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:

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 Q3 of 2019. The Wi-Fi Alliance has a similar timeline for an 802.11ax certification. As was done with 802.11n and 802.11ac, WLAN vendors such as Aerohive will be releasing 802.11ax products in 2018 prior to the ratification of the amendment.

4. 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. 

5. 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.

6. 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.

7. When will we see 802.11ax clients?

70% of client radios are manufactured by Broadcom. We expect to see 802.11ax clients from Broadcom and other chipset vendors as early as Q3 in 2018.

8. 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.

9. 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.

10. 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 a 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. 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.

11. 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, An 802.11ax 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.

12. 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.

13. Is ODMDA 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

14. 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.

15. 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 has still not occurred and currently is not practical:

  • 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:

Increased efficiency Increased capacity
Reduced latency Higher speeds per user
Best for low bandwidth applications Best for high bandwidth applications
Best with small packets Best with large packets

Broadcom is betting bigger on wide OFDMA implementation. However, Qualcomm is betting on wider acceptance of MU-MIMO.

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

In theory, BSS coloring 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

17. 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 (~ 37dB) 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.

18. 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/days and conserve battery life.
19. What is BSS Coloring?

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 header. 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 Coloring potentially decreases the channel contention problem that is a result of existing 4 dB signal detect (SD) thresholds.

20. 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.

21. 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 Qualcomm IPQ8074 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.
  • Aerohive APs will be using 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 dual-frequency meaning you can have two dual-5 GHz 4×4:4 802.11ax radios in a single AP. The Aerohive AP650 and AP650X will both have dual-5 GHz capability.
  • Leverage the Dual 5 GHz story is where Aerohive has been a leader. 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.
  • Furthermore, Aerohive’s unique architecture allows for salt and pepper design, where APs can be seamlessly added to existing infrastructure. Most other vendors will likely support 802.11ax only on latest generation controller.

22. Will there be any 8X8:8 Clients? 

The battery life of an 8×8:8 client will be about 5 minutes 😉 – The Broadcom BCM4375, Qualcomm QCA6290 and Qualcomm WCN3998 are the first two 802.11ax client radios targeted for mobile devices. Both are 2×2:2 dual-band radios. There are currently no 4×4:4 802.11ax clients in the near future let alone any 8×8:8 802.11ax clients.

23. 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?

  • Everyone thought we were going to need aggregate GIG ports with two cables when 802.11n debuted – Did not happen.
  • Everyone thought we were going to need aggregate GIG ports with two cables when 802.11ac debuted – Did not happen.
  • Cisco is telling everyone we need 2.5G ports with Wave-2 802.11ac APs, so they can sell switches. It’s nonsense because of medium contention overhead.
  • Any talk of the need for 10 Gbps uplinks from the APs is nonsense.

Will we need 2.5G Ethernet ports for 802.11ax?  The answer is probably not for a very long time. The whole point of 802.11ax is to cut down on medium contention and airtime consumption. Logically that means bigger uplink ports will be needed. However, in the field, we probably will still not exceed 1G for a very long time because of these two reasons:

  • Even though the chipset vendors appear to be aggressive with making 802.11ax client radios available at the same time as AP radios, it will be very long time before there is a wide proliferation of 802.11ax clients in the enterprise client population.
  • 802.11ax requires backward compatibility with 802.11a/b/g/n/ac which means that RTS/CTS protection mechanisms must be used. RTS/CTS creates overhead and consumes airtime.

That being said the AP650 and AP650X will each have a 2.5G and 1G (full duplex) Ethernet uplink port. These 802.3bz MultiGig ports will provide:

  • A checkbox for RFPs
  • Future-proofing

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

Aerohive and other WLAN vendors will be adding more radio chains to 802.11ax access points. For example, all three of Aerohive’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 MIMO APs should be considered a standard requirement.
25. What are the Aerohive 802.11ax APs?

  • AP630:  4×4:4 + 4×4:4
    • Dual frequency
    • 1G + 1G Ethernet ports
    • BLE
    • USB
  • AP650:  4×4:4 + 4×4:4
    • Dual 5 GHz
    • 2.5G + 1G Ethernet ports
    • BLE
    • USB
  • AP650X:  4×4:4 + 4×4:4
    • External Antenna
    • Dual 5 GHz
    • 2.5G + 1G Ethernet ports
    • BLE
    • USB

26. What chipsets are in the Aerohive APs?

Aerohive 802.11ax APs will 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
  • ZeroWait DFS
  • AirIQ interference identification
  • Full compliance with IEEE and WFA 802.11ax specifications

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