802.11ac: Inside the heart of state of the art

Computer Weekly explains the new 802.11ac wireless standard and whether it is ready for businesses to adopt

For a technology that is now part of the landscape, the 802.11 wireless network family is surprisingly young. Born five years after the invention of the web, the 1997-vintage original 802.11 standard was expensive, tricky to set up and ran at a mere 2Mbps.

Much alphabet soup later, we have arrived at 802.11ac. With headline figures in excess of 1Gbps, the new standard claims to be a good fit for 2013’s promised land of high-performance mobile devices, gigabit broadband and streaming high-definition (HD) video.

Yet wireless veterans will know the numbers on the box rarely ever match reality. 802.11ac has much potential for consumer and business, but getting the most out of it will not be simple.

At heart, 802.11ac still has much in common with its earliest ancestor. The basic unit of connection between access point and device is still the stream. While 802.11 had one stream of 2Mbps using 20MHz of bandwidth in the then newly available 2.4GHz radio band, 802.11ac’s basic stream is also 20MHz wide – but it has a throughput of 86.7Mbps and uses the 5GHz band.

802.11ac then combines multiple streams in ever-larger combinations of channels to increase raw throughput. In addition, it can run simultaneous connections on the same channels to different devices in different directions, making it a good fit for the office environment with multiple Wi-Fi-enabled portable devices.

Can 802.11 deliver on its promise of increased data throughput?

Like its immediate predecessor 802.11n, 802.11ac uses QAM – quadrature amplitude modulation – to put data onto radio signals. This combines two carrier signals that can change their relative frequencies and sizes, with each combination representing one lot of transmitted data.

The precision of these changes sets how much data can be transmitted with each change. 802.11n used 64-QAM – there are 64 possible combinations of the two signals, meaning each combination can represent a six-bit binary number. 802.11ac increases this to 256-QAM, which is eight bits per combination, and that can increase throughput by around 30%.

Physics is unkind, though. Every radio channel has noise, and the same level of noise will disrupt the more delicately composed 256-QAM signal more than it would the more robust 64-QAM. While 802.11ac has a theoretically higher maximum throughput per channel, environmental factors will often reduce it to much the same performance as earlier systems.

The other major factor generally affecting 802.11ac’s real-world versus theoretical performance is its use of the 5GHz band. Anyone who has deployed 2.4GHz Wi-Fi, which is the vast majority of installations to date, will know it can work through walls and over a moderate distance through free space, but that it is prone to shadows, dead zones and speed fluctuations as people and objects move about.

But 5GHz is not as a good at going through walls or other obstructions, so you will not in general get the same range. However, it is better at bouncing off surfaces; radio engineers know that higher frequencies are often better than lower ones at travelling through tunnels. So in some circumstances – a corridor, for example – it may do better.

Detecting the best path of communication

Other advantages of 5GHz include greater isolation between neighbouring access points, and far less interference from other radio sources – 2.4GHz is also home to Bluetooth, baby alarms and a wide variety of other consumer devices.

The 5GHz band does have other users, such as military radar, and it also has varying numbers of channels available in different parts of the world. But regulators and other users are clearing and harmonising the band worldwide; in general, 802.11ac should be no harder to deploy than its predecessors, even if getting the most out of it will need some thought.

802.11ac has much potential for consumer and business, but getting the most out of it will not be simple

802.11ac communicates with each of its devices on one primary channel, which it uses not only to pass data, but also to tell the devices what other channels are available – it can use one, two, four or eight 20MHz channels giving 20MHz, 40MHz, 80MHz or 160MHz of radio bandwidth to provide up to 867Mbps in one stream. An access point will allocate these channels through a combination of what the device says works well and through sensing other interference sources – such as other access points within range, and whether they too are 802.11ac or an older, dumber standard.

It also probes the space between it and the devices to combine the best of multiple transmissions. An 802.11ac access point has multiple physically separate antennas and each will have a slightly different physical relationship to a particular device. This means the radio waves from each antenna will follow a slightly different path to its neighbours, and arrive at the device at a different strength and timing – with some frequencies being better and some worse.

An 802.11ac device can detect these differences and send the results back to the access point, which can then change its transmissions to make the best use of the physical paths available.

This is called beamforming, as it has the same effect as taking a directional antenna to send a beam of signals instead of radiating them in all directions. There are two advantages: you get better range with less interference and you can run multiple different beams – what 802.11ac calls spatial streams – to different devices on the same frequencies. This is how the higher theoretical speeds for 802.11ac are achieved, with a theoretical limit of eight spatial streams of 160MHz bandwidth apiece to clock up nearly 7Gbps.

The only thing that is certain about the 7Gbps scenario is it will never happen in the real world. Current 802.11ac access points have a headline figure of 1.3Gbps, which is what you get with three access point antennas and 80MHz channels. This is called Wave 1 and is an intermediate standard on the way to full 802.11ac. That is called Wave 2 and includes the 160MHz channel specification; working implementations of that are expected late next year or in 2015.

Lack of supported devices

Wave 2 also includes MU-MIMO (multi-user, multi-input, multi-output), which increases the number of ways multiple antennas on both access point and devices can be used – necessary for better utilisation of both space and time.

As the maximum 160MHz channel use will be highly dependent on low interference and band use, and MU-MIMO will not come into its own until a new generation of devices are in use, the change-over is unlikely to signal an enormous step in immediately usable bandwidth, in most cases.

There are also a multitude of other, hidden factors, which make it very hard to predict 802.11ac’s actual abilities in any given installation. Receiver and transmitter design – both things that are hard to measure without expensive test equipment and the right engineers – can have a dramatic effect on performance.

Receivers can be affected by strong unwanted signals, or perform badly in crowded bands; transmitters can emit high levels of noise with the wanted transmission, which can affect nearby receivers no matter how good they are. Only experience and time will identify which suppliers come closest to matching the promise of 802.11ac.

Experiment with 802.11ac

A final but major question for business or commercial 802.11ac deployment is the state of the network planning and diagnostic tools. To get the best out of the standard, a high level of understanding will be required – not only of the techniques used, but of the physical space in which the network will be deployed, the existing wireless infrastructure and the likely mix of devices that will be using the network.

New techniques, such as creating a computer model of the environment and analysing it through tracing the paths individual beams will take, will replace the old ones of estimating range through transmitter strength and placement.

While many suppliers say they have 802.11ac awareness in their tools, there are sufficient new variables to counsel caution and a thorough practical familiarisation with working systems before making a major investment.

It is too early to treat 802.11ac as a simple upgrade, let alone a replacement for high-speed wired systems, in performance-critical environments

That is true in general for this new technology. While there are a number of available access points and a growing number of mobile devices that can use 802.11ac, experience says that in these early days there will be unexpected incompatibilities and occasions where things will not work as planned.

Combining the newness of the standard and the imminent arrival of the final Wave 2 systems, it is too early to treat 802.11ac as a simple upgrade, let alone a replacement for high-speed wired systems, in performance-critical environments.

For less sensitive installations, though, there are enough 802.11ac suppliers and systems out there at competitive prices to make it worth starting an evaluation – or for small businesses or individual users to choose an 802.11ac system for their next upgrade. The standard includes backward compatibility with all currently deployed 802.11 systems and will provide useful and significant speed increases in many situations.

For those who have put off trying 5GHz, now is a very good time to experiment; it is standard in at least the higher-end mobile phones and tablets, and will behave sufficiently differently to your 2.4GHz installation to require investigation.

The message here is give it a try, but don’t bet your shirt on the new standard just yet.

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