The buzz about 802.11ac is in full swing, but don’t believe everything you read. The newest of Wi-Fi innovations, 802.11ac (still in draft form) looks like it will start making it into enterprise Wi-Fi products as early as 2013 and home products even earlier. It’s already being flaunted as Gigabit Wi-Fi. And for the largest Wi-Fi market, the home, it will be. But will it deliver gigabit speeds for the enterprise? Not a chance.
Defined for the capacity-rich 5GHz spectrum (495MHz) only, 802.11ac introduces a number of new techniques like advanced modulation and encoding, multi-user MIMO and channel bonding, that theoretically, if you’re talking to a vendor anyway, has the potential to dramatically increase Wi-Fi capacity. The question is, REALLY?
Make no mistake, 802.11ac is a great innovation. But like any great innovation, the devil is often in the details. So here are some details that should help demystify 802.11ac. Here are the key differences to understand:
* Eight spatial streams. One of the biggest Wi-Fi innovations came with 802.11n in the form of spatial multiplexing using a technique called MIMO (multiple input, multiple output). MIMO is the use of multiple antennas at both the transmitter and receiver to increase data throughput without additional bandwidth or increased transmit power. Basically it spreads the same total transmit power over multiple antennas to achieve more bits per second per hertz of bandwidth with the added benefit of greater reliability due to more antenna diversity.
In essence, MIMO lets an access point send multiple spatial streams to one client at a time to increase capacity. 802.11n specified up to four spatial streams.
Now in glorious one-upmanship, 802.11ac will support up to eight spatial streams. Historically it has taken chip manufacturers about two years to add an additional spatial stream (802.11n is only at three right now). While that will surely improve with 802.11ac, don’t look for it to ever get to eight. However it would be a funny sight to see. Just picture an AP with 12 omni-directional antennas (eight for 11ac in 5GHz and four for 11n in 2.4GHz) sticking out of it. Not a pretty picture.
* Multi-user MIMO. Another difference with 11ac is support for “multi-user MIMO” (MU-MIMO). With 802.11n, MIMO could only be used for a single client at any given time, while 802.11ac tries to improve on this by supporting multiple clients.
This allows an 802.11ac AP to transmit two (or more, depending on the number of radio chains) spatial streams to two or more client devices. This has the potential to be a good improvement but is optional. And it’s expected that the first 802.11ac chips out the door won’t support this. What’s more, there’s is a good chance that MU-MIMO won’t ever be supported due to the radio and MAC complexity required.
* 256 Quadrature Amplitude Modulation (QAM). QAM is a way to modulate radio waves to transmit data. 802.11n maxed out at 64QAM so the advent of 256QAM should deliver big improvements in maximum throughput. However, the more complex a modulation scheme, the more difficult it is to achieve. In realistic situations it is highly unlikely that any percentage of client devices would consistently achieve 256QAM. Ouch.
* 5GHz only. Due to how 11ac really achieves all this speed (channel bonding) it doesn’t make sense for 11ac to support 2.4GHz which only has three (of 11) non-overlapping channels. What this means is that devices that want to have 11ac (they all will) will be 5GHz capable. Right now it is a very low percentage that are capable of 5GHz and that is a real shame. Now they’ll be required to do it.
* Channel bonding. An easy and effective method to increase the speed of any radio link is to give it more frequency, or bandwidth. To get more bandwidth, 802.11n introduced us to channel bonding: the ability to take two 20MHz channels and make them work as one — basically a bigger Wi-Fi pipe. This effectively doubled throughput that could be achieved.
Now 802.11ac has mandated the support of 80MHz channels with options to bond eight channels for a total channel of 160MHz.
Even with 802.11n, channel bonding is a double-edged sword. In North America, the 2.4GHz band has 83.5MHz (three non-overlapping channels) of total bandwidth, while the 5GHz bands have a total of 495MHz. That means that 5GHz can carry almost six times the traffic of 2.4GHz plus the added benefit that (for now) the 5GHz band is a much cleaner spectrum.
But don’t count your bits quite yet. What most people don’t realize is that by enabling channel bonding you are actually reducing your overall capacity.
When designing and deploying a Wi-Fi network for high density, more channels are preferred to fewer, larger channels. Increasing the number of devices occupying one channel in a given area makes reduces the efficiency of Wi-Fi.
This is why people like wires. Because each device effectively has its own channel and there are no other devices occupying that channel (the copper or fiber). So we see staggering amounts of throughput.
If Wi-Fi could have hundreds of channels, and each client would get their own, this would be wireless nirvana. But, as you can see from that chart, we don’t have that many channels, and we sure don’t want to exacerbate the problem by bonding them together if it reduces the overall efficiency of the wireless LAN. Using 802.11ac in the home is a different story. Bonding channels all the way to 160MHz is preferred given there are few devices trying to access a single AP.
The enterprise is just the opposite. Here, numerous APs are required to support hundreds or thousands of users. And, as much as is possible, those APs should be on different channels.
Ultimately, 802.11ac offers improvements for the Wi-Fi industry primarily because it forces clients to add support for the capacity-rich 5GHz spectrum. Current enterprise APs already support both bands.
Ironically, 802.11ac will prolong the viability of current 802.11n networks. As more and more clients become 5GHz capable, capacity and performance will increase without touching the infrastructure. This is the best news of all.
(This primer was contributed by Ruckus Wireless, which markets and manufactures a wide range of indoor and outdoor “smart Wi-Fi” products for mobile operators, broadband service providers and corporate enterprises around the world.)
