802.11ac and Wi-Fi® Client Devices

September 21, 2012, 10:26 am

802.11ac is a draft standard for Wi-Fi® operation. It is a superset of 802.11n and, more importantly, is interoperable with 802.11n and previous standards.

According to a white paper from Ixia:

  • Video content is expected to exceed 70% of all internet traffic.
  • Increased demand for video content is driving the need for greater throughput over Wi-Fi.  Without it, videos take longer to load, and they can be choppy when viewed.
  • In addition to video streaming, data syncing between mobile devices and data backup require higher throughput than is available with today’s 802.11n networks.
  • 802.11ac is designed to meet the need for higher throughput over Wi-Fi.

802.11n introduced several throughput-boosting technologies, including:

  • MIMO (multiple-in, multiple-out), which enables traffic delivery over multiple spatial streams at the same time
  • Packet aggregation, or the delivery of multiple packets in a single transmission

802.11ac further boosts performance through four enhancements:

  1. 256-QAM: The highest-rate encoding scheme supported by 802.11n is 64-QAM.  To increase the transport rate at the physical layer, 802.11ac supports 256-QAM, which transmits 33% more data than 64-QAM. Signal-to-noise ratios that worked for 802.11n are no longer sufficient for the higher speeds in 802.11ac because the difference in detectable signal level is now significantly smaller.
  2. Wider channels: Before 802.11n, all Wi-Fi channels had a width of 20 MHz.  802.11n supports channel widths of 20 MHz and 40 MHz, where the latter supports transmission of twice as much data per second.  802.11ac supports channel widths of 20, 40, 80, and 160 MHz. Because the 2.4 GHz band supports a maximum of three non-overlapping 20-MHz channels, 802.11ac can be used effectively only in the 5 GHz band, which supports up to 24 non-overlapping 20-MHz channels.
  3. More spatial streams: 802.11n accommodates up to four spatial streams; 802.11ac allows for up eight spatial streams.
  4. MU-MIMO: With 802.11n and its predecessors, an access point (AP) can direct traffic to only one client at a time.  When an AP is transmitting to a mix of fast and slow client devices, the “fast traffic” is delayed by transmissions to slower clients. With 802.11ac, an AP can use Multi-User MIMO (MU-MIMO) to transmit data to multiple client devices on the same channel at the same time. MU-MIMO works by directing one or more spatial streams to one client and other spatial streams to a second client.

As with all Wi-Fi standards, the highest data rates for 802.11ac will be available only at relatively short distances. As the distance between the client device radio and the infrastructure device radio increases, the data rate drops.

Infrastructure devices such as APs and home routers can support all four enhancements.  Expect to see broad 802.11ac support on infrastructure devices in the first half of 2013, even before the standard is ratified and the Wi-Fi Alliance begins conducting interoperability tests.

Client devices that want to use streaming video, such as smartphones and tablets, will be the first in line for 802.11ac support.  But initial 802.11ac support on many of these client devices may prove disappointing.

One reason is that, because of its support for channels as wide as 160 MHz, 802.11ac will be available only in the 5 GHz band.  Today, the Wi-Fi radios in the vast majority of smartphones, tablets, and other consumer devices operate only in the 2.4 GHz band.  As detailed in our white papers, 5 GHz operation is very different than 2.4 GHz operation, and most device makers have little or no experience with 5 GHz operation.  Expect initial implementations to be buggy.

Another reason is that most client devices will not support multiple spatial streams, because:

  • Such support requires the use of more than one antenna simultaneously, something that most client device makers have never attempted.
  • Antennas must be over 5 cm  (one 5 GHz wavelength) apart, which is difficult to accomplish on a smartphone or other small device.
  • Spatial streams work only when a client device is stationary; smartphones often are mobile when in use.

The most compelling 802.11ac enhancement, MU-MIMO, does not require client devices to support 802.11ac.  But initial 802.11ac infrastructure devices may lack support for MU-MIMO; support is not expected to be widespread until 2014 or 2015.

802.11ac offers a lot of promise, but 802.11n remains the safe bet, at least in the client realm, for the next few years.

For more information on 802.11ac, check out the Network World® article, 11ac Will be Faster, but How Much Faster Really?