Toward a Gigabit Wi-Fi Nirvana: 802.11ac and 802.11ad

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.

Today's existing state-of-the art wireless LAN can achieve 300 Mbps using 802.11n with two spatial streams. Future developments will deliver three- and four-stream speeds of up to 600 Mbps. But the 802.11 working group has set its sights on a more ambitious milestone: 1 Gbps throughput.

After considering several approaches for getting to gigabit speeds, the 802.11 WG settled on two related approaches, and formed two task groups to produce future gigabit standards: 802.11ac and 802.11ad. While both groups share the same goal, the approaches taken are different because the groups have fundamentally different purposes.

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Fundamentally, all wireless LAN standards depend on access to radio spectrum. 802.11ac will be designed for use at frequencies under 6 GHz, which in practice refers to the existing radio spectrum available today in the 2.4 GHz and 5 GHz bands used by 802.11a/b/g/n. Therefore, an important component of the work in Task Group AC will be to design backward-compatibility mechanisms to peacefully coexist with existing networks.

Higher data rates in 802.11ac are supported by a set of familiar techniques. Once again, the speed will be supported by well-understood OFDM techniques, another bump up in the size of radio channels, and MIMO. Advances in both chip manufacturing technology and processing power have also made it possible to use more sensitive coding techniques that depend on finer distinctions in the received signal as well as more aggressive error correction codes that use fewer check bits for the same amount of data.

Wider radio channels support higher speeds. Just as 802.11n provided a leap in speed by doubling channel width from 20 MHz to 40 MHz, 802.11ac provides a bump in throughput with still-wider 80 MHz channels. At 80 MHz, channel layout once again becomes a challenge, even in the relatively expansive 5 GHz spectrum. Manufacturers will need to adapt automatic radio tuning capabilities to offer higher-bandwidth channels only where necessary to conserve spectrum.

Increasing data rates through efficiency is an important goal of every new 802.11 standard. One common measure of efficiency is the number of megabits transmitted per megahertz of spectrum (Mbps/MHz). 802.11 began life at 0.1 Mbps/MHz, and current 802.11n standards have pushed that figure to 7.5 Mbps/MHz. Several efficiency enhancements are on the drawing board for 802.11ac, and the most interesting of these is multi-user MIMO (MU-MIMO).

MU-MIMO builds on the beamforming capabilities of 802.11n and enables the simultaneous transmission of different data frames to different clients. Correctly using MU-MIMO requires that vendors develop spatial awareness of clients and sophisticated queuing systems that can take advantage of opportunities to transmit to multiple clients when conditions are right.

802.11ad has the same gigabit goal, but is intended for use with new spectrum around 60 GHz to use. Range will be shorter, but the spectrum is "cleaner" because many fewer devices use it today. The open spectral band is large enough that the current 802.11ad draft supports nearly 7 Gbps throughput.

The higher data rates of 802.11ac and 802.11ad will have far-reaching influences into other areas of the protocol. CCMP, the existing encryption protocol first standardized in 802.11i, requires two AES encryption operations for every 16 bytes of data. To encrypt a 1,500-byte frame requires roughly 200 AES encryption operations. To make matters worse, CCMP is based on a "chained" mode of operation that requires in-order processing of the 16-byte chunks because chained cryptographic modes require the output of one stage to be used as the input to the next. Many engineers within the 802.11 working group expect that the high data rates of 802.11ac and 802.11ad will be too high for CCMP.

Fortunately, a solution is readily available in the form of the Galois/Counter Mode Protocol (GCMP), which has been incorporated into the 802.11ad draft. GCMP uses the same AES cryptographic engine, but embeds it into a more efficient framework. Compared with CCMP, GCMP requires only half the number of encryption operations, and, more importantly, is not chained so that GCMP cryptographic acceleration can be applied to an entire transmitted frame in parallel. The downside of the adoption of GCMP is that it is a new protocol and will only become available in new radio chips that support it, and an entire generation of centralized cryptographic equipment, such as the security processors in WLAN controllers, will become obsolete.

As with every jump in speed that has occurred in Wi-Fi, 802.11ac and 802.11ad present challenges for the network administrator. The move to gigabit Wi-Fi is needed to keep up with demand for Wi-Fi network capacity and enable Wi-Fi to remain the technology of choice at the edge.

Aerohive Networks is the leader in fully distributed, intelligent Wi-Fi systems.

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