What is 802.11ac, anyway?

-September 09, 2014

IEEE 802.11ac has a lot to add to the wireless family. It brings a significant improvement over 802.11n. What are the differences between 802.11n and 802.11ac?

802.11n uses a system called MIMO (Multiple-Input Multiple-Output) to transmit multiple spatial streams to another device that implements 11n. This is accomplished by using spatial multiplexing, a technique used in wireless MIMO technology to transmit multiple data streams simultaneously.

With MIMO technology, 802.11n devices could reach a maximum theoretical data rate of 600 (Mbits/s). 802.11ac outlines the use of eight spatial streams, which has a theoretical data rate of 6933.3 Mbits/s. This is a huge boost over 802.11n because it allows for gigabit speeds over wireless. These are, of course, the best case rates. Most 802.11n devices will perform at 405.0-450.0 Mbits/s because vendors typically only implement up to three spatial streams. In 802.11ac, three spatial streams will perform at about 2106-2340 Mbits/s, which still represents a large increase over 802.11n.

In addition to the number of spatial streams, there are other optional features of 802.11n and 802.11ac that help them reach their data rates. 802.11n devices normally operate on 20 MHz channels, though 802.11n allowed for channel bonding, turning two channels into one. Three 20-MHz spatial streams produce about 195-216.7 MBits/s, and three spatial streams at 40 MHz is roughly 405.0-450.0 Mbits/s. 802.11n has been restricted to a 40 MHz channel width as a result of the limitation of the 2.4 GHz band on which it operates. 802.11n can also operate on the 5 GHz band, which is a much larger spectrum, but because the IEEE 802.11n standard also outlines implementation in the 2.4 GHz band, there was no way to increase channel width.

802.11ac operates exclusively in the 5 GHz band, which lets it create a channel width of up to 160 MHz. This is a benefit because the 2.4 GHz band has become congested as a result of the number of devices using it. 802.11ac accommodates 80 MHz, 160 MHz, and 80+80 MHz channels. Mandatory channel widths include 20 MHz, 40 MHz, and 80 MHz. Three spatial streams at 80 MHz produces about 1170–1300 Mbits/s. 802.11ac also adds 256-QAM to the OFDM (orthogonal frequency-division multiplexing) modulation scheme. This addition allows for more data bits to be transmitted per SS allowing for increase in rates. The figure below shows the channel for two center frequencies separated by 160 MHz.

IEEE 802.11ac doesn't just bring higher data rates to the table. Theoretically, it also should let every user take advantage of the wireless medium. 802.11ac's "big thing" is MU-MIMO (Multiple-User Multiple-Input Multiple-Output). This means that devices should be able to simultaneously transmit traffic. In other 802.11 technologies, devices typically have to wait to capture a TXOP (transmit opportunity) before they can take their turn to transmit. 802.11ac devices allow for multiple users to transmit simultaneously using what is called beamforming, or beam-steering. In this situation, two devices exchange spatial verities and angles to determine where to steer their beams. One is called the beamformer and the other is called the beamformee. They are constantly exchanging spatial information so they don't lose track of each other. This tracking allows devices to transmit at the same time because their beams are directed toward whomever they want to receive the traffic. Beamforming allows for a very efficient use of the wireless medium because devices don’t spend time contending for it.

While all of these things are very impressive and help make wireless reliable and fast, consumers should also be aware of the limitations of IEEE 802.11ac.

Although 802.11ac is backwards compatible with 802.11n, this does not mean that a 802.11n wireless card can use AC. A computer with an 802.11n card connected to an AC router is limited by that card. It will only be capable of using 802.11n, not 802.11ac. In order to take advantage of 802.11ac, a wireless card capable of using the technology is necessary. Once this type of wireless card is in use, throughput will increase drastically.

The wireless card's supported features must match that of the router to reach its maximum potential. For example, in the case of an 802.11ac wireless card capable of one spatial stream paired with a router capable of three spatial streams, the router would only transmit to the wireless card using one stream because that is the maximum that card supports. This concept is the same for channel width. Overall channel width is limited by the highest channel width that the wireless card supports. This incompatibility of technologies is a common misconception of wireless in general.

The UNH-IOL’s Wireless Consortium works closely with this new technology. It has many of the latest 802.11ac devices from major companies.

My colleagues and I are implementing an 802.11ac Interoperability test plan to help vendors test against other 802.11ac devices. Our consortium also offers AC pre-certification for the Wi-Fi Alliance's new AC certification program using the Wi-Fi Alliance's SIGMA software.

IEEE 802.11ac is a powerful wireless technology that improves on the ideas of 802.11n to help users have fast and reliable wireless connections. This technology presents new opportunities for vendors, and it will be fascinating to see how it is utilized.

Also See
Measure throughput of cellular and WiFi MIMO radios, part 1
Measure throughput of cellular and WiFi MIMO radios, part 2
Measure throughput of cellular and WiFi MIMO radios, part 3
Measurements for the new WLAN standard: IEEE 802.11ac
Introduction to IEEE 802.11ac manufacturing test requirements
802.11ac Technology Introduction
WLAN 802.11ac Technology Introduction and Measurement Solutions

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