Next Generation Wi-Fi - A Brief on 802.11ac

    At the end of last year, 802.11n completely crushed all previous Wi-Fi standards (at least in terms of chipset deliveries) - some experts voiced figures of about 70%. Having tested high speeds, the public wants more, and 802.11ac is promised to the heirs, promising up to gigabit speeds. Let's see what we get in reality.

    What is the speed?
    802.11n used the following to increase throughput:
    • Optimized modulation and packet transfer mechanisms allowed breaking through from 54Mbps to ~ 75Mbps
    • Then Channel Bonding was turned on - channels with a width of 40Mhz (twice as wide as the traditional 22Mhz) ensured a doubling of speed - up to 150Mbps.
    • Then, the Multiple Spatial Streams mechanism was turned on, of which there can be up to four according to the standard, which allows reaching 150 * 4 = 600Mbps in theory
    802.11ac is going to catch up and overtake in the following ways:
    • Channels 80Mhz and 160Mhz wide, which allows you to instantly double / quadruple the results of 802.11n.
    • The maximum number of Spatial Streams was increased to 8, which allows you to double the speed once again.
    • Optimization of modulation and packet transmission methods allows you to squeeze out a little more resource and ensure that high speeds will be available not only within a radius of 4 m from the access point.
    Total, adding all the factors, we can get the speed in theory eight times more than the performance of 802.11n - about 5Gbps. Marketers are jubilant, people are waving. In practice, however, such a speed is practically unattainable:
    • New channels no longer fit into the 2.4Ghz range, so 802.11ac will only work in 5Ghz. But 5Ghz is not so simple. In the same Europe, it is possible to work without problems only on the first four channels: 36/40/44/48 - on the rest it is necessary to include DFS / TPC (coexistence with radars), which excludes the possibility of building a more or less reliable network. And in these 4 channels only 1 802.11ac channel will fit, and even then “only something” at 80Mhz. As an exercise, try to calculate how many 160Mhz channels fit into the entire 5Ghz range in different regions. Some hopes are placed on 802.11ad, a version of the standard that runs at 60Ghz, where more megahertz is available, but this frequency range is so far clearly defined only in the United States, and the communication range in it is measured in tens of meters (which is very good in order to reset video from the phone on a TV or using a wireless mouse, but not for serious networks). So, divide the speed in half.
    • 8 Spatial Streams require a radio module with 8 antennas and a suitable environment (so that all 8 streams eventually converge on the client). What the Dual-Radio access point with 8x8: 8 MIMO (16 antennas with Dual Radio!) Will look like can only be guessed. :) Mobile devices, most likely, will not go beyond 4x4: 4 due to the need to save electricity and space in the case. So, we cut the speed in half.
    So we get a maximum doubling of 802.11n speeds - which in itself is still not bad.

    Why are we switching to 802.11ac anyway?
    In addition to speeds, 802.11ac offers two key improvements:
    • Beamforming - the ability to dynamically change the antenna pattern (which is real for an antenna array of 8 elements). Ideally, this means that the coverage area of ​​the access point optimally adapts to the current location of customers. Beamforming is not new to Wi-Fi; it has even been made part of the 802.11n standard. But part optional! In 802.11ac, it will become part of the required. It is not yet clear how Beamforming will work in 802.11ac and whether it will be of any use in the end , but it is clear that it is introduced to maximize the effect of the next (and main) improvement.
    • MU-MIMO finally appears on 802.11ac !Wi-Fi networks are half-duplex: while one is transmitting, the others are listening. Packets are transmitted sequentially - at one point in time, one packet is transmitted. If the “pipe” in 450Mbps (802.11n 3x3: 3 MIMO) flows in 1Mbps, then 1/450 bandwidth is used. If at the same time data arrives for another client, it will not be possible to use unused bandwidth. As a result, the use of ultra-high speeds 802.11n in networks with a large number of slow clients (i.e., corporate) is very small. MU-MIMO allows you to split the "pipe" into several "tubes of smaller diameter" and transmit data through them in parallel. This technology is well known to telecom operators. So far, they are talking about two options for implementing MU-MIMO in 802.11ac: SDMA (Space Division Multiple Access) allows you to transfer data to different clients on different Spatial Streams (that's where Beamforming is needed!), Downlink MIMO allows you to break OFDM subcarriers into groups, and dynamically (seemingly) allocate the right number of subcarriers to each client. Thus, even if 2x2: 2 MIMO clients are sitting on the access point, you can still use the full potential of the “pipe”.
    As you can see, even if you limit the maximum speed to one Gbps, the 802.11ac standard promises significant benefits for both home (high speed) and corporate networks (efficient utilization of these highest speeds in networks with a large number of clients). In addition, to support new radio modes, it is necessary to completely and completely change all the equipment, which promises significant benefits to vendors and integrators - everyone is happy, in general :) It is currently expected that the standard will be ratified at the end of 2012 / beginning of 2013, but it is more optimistic configured vendors have already introduced chipsets and products based on them at CES-2012 . Whether 802.11ac is prepared for the fate of 802.11n, adopted a few years later than expected, time will tell. We are waiting, sir.

    UPD: The first hardware and tests appeared

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