New wireless networking standard IEEE 802.11ac

9 Sep


What is 802.11ac?

802.11ac is a brand new, soon-to-be-ratified wireless networking standard under the IEEE 802.11 protocol. 802.11ac is the latest in a long line of protocols that started in 1999:

  • 802.11b provides up to 11 Mb/s per radio in the 2.4 GHz spectrum. (1999)
  • 802.11a provides up to 54 Mb/s per radio in the 5 GHz spectrum. (1999)
  • 802.11g provides up to 54 Mb/s per radio in the 2.4 GHz spectrum (2003).
  • 802.11n provides up to 600 Mb/s per radio in the 2.4 GHz and 5.0 GHz spectrum. (2009)
  • 802.11ac provides up to 1000 Mb/s (multi-station) or 500 Mb/s (single-station) in the 5.0 GHz spectrum. (2013?)

802.11ac is a significant jump in technology and data-carrying capabilities. The following slide compares specifications of the 802.11n (current protocol) specifications with the proposed specs for 802.11ac.

What is new and improved with 802.11ac?

For those wanting to delve deeper into the inner workings of 802.11ac, this Cisco white papershould satisfy you. For those not so inclined, here’s a short description of each major improvement.

Larger bandwidth channels: Bandwidth channels are part and parcel to spread-spectrum technology. Larger channel sizes are beneficial, because they increase the rate at which data passes between two devices. 802.11n supports 20 MHz and 40 MHz channels. 802.11ac supports 20 MHz channels, 40 MHz channels, 80 MHz channels, and has optional support for 160 MHz channels.

More spatial streams: Spatial streaming is the magic behind MIMO technology, allowing multiple signals to be transmitted simultaneously from one device using different antennas. 802.11n can handle up to four streams where 802.11ac bumps the number up to eight streams.

MU-MIMOMulti-user MIMO allows a single 802.11ac device to transmit independent data streams to multiple different stations at the same time.

BeamformingBeamforming is now standard. Nanotechnology allows the antennas and controlling circuitry to focus the transmitted RF signal only where it is needed, unlike the omnidirectional antennas people are used to.

What’s to like?

It’s been four years since 802.11n was ratified; best guesses have 802.11ac being ratified by the end of 2013. Anticipated improvements are: better software, better radios, better antenna technology, and better packaging.

The improvement that has everyone charged up is the monstrous increase in data throughput. Theoretically, it puts Wi-Fi on par with gigabit wired connections. Even if it doesn’t, tested throughput is leaps and bounds above what 802.11b could muster back in 1999.

Another improvement that should be of interest is Multi-User MIMO. Before MU-MIMO, 802.11 radios could only talk to one client at a time. With MU-MIMO, two or more conversations can happen concurrently, reducing latency.

What do experts say about 802.11ac?

There is a lot of guessing going on as to how 802.11ac pre-ratified devices are performing. I don’t like to guess, so I contacted Steve Leytus, my Wi-Fi guy who also owns Nuts about Nets, and asked him what he thought:

Regarding 802.11ac, we are testing wireless game consoles for a large company in the Seattle area. We test performance using 20, 40, and 80 MHz channels. During the tests, we stream video data and monitor the rate of packet loss in the presence of RF interference or 802.11 congestion.

802.11ac’s primary advantage is support for the 80 MHz-wide channel. And without question, the wider channel can stream more data. But, as with everything, there are trade-offs.

I asked Steve what the trade-offs were:

  • I don’t think you’ll find 802.11ac clients as standard equipment for computers. So, you need to buy one, connect it to the computer via Ethernet, configure the client, and finally pair the client with the router/access point.
  • Unless your application requires streaming large amounts of data, you probably will not experience a noticeable improvement in performance.
  • The 80 MHz-wide channel is more susceptible to RF interference or congestion from other Wi-Fi channels by virtue of its larger width.
  • The 80 MHz channel eats up four of the available channels in the 5.0 GHz band. Some routers implement DCS (dynamic channel selection) whereby they will jump to a better channel in the presence of RF interference. But if you are using 80 MHz channels your choices for better channels are few or non-existent.

Transmission testing results

[UPDATE] Steve Leytus finally was able to break away from his testing long enough to grab screen shots of the three channel widths. I haven’t seen this anywhere else, so I thought I’d pass his explanation and slides along:

The three images are of iperf transmitting from one laptop to another at 20 Mbps; both laptops are connected to the same Buffalo 802.11ac router — one laptop is connected via Ethernet, and the other is associated wirelessly. The transmission test was repeated three times using channel widths of 20 MHz, 40 MHz, and 80 MHz.

You can clearly see how the width of the spectrum trace increases with channel width. The other thing to notice which might not be so apparent is the power level — as the channel width increases the power level decreases.

This is expected since the transmit power has to be spread out over a wider frequency range. The implication is that as the channel width increases then the distance the signal can reach probably decreases.

20 MHz


80 MHz


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