WiFi Basics Part 3 – 802.11ac

Here’s Part 3 of this useful, interesting overview of WiFi standards and history …

Like 802.11n, the IEEE released a draft of the amendment before its final release. These early devices are known as “Wave 1”. Unless you 11ac router is less than a year old, it falls into this category. The final release was in January 2014 and the subsequent equipment known as “Wave 2”. The majority of this article will compare 11ac to 11n because it really is an enhancement of 11n.

Let’s start with what 11ac doesn’t do:

  • Does not work in the 2.4GHz band; 5GHz band only
  • Does not provide more range/coverage

The overall goal for 802.11ac is to provide significantly more bandwidth. It does this using several methods.

More Channel Bonding. Remember 11n allows up to 4 channels to be bonded. 11ac allows up to 8 channels. If used to the fullest extent, throughput is doubled by this method alone. The one downside to this is channel planning. In the current allowable 5GHz band, only 2 channel groups can be used. This makes planning for multiple APs in an environment fairly difficult.

Enhanced modulation techniques. 11ac can use 256-QAM (Quadrature Amplitude Modulation) whereas 11n is only capable of 64-QAM. The technical details are far beyond this article, but the result is 33% more throughput.

More MIMO (Multiple Input Multiple Output). As with channel bonding, 11ac doubles the number of spatial streams from 4 to 8. Of course, this again doubles the throughput.

But don’t expect to see any 8×8 11ac WiFi equipment available in the consumer market due to design constraints and cost. Although at least one chip manufacturer is producing 8×8 SoC, there are no routers or APs available yet. For now, the fastest are the 4×4 devices.

d-link ultra wifi router

I know what you’re thinking…those aliens finally broke out of Area 51.

Sadly no, this is an example of the newer Wave 2 routers; D-Link’s DIR-895. I just couldn’t resist the picture. Actually the spec sheet doesn’t specify whether it is a 3×3 or a 4×4, but it is a visual reminder of what the designers have to deal with. Imagine a router capable of 8 spatial streams…

While we’re on the topic of antennas, there are a couple of critical requirements. While 802.11ac is fully backward compatible with previous standards, you will only get the 11ac benefits if you’re connecting from an 11ac device. Another critical factor is the capability of your device. Does it have 1 antenna (1×1), 2 antennas (up to 2×2), etc? Below are typical antenna configurations:

  • Smartphones: 1 antenna
  • USB Adapters: 1 or 2 antennas
  • Tablets: 1 or 2 antennas
  • Laptops: 2 or 3 antennas
  • Desktops (with PCI Express cards): 3 or 4 antennas

The end result is the lowest common denominator. If you’re connecting with a Smartphone, you will only get 1 spatial stream, no matter the capability of the AP.

WiFi Standard # of Channels Bandwidth (MHz) # of Spatial Streams Modulation Guard Interval PHY Data Rate (Mbps) Calculated Throughput (Mbps)
802.11a 1 20 1 64QAMr3/4 Long 54 24
802.11n 1 20 1 64QAMr5/6 Long 65 46
1 20 1 64QAMr5/6 Short 72 51
2 40 2 64QAMr5/6 Short 300 210
2 40 3 64QAMr5/6 Short 450 320
2 40 4 64QAMr5/6 Short 600 420
802.11ac 4 80 1 64QAMr5/6 Long 293 210
4 80 1 256QAMr5/6 Short 433 300
4 80 2 256QAMr5/6 Short 867 610
4 80 3 256QAMr5/6 Short 1300 910
4 80 8 256QAMr5/6 Short 3470 2400
8 160 1 256QAMr5/6 Short 867 610
8 160 2 256QAMr5/6 Short 1730 1200
8 160 3 256QAMr5/6 Short 2600 1800
8 160 4 256QAMr5/6 Short 3470 2400
8 160 8 256QAMr5/6 Short 6930 4900

Beamforming. 802.11ac has enhanced the beamforming capabilities. The technical details of beamforming are fascinating, but beyond the scope of this article. In summary, it is a method of transmitting RF through multiple antennas (using modulation amplitude and phase shifting) to create a physical zone where the signal appears stronger to the receiving device. This results in a more reliable signal, capable of higher throughput. Theoretically, it should also increase the coverage, but I have seen no tests that bear this out.

MU-MIMO (Multi User – Multiple Input Multiple Output). We’ve discussed MIMO before, but this is much better. We know that standard MIMO can communicate with an end device using up to 4 spatial streams, and 11ac increases that to 8 streams. The big difference is that MU-MIMO can use those (up to) 8 streams to communicate with up to 4 different devices simultaneously. This is a huge advantage in high-density environments. Note that this capability is only on Wave 2 devices only.

More power efficiency. Because of the dramatically increased throughput of 11ac, devices spend less time transmitting, thus less power is used.

Despite the cost (20-50% more than 11n), there is no doubt 11ac will spread quickly. For instance, I know there are several major hotel chains deploying 802.11ac access points right now.

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