What you really need to know about 802.11ac WiFi
Christian Karasiewicz 270005XS4E Visits (3594)
This blog post is contributed by Justin Ndreu, a Senior IT Architect for IBM within IBM Global Technology Services.
It's amazing how rapidly technology standards evolve these days. For wireless LANs (WLANs), 802.11ac is now the standard for new deployments or upgrades, even though the previous standard, 802.11n, never reached its full potential—and now never will.
What has really changed, though, with 802.11ac? And what are the key things you need to know if you are planning a new deployment or WLAN upgrade? We hear promises of gigabit-per-second speeds, but, realistically, what is required to achieve that kind of wireless throughput on your mobile device?
802.11ac versus 802.11n
802.11ac builds upon many of the enhancements that were introduced with 802.11n, while providing full backwards compatibility for legacy WiFi devices. Some of the key changes include the following:
All of this sounds great, right? Well, yes, it is, but there are several caveats that you should be aware of. Let's step through each one of these enhancements and discuss the "real world" implications:
160 MHz channels
First off, you need to know that 802.11ac is for 5 GHz only. 2.4 GHz is not supported. Period. Why, you ask? The reason is because in the 2.4 GHz band, there is only 83 MHz of available spectrum, and actually less than that when you factor in something called "side-lobe emissions." So, 40 MHz is the largest channel that could be used, but even that would limit us to a single nonoverlapping channel, meaning two adjacent access points would interfere with each other. For this reason, even 802.11n was limited to 20 MHz channels in the 2.4 GHz spectrum by most WLAN vendors.
In the 5 GHz band, there is more spectrum available; however, even with the Dynamic Frequency Selection (DFS) band enabled, only two 160 MHz channels are possible. Efforts are underway globally to expand the availability of additional 5 GHz spectrum, but for now that's all we have to work with. What this means is that 160 MHz channels are simply not practical for multi-access point deployments, and you will likely never see an enterprise WLAN that supports 160 MHz channels anytime soon.
Finally, we'll need to wait for access points to support 802.11ac Wave 2 for 160 MHz support. Those access points aren't expected to be available until sometime in 2015. So for now, 80 MHz will get us up to 1.3 Gbps using three spatial streams with 802.11ac Wave 1.
Eight spatial streams
Current 802.11ac access points support up to three spatial streams. Adding more spatial streams will generally increase mobile device throughput proportionally; however, this will only be possible at shorter distances from the access point due to multipath interference and other factors.
There is no doubt that processor and radio chipset technology will advance to support additional spatial streams. Unfortunately, there are some physical challenges that need to be overcome as well. Eight spatial stream performance will only be possible when both devices—access point and mobile device—have eight antennas. Without new, innovative antenna designs, this will probably preclude most handheld devices such as smartphones and tablets. In fact, the majority of these types of devices today support only a single stream, limiting them to speeds of less than 150 Mbps, assuming that they also support 40 MHz channel operation.
Modulation is a technique whereby the properties of a waveform such as phase and amplitude are varied to transmit a digital bit stream. As the modulation technique becomes more complex, more bits of data can be sent with each wave form. The use of 256-QAM modulation in 802.11ac can offer up to a 30 percent increase in data rates over 802.11n using 40 MHz channels.
The challenge with using more complex modulation schemes like 256-QAM, however, is that any "noise" in the air will have a greater impact toward introducing errors into the bit stream. So, achieving 256-QAM data rates will require the mobile device to be much closer to the access point. This is the reason why all 802.11 devices will "rate shift" down to lower data rates as they move further away from the access point they are associated with.
To ensure consistently high data rates in enterprise WLANs, we therefore need a more "dense" deployment of access points. Density has its limits though, since positioning access points too close to each other can result in increased co-channel interference and poor mobile device roaming. Realistically, achieving 256-QAM data rates will only be consistently possible for mobile devices that have a clear line of sight and are within 25 feet from their associated access point.
Building on the technique of using multiple radios per band (enabling multiple spatial streams) that was introduced with 802.11n, 802.11ac adds the ability to dedicate up to four spatial streams to a specific mobile device. This technique will allow up to four mobile devices to communicate simultaneously without interfering with each other, significantly increasing the client capacity of an access point.
Again, there are a few drawbacks. First, MU-MIMO is another 802.11ac Wave 2 feature, so it's not currently available on any enterprise access point.
Second, to take full advantage of MU-MIMO will require the use of 80 MHz channels, since clients must share the available bandwidth. An 80 MHz channel will allow up to four clients using 20 MHz channels. A 40 MHz channel reduces the number of simultaneous clients by a factor of two and provides only incremental capacity increases.
Third, MU-MIMO does not increase the aggregate bandwidth of an access point. Bandwidth is simply partitioned across multiple clients. In addition, the multi-user capability is only half-duplex—that is, it works in the downlink direction (from access point to client) only, while upstream transmissions remain single-threaded.
In summary, 802.11ac adds a number of exciting enhancements to the previous WiFi standard that will go a long way toward improving mobile device performance. As long as you understand and take into account the associated design constraints when building or upgrading your WLAN, you can expect a significant performance improvement over 802.11n.
What is your experience in working with 802.11ac? Have you noticed a considerable performance improvement? Connect with me on Twitter @JustinNdreu to share your thoughts!