802.11ac Advances
Gigabit Wireless. This is the phrase that pays when it comes to marketing 802.11ac, because finally the wireless providers have a technology able to compete against structured CAT5e or CAT6 wiring. Why would you hassle with the deployment and location restrictions of wired networking if you can get the same results from Wi-Fi? You wouldn’t...if the promise held true, that is.
We saw in Gigabit Ethernet: Dude, Where's My Bandwidth? that you can get 100 MB/s+ throughput on a gigabit network using 28 feet of cable the same as 50 feet. The same story showed us it's nearly impervious to environmental interference. So, unlike wireless, we’re not left thinking, "Well, it says 1000 Mb/s, but I'm really only getting 30 MB/s." Providing you don't have any bottlenecks, gigabit is gigabit, period. As we’ll see, 802.11ac is not gigabit-class wireless. That's marketing. But is it better than 802.11n? Oh, most definitely.
To understand why 802.11ac is superior, we need to delve into some of its key advances over the prior Wi-Fi technology.
Exclusive use of the 5.0 GHz band. 802.11n makes use of either 2.4 GHz or 5.0 GHz, but we know that the 2.4 GHz range is already congested. It works, but 2.4 GHz is unreliable, and the more we want to trust it with high-bandwidth data, such as streaming HD video, the more reliability becomes a factor. Simply put, 2.4 GHz is almost tapped out, at least with current-gen approaches. You can force it into providing more performance, through “bad neighbor” tactics like channel bonding, but this has counterbalancing negative effects for others in the wireless community. The 5.0 GHz range is largely pristine land for wireless drilling, if you will, and the IEEE forces behind the new wireless standard opted to open it up for its next-gen resources.
Wider channel bandwidth. The 802.11n standard allows for the combination of two 20 MHz channels into a single 40 MHz bonded channel. In the 2.4 GHz range, already having 40 MHz channels dropped the number of effective channel options to just three. With 5.0 GHz Wi-Fi, we have 23 possible 20 MHz channels. This yields 11 effective 40 MHz channels. And now, with 802.11ac, we’re starting with 80 MHz channels, of which there are five non-overlapping options. And yes, the 802.11ac spec does scale up to 160 MHz bonded channels, but there will only be two such channels available. We’ll suspend judgment on whether 160 MHz is a good thing when we start hearing reports of how such super-wide channels perform in residential areas, particularly in the company of competing HDTV sets and smartphones.
Mo’ MIMO. Multiple-in, multiple-out (MIMO) technology provides for the separation of a single data stream into more than one sub-stream able to travel along different radio paths. This separation and recombination of signals yields higher total data throughput in many circumstances. However, more sub-signals (properly called spatial streams), results in the need for more transmit (Tx) and receive (Rx) antennas. The 450 Mb/s rates advertised in the latest, highest-end 802.11n products are only possible with a 3x3:3 (three transmit, three receive, three stream) antenna array. While 802.11n provides for up to four spatial streams, 802.11ac can use up to eight.
MU-MIMO. Multiple-user MIMO can turn multiple users into spatially disparate, but wirelessly linked transmission resources. In other words, with multiple radio terminals in a given area, all can cooperate in order to improve each terminal’s performance. The singer-user MIMO found in 802.11n can only operate with the multiple antennas hard-wired into a single terminal. With MU-MIMO, 802.11ac access points will be able to process MIMO signals from multiple clients simultaneously, rather than having to hop quickly (and inefficiently) from one to the next. This should dramatically help with airtime fairness issues in highly-populated client environments.
Optional beamforming. In Why Your Wi-Fi Sucks and How It Can Be Helped, we spent considerable time delving into beamforming and the circumstances in which it can dramatically improve wireless throughput. At the time of that writing, there were no industry standard approaches to beamforming, leaving buyers to pick among a few vendors who chose to improve their 802.11n products with proprietary approaches to the technology.
