New 802.11ah standard (High Efficiency WLAN), what's new in it and when to wait for it?

    The working group began work on the standard back in 2014 and is now working on draft3.0. That is somewhat different from previous generations of 802.11 standards, because there all the work fit into two drafts. This happens because of a sufficiently large number of planned complex changes, which accordingly require more detailed and complex compatibility testing. Initially, the group was faced with the task of improving spectrum efficiency to increase the capacity of a high-density WLAN subscriber stations and access points. The main drivers for the development of the standard were: an increase in the number of mobile subscribers, live broadcasts in social networks (emphasis on upload traffic) and of course IoT.

    Schematically, innovations are as follows:


    MIMO 8x8, more spatial streams

    Support for MIMO 8x8, up to 8SS (Spatial Streams) will appear. The 802.11ac standard also described support for 8 SS in theory, but in practice, 802.11ac “wave 2” access points were limited to supporting 4 spatial streams. Accordingly, access points with MIMO 8x8 support will be able to simultaneously service up to 8 1x1 clients, four 2x2 clients, etc.

    MU-MIMO DL / UL (Multi-User MIMO Downlink / Uplink)

    Simultaneous support for multiuser mode for both download and channel upload. The possibility of simultaneous concurrent access to the upload channel, grouping both date and control frames will significantly reduce the “overhead”, which will lead to an increase in throughput and a decrease in response time.

    Long OFDM symbol

    OFDM has been working in the 802.11a / g / n / ac standards for ~ 20 years without any changes. According to the standard, a channel width of 20MGz accommodates 64 subcarriers spaced at 312.5 kHz (20MHz / 64) intervals. Since during this time, the semiconductor industry has advanced far ahead in 802.11, a 4-fold increase in subcarriers to 256 is proposed, with an interval of 78.125 kHz between subcarriers. The length of the OFDM symbol (time) is inversely proportional to the frequency, and it accordingly also increases 4 times from 3.2 µs to 12.8 µs. This improvement will increase the efficiency and reliability of data transmission, especially in the “outdoor” WLAN.

    Extended range

    Added new values ​​of guard intervals between frames, which can now be equal to 1.6 μs and 3.2 μs for the “outdoor” WLAN, for the “indoor” interval left 0.8 μs. New packet format with a more reliable (long) preamble. All of the above will allow you to get up to 4-fold increase in connection speed at the network edge.

    OFDMA DL / UL (Orthogonal Frequency Division Multiple Access)

    One of the major changes is the introduction of OFDMA instead of OFDM. OFDMA technology is used in LTE networks and has proven to be highly efficient. The difference is that when transmitting in OFDM, the entire frequency channel is occupied and until the transmission is completed the next client cannot occupy the frequency resource. In OFDMA, this problem is solved by the fact that the channel is divided into subchannels of various widths, the so-called RU (Resource Units). In practice, this will mean that the 256 subcarriers of a 20MHz channel can be broken into RU by 26 subcarriers. For each RU, you can assign your own MCS coding scheme as well as transmit power.
    In general, this will bring a significant increase in network capacity in general, as well as throughput for each individual client.

    1024 QAM

    Added new MCS (Modulation and Coding Sets) 10 and 11 to modulate 1024-QAM. That is, now one character in this scheme will carry 10 bit of information, and this is a 25% increase compared to 8bit in 256-QAM.

    TWT (Target Wake Time) - "Up Link resource scheduling"

    The power saving mechanism that has established itself in the 802.11ah standard and is now adapted to 802.11ax. TWT allows access points to inform customers when to go into power saving mode, and provides a schedule when you need to wake up to receive or transmit information. These are very short periods of time, but being able to sleep a bunch of short periods will go a long way in battery life. Reduction of “contention” and collisions between customers will increase the time spent in the power saving mode. Depending on the type of traffic, energy improvements can range from 65% to 95% (according to Broadcom tests). TWT support is crucial for IoT devices.

    BSS Color - Spatial Reuse

    To increase the network capacity of high-density WLANs, you need to increase the frequency of channel channel reuse. In order to reduce the influence of neighboring BSS operating on the same channel, it is proposed to mark them with the help of “color-bit”. This will allow you to dynamically adjust the sensitivity of the CCA (clear channel assessment) and transmitter power. The network capacity will increase due to compaction of the channel plan, while the existing interference will have less influence on the choice of MCS.

    In connection with the next update of security standards to WPA3 , not everyone will have the opportunity to solve security issues with a simple software update, so Extreme Networks will introduce access points with hardware support for 802.11 and WPA3 in the fourth quarter of 2018.

    More about 802.11ah .

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