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Not much has happened to good old Wi-Fi since 802.11n arrived on the scene about six years ago, but a new protocol that the 802.11 WG (Working Group) is currently stirring up might turn out to be much bigger and way faster than 802.11n. It’s called 802.11ac, and it promises a whopping 1Gb/s throughput by improving modulation and extending 802.11n’s MIMO scheme to extreme levels. The only real bad news is that we may have to wait a while to experience it. We’ll explore the specifications of this budding standard and its potential availability below.
Where 802.11n offered a dual-band solution (2.4GHz and 5GHz), 802.11ac operates solely in the 5GHz (VHT, or very high throughput) band. This is still considered a cleaner spectrum than 2.4GHz, despite its use in 802.11n, because few 802.11n access points actually use much of the higher band.
The basic specifications for 802.11ac, as currently defined, are as follows:
Wider channel bandwidths: 80MHz and 160MHz channel bandwidths (vs. 40MHz maximum in 802.11n). The 80MHz channel is mandatory for stations (STAs); 160MHz is optional.
More MIMO spatial streams: Support for up to eight spatial streams (vs. four in 802.11n).
Multiuser MIMO: Multiple stations (STAs, typically handheld or mobile devices), each with one or more antennas, can transmit or receive independent data streams simultaneously. Downlink MU-MIMO (a single transmitting device with multiple receiving devices) is an optional mode within the specification. The upside of these multistation enhancements is that routers or host computers will be theoretically capable of streaming HD video to multiple clients throughout a networked environment.
Space Division Multiple Access (SDMA): Streams of data are resolved spatially as opposed to by frequency. This is similar to 802.11n’s MIMO approach and boosts throughput while also ensuring signal strength and fidelity.
Modulation: 256-QAM (quadrature amplitude modulation), rate 3/4 and 5/6 is used to carry data, as opposed to 64-QAM, rate 5/6 in 802.11n. The result should be considerably improved throughput. (This is not the same as the digital television QAM standard.)
The chart above describes a series of possible 802.11ac usage scenarios based on device and network configurations.
Other features include improved beamforming, which will enable the multiple signal emissions to work together, and MAC modifications to support the multiclient changes noted above. The standard as currently specified is also backward compatible for 20/40/80/160MHz channels as well as 802.11a/b/n devices.
It’s worth noting that while 802.11ac’s goal is to produce transfer rates as high as 1Gb/s, rates will vary depending on the exact scenario. We’ll insert our usual caveat here: Real-life transfer rates are always lower than theoretical throughput rates—sometimes embarrassingly so. 802.11ac will be faster than 802.11n, but probably not as fast as the throughput rates claim. For example, 802.11ac will probably operate in the 350Mb/s range, not 1Gb/s—which is still a huge step up from 802.11n’s 160Mb/s (or so).
This said, given the use of multiple signals, it’s theoretically possible that 802.11ac might even be able to exceed the maximum given exaggerated MU-MIMO conditions. At the very least, this architecture will permit much faster file synchronization and backup, and may even permit direct transmission of wireless video signals.
As if we haven’t had enough of competing standards over the years, the 802.11 Working Group is also working on an 802.11ad specification that operates in the 60GHz bandwidth spectrum. Fortunately, it and 802.11ac are not competitive. They can, in fact, be used in complementary situations. For example, using both 5GHz and 60GHz interfaces, it’s possible to carry typical network data on the 802.11ac portion throughout the house while using the 802.11ad specification for streaming media within rooms. Assumptions, at this point, indicate that 802.11ad and its potential 6Gb/s transfer rate should be able to handle as many as three HD videos simultaneously.
The semi-bad news is that 802.11ad parallels WiGig’s goals. And while 802.11ad is still to come, WiGig already enjoys support from Atheros, Broadcom, and Intel. Despite the considerable stature of these three companies, this is only semi-threatening to 802.11ad because support and alliances are routinely abandoned and/or assimilated with frightening regularity for a variety of reasons.
As always, backward compatibility is a mixed bag. Its presence is understandable, but insisting on it often ensures that weaknesses built into prior technology limits performance. With 802.11n equipment already in use, it would be interesting to see the spec architects draw a line in the ether and offer a fresh starting point for a new class of WLAN. This is not likely.
Assuming that the ISPs don’t start throttling bandwidth—a valid concern given the recent data limit edicts by AT&T—the implications of real-world data transfer rates of 350Mb/s are potentially revolutionary, particularly when used in tandem with 802.11ad devices. Video transmission, networked virtualization, remote control, and basic large-file transfers all suddenly become much more practical.
So when will we get our hands on 802.11ac tech? The sad answer is not anytime soon. The standard will likely be finalized in late 2012. Assuming this is the case, Working Group approval probably won’t come until a year or so later in late 2013, which means we probably won’t see the release of officially sanctioned 802.11ac consumer devices until then.
But, just like with 802.11n devices, it is likely that we’ll be faced with confusing standards before the final 802.11ac spec is approved. Remember “draft-n” and its variants? We’ll probably face the same coin toss with the same probability of buying noncompatible gear. Our take is that it’s a small price to pay for doubling our wireless transfer rates.
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