as a technology that partitions a channel into smaller sub-channels so that simultaneous multiple-user transmissions can occur. For example, a traditional 20 MHz channel might be partitioned into many as 9 smaller channels. Using OFDMA, an 802.11ax AP could simultaneously transmit small frames to nine 802.11ax clients. Intially the Wi-Fi Alliance will be testing for simultaneous transmissions of four RUs using OFDMA on both the downlink and the uplink. OFDMA is much more efficient use of the medium for smaller frames. The simultaneous transmission cuts down on excessive overhead at the MAC sublayer as well as medium contention overhead.

13. What size are the Resource Units (RUs) that will function as OFDMA sub-channels?

When subdividing a 20 MHz channel, The AP can designate 26, 52, 106, and 242 subcarrier Resource Units (RUs), which equates roughly to 2 MHz, 4 MHz, 8 MHz, and 20 MHz channels. The 802.11ax AP dictates how many RUs are used within a 20 MHz channel and different combinations can be used. For example, a Wi-Fi 6 AP could simultaneously communicate with one 802.11ax client using 8 MHz of frequency space while communicate with two other 802.11ax clients using 4 MHz sub-channels.

14. Is OFDMA downstream or upstream?

Both! The AP coordinates OFDMA transmissions both downstream and upstream using a trigger frame mechanism. For the first time in 802.11 technology, an access point can coordinate upstream client transmissions. The AP uses a trigger frame to allocate client resource units (RUs) and set transmit timing for each client.

15. Is OFDMA the same thing as MU-MIMO?

No! Do not confuse OFDMA with MU-MIMO. OFDMA allows for multiple-user access by subdividing a channel. MU-MIMO allows for multiple-user access by using different spatial streams. Access points will send unique steams of data to multiple clients simultaneously. The 802.11ax standard also allows for the combined use of MU-MIMO and OFDMA but it is not expected to be widely implemented

16. What about MU-MIMO? Will it be supported in 802.11ax?

Downlink MU-MIMO was introduced with Wave-2 802.11ac access points. 802.11ax will continue to support downlink MU-MIMO may also define uplink MU-MIMO. Support for uplink MU-MIMO will not be included in any of the first generation of 802.11ax radios. Real-world adoption of MU-MIMO, in general, has yet to take place.

17. Which is better, OFDMA or MU-MIMO?

Most industry experts believe that OFDMA will be the most relevant technology that 802.11ax offers. Downlink MU-MIMO was introduced with Wave-2 802.11ac access points, however, real-world implementation of MU-MIMO for indoor environments is rare:

• Hardly any MU-MIMO capable clients exist in the current marketplace and the technology is rarely used in the enterprise.
• MU-MIMO requires spatial diversity, therefore the physical distance between the clients is necessary. Most modern-day enterprise deployments of Wi-Fi involve a high density of users that is not conducive for MU-MIMO conditions.
• Because MU-MIMO requires spatial diversity, a sizable distance between the clients and the AP is necessary. Most modern-day enterprise deployments of Wi-Fi involve a high density of users that is not conducive for MU-MIMO conditions.
• MU-MIMO requires transmit beamforming (TXBF) which requires sounding frames. The sounding frames add excessive overhead, especially when the bulk of data frames are small.
• MU-MIMO would only be a favorable option in very low density, high bandwidth environments.

If all things were equal, a quick comparison of potential benefits from each technology:

A very good use case for MU-MIMO is a point-to-multipoint (PtMP) bridge links between buildings. The spatial diversity that is required for MU-MIMO exists in this type of outdoor deployment.

18. Will there be any advantage to use 80 MHz or 160 MHz channels with 802.11ax in the enterprise?

In theory, BSS color could provide the capability to take advantage of 80 MHz channels. However, this is assuming no legacy devices exist. In reality, designing for 20 MHz channels will still be the best practice. If deploying, 40 MHz channels, design best practices will also most likely remain the same:

• Only use if DFS channels are available
• Thick walls
• Low transmit power

19. I thought you said 802.11ax was all about high efficiency and not higher data rates. If 802.11ax introduces 1024-QAM modulation, will there then be higher data rates?

Okay, there is always an exception. First generation 802.11ax radios will support 1024-QAM modulation which will also mean some new Modulation and code schemes (MCS) that define some higher data rates. Much like 256-QAM, we anticipate that very high SNR thresholds (~ 35 dB) will be needed in order for 802.11ax radios to use 1024-QAM modulation. Pristine RF environments with a low noise floor and close proximity between an 802.11ax AP and 802.11ax client will be needed.

20. Will 802.11ax be good for the Internet of Things (IoT)?

802.11ax also includes Target Wake Time (TWT) that will be very useful for IoT devices. The TWT has first proposed under 802.11h. TWT uses negotiated policies based on expected traffic activity between 802.11ax clients and an 802.11ax AP to specify a scheduled wake time for each client. 802.1ax IoT clients could potentially sleep for hours and conserve battery life.

21. What is BSS Coloring?

BSS Color (also referred to as BSS Coloring) is a method for addressing medium contention overhead due to overlapping basic service set (OBSS). 802.11ax radios can differentiate between BSSs by adding a number (color) to the PHY and MAC headers. Same color bit indicates an intra-BSS. Different color bits indicate inter-BSS. Inter-BSS detection means that a listening radio treats the medium as busy and must defer. Adaptive CCA implementation could raise the signal detect (SD) threshold for inter-BSS frames while maintaining a lower threshold for intra-BSS traffic. BSS Color potentially decreases the channel contention problem that is a result of existing 4 dB signal detect (SD) thresholds.

22. Are there really three Guard Intervals with 802.11ax?

Yes, 802.11ax defines three-guard intervals of .8us, 1.6us and 3.2us. The longer guard intervals will enhance delay spread protection. Better resiliency in outdoor environments is expected.

23. Which is better 4×4:4 access points or 8×8:8 access points?

Some of our competitors may manufacture an 8×8:8 access point using a non-Broadcom chipset. It will support eight 5 GHz streams and four 2.4 GHz streams. Some key points should be understood about 8×8:8 APs versus 4×4:4 802.11ax APs:

• 8×8:8 APs will initially be more expensive and require more power. The main advantage of 8×8:8 AP is to take advantage of MU-MIMO capabilities which 802.11ax clients will need to support. Regardless of stream count, all APs will support the same number of 11ax OFDMA clients. There is no real advantage with an 8×8:8 AP over a 4×4:4 when using the OFDMA technology.

• Extreme APs use a Broadcom chipset. The chipset provides for 4×4:4 2.4 GHz radio and a 4×4:4 5 GHz radio. However, one of the radios is an adaptive software-defined radio (SDR), meaning you can have two dual-5 GHz 4×4:4 802.11ax radios in a single AP. Almost all of the Extreme 802.11ax APs have software-defined radios (SDRs) that allows for dual 5 GHz functionality.

• Extreme has been a leader in dual 5 GHz technology. Two dual-5 GHz 4×4:4 802.11ax radios transmitting on two separate 5 GHz 20 MHz channels will provide better performance and efficiency. The competitor’s AP 8X8:8 radio will transmit on a single 5 GHz/20 MHz channel and a single 2.4 GHz/20MHz channel.

24. Will there be any 8x8:8 Clients?

The battery life of an 8×8:8 client will be about 5 minutes. Most Wi-Fi mobile client devices such as smartphones will use dual-frequency 2x2:2 radios because an 8x8:8 radio would drain battery life. In the future, you might will some 4x4:4 client radios in high-end laptops.

25. Some of our competitors are claiming we will need 10 Gbps uplinks from 802.11ax APs. Is this true? Will we at least need 2.5 MultiGig (802.3bz) Ethernet ports? Let’s look at this historically:

• 802.11n: Claims were made in 2009 that we were going to need aggregate GbE ports with two cables when 802.11n debuted. Did not happen.
• 802.11ac: Claims were made in 2013 that we were going to need aggregate GbE ports with two cables when 802.11ac debuted. Did not happen.
• 802.11ac – Wave 2: When the second generation of 802.11ac chipsets debuted in 2016, several enterprise switch vendors made claims that everyone needed to upgrade their switches to support 2.5 GbE uplinks with 802.3bz MultiGig technology. Did not happen.

Will we need 2.5 GbE or 5 GbE ports for 802.11ax? The whole point of Wi-Fi 6 (802.11ax) is better spectrum efficiency and a reduction in airtime consumption. Logic dictates that if Wi-Fi becomes more efficient, the user traffic generated by a dual-frequency Wi-Fi 6 AP could potentially exceed 1 Gbps. The fear is that a standard Gigabit Ethernet wired uplink port could be a bottleneck, and therefore 2.5 Gbps uplink ports will be needed. As a precaution, WLAN vendors’ Wi-Fi 6 APs will include at least one 802.3bz Multi-Gig Ethernet port capable of a 2.5 or 5 Gbps wired uplink. Think of this as future-proofing.

Prior to Wi-Fi 6 (802.11ax), the only time a 1 Gbps uplink has not been sufficient is in laboratory test environments or very unique corner cases. Bandwidth bottlenecks almost never occur at the access layer. However, bandwidth bottlenecks can certainly occur on the wired network due to poor wired network design. The number one bandwidth bottleneck is usually the WAN uplink at any remote site.

Although past gloom and doom predictions of access-layer bottlenecks have not come true, as Wi-Fi 6 client populations grow and as WLAN vendors add tri-band radios into their APs, 1 GbE uplinks may no longer be sufficient. Although historically 1 Gbps uplinks have been more than enough, eventually at least 2.5 Gbps uplinks and maybe 5 Gbps uplinks may be needed. Any vendor claims that 10 Gbps uplinks will be needed are fantasy.

26. Will 802.11ax APs work with 802.3af PoE?

Extreme and other WLAN vendors will be adding more radio chains to 802.11ax access points. For example, all three of Extreme’s 802.11ax AP family line will be 4X4:4. The extra radio chain and quad-4 processor will require more power. 802.3at PoE Plus power will be required. PoE Plus requirements for 4×4:4 Wi-Fi 6 APs should be considered a standard requirement. 802.3af power of 15.4 watts will be sufficient to power 2x2:2 Wi-Fi 6 APs as they enter the marketplace.

27. What chipsets are in the Extreme 802.11ax APs?

Extreme 802.11ax APs use the Broadcom chipset.

• Support for four streams of 802.11ax
• 4.8 Gbps PHY rate
• 160 MHz channel bandwidth
• 1024 QAM modulation
• Uplink & downlink OFDMA
• MU-MIMO
• ZeroWait DFS
• AirIQ interference identification
• Full compliance with IEEE and WFA 802.11ax specifications