Another milestone for 802.11ac and gigabit wireless LANs

This vendor-written tech primer has been edited by Network World to eliminate product promotion, but readers should note it will likely favor the submitter's approach.

One-gigabit wireless LAN products should enter the retail channel for the 2012 holiday season thanks to the fact that the 802.11 working group concluded balloting on the proposed 802.11ac standard last summer, meaning enterprise-focused products should follow in 2013.

Interest in 802.11ac is explained simply by the desire for speed. In homes, the complexity and challenges of A/V or other multimedia distribution and streaming is a catalyst for higher-speed networking, as is anything that improves gaming performance. Enterprise networks may not have as glamorous a need for higher speed, but depend on increasing bit rates to improve user experiences in high-density areas such as conference rooms and auditoriums, improve data service to mobile Internet devices, background synchronization between devices, and support more lifelike video systems.

802.11ac is an evolutionary step in the development of wireless networking (see our earlier article on development of the standard). Previous 802.11 technologies have operated in the now-familiar 2.4GHz and 5GHz bands, and the 802.11ac project began with a project authorization at the IEEE that focuses the effort at less than 6GHz.

IN DEPTH: Wi-Fi devices crowd 2.4GHz band; IT looks to 5GHz

Even with a wide project authorization that covers both the 2.4GHz and 5GHz frequency bands, the task group developing 802.11ac has decided to focus the new standard on only the existing 5GHz band because much of the benefit from 802.11ac's underlying technologies is derived from wider channels. We don't expect that dual-band APs will go away any time soon because there are so many devices that are 2.4GHz-only. To continue to build dual-band APs, the industry will build dual-technology APs with 802.11n remaining the capstone technology at 2.4GHz, and 802.11ac offering higher speeds in the 5GHz band.

Like 802.11n, 802.11ac is a complex standard with many features. For example, 802.11ac specifies more than 300 data rates, though not all will be available in early products. As a result, the industry will follow a similar pattern of adoption. Early implementations will offer a basic set of high-value features and as hardware engineers refine and perfect designs more advanced features will follow.

In the initial wave of products, devices will offer speed improvements over existing 802.11n devices by increasing channel width. 802.11ac includes both the familiar 20MHz and 40MHz channels, and adds new options for 80MHz and 160MHz channels.

As the channel width increases, the challenge of clearing the entire band for transmission becomes more difficult. Part of the challenge is technical, due to the need of radio devices to perform more computationally intensive Fourier transforms to cover the wider band, and part of the problem is that the load on the spectrum is much higher.

With many new devices supporting 5GHz channels, it becomes comparatively more difficult to identify wide swaths of spectrum for wideband transmissions. Nevertheless, a move to 80MHz channels is assured, and it offers readily accessible speeds that are faster than any 802.11n device will ever achieve.

THE FUTURE: Major Wi-Fi changes ahead

A second protocol feature used to increase speed is the use of more aggressive modulation techniques. Development of higher data rates in 802.11 has used a technique called Quadrature Amplitude Modulation (QAM). QAM works by packing multiple bits into a single time slice through the clever use of phase shifts and amplitude changes. For each transmission interval, a set of bits is plucked out from a "constellation" of symbols. To increase the bit rate, the constellation needs to get more data points.

After being pioneered by 802.11a more than 10 years ago, the size of the constellation has remained at 64 points. 802.11ac is increasing the maximum constellation size to 256 points. Moving from 64-QAM to 256-QAM provides a speed boost of 20%. However, nothing is free. With more targets in the constellation, the hardware must work significantly harder to control a parameter called Error Vector Magnitude (EVM). EVM is difficult to explain in the space constraints here but if you want to learn more see this blog post.

802.11ac also has taken the opportunity to simplify one promising 802.11n feature: Most access points use omnidirectional antennas that direct radio energy equally in all directions no matter where the intended receiver of a transmission may be located. With the multiple-antenna arrays required for 802.11n, access points gained the capability of selectively focusing radio energy in a particular direction using a technology called explicit beamforming. By taking a measurement of the radio channel between two devices, it was possible for an AP to determine how to transmit a signal to focus the energy. 802.11n had multiple ways of performing the explicit measurement, with the unfortunate side effect that none was ever widely adopted. 802.11ac has simplified beamforming by settling on one protocol for explicit measurement.

Even as preparations are underway for the first wave of 802.11ac devices, momentum is building for a second wave following closely behind. An additional bump in channel width to 160MHz is possible, though the availability of such wide channels is subject to regulatory developments. 160MHz channels offer multi-gigabit throughput, but at significant costs in terms of battery life and limited spectral availability.

Beamforming is a key to increasing the efficiency of the radio channels used by 802.11ac. The 802.11ac standard includes up to eight spatial streams (compared to just four in 802.11n today). However, 802.11ac specifies that a maximum of four streams are available to a single client device.

In the first wave of 802.11ac devices, this restriction does not come into play, but it offers interesting possibilities for the future. In Multi-User MIMO (MU-MIMO), the AP can divide its transmit streams between multiple devices. As an example, say that an AP allocates four of six transmit streams to a laptop performing a high-speed file transfer, and each of its two remaining streams to two tablet devices. All three streams are active simultaneously, and can have different beamforming steering information applied.

With the rise of smartphones and tablets, the existence of pervasive wireless LANs is just assumed to exist by most users. The continued development of robust, high-speed technologies such as 802.11ac will meet the needs of users for the next several years. [Also see: "Wi-Fi client surge forcing fresh wireless LAN thinking"]

Matthew Gast is a product manager at Aerohive. Praveen Mehrotra is a senior engineer at Aerohive and a participant in the development of the Wi-Fi Alliance VHT5 certification program.

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