Specifications

R&D
examining technology and putting it to use
50
JUL 2011 maximumpc.com
MAXIMUMPC
BY BILL O’BRIEN
With eight transmitting antennas and theoretical data transfer
rates of 1Gb/s, Wi-Fi is about to become turbocharged
802.11ac
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 specifi cations of this budding standard and its
potential availability below.
THE CURRENT STATE OF AC
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 specifi cations for 802.11ac, as cur-
rently defi ned, 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, typi-
cally 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 specifi cation. The
upside of these multistation enhancements is
that routers or host computers will be theoreti-
cally 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 through-
put while also ensuring signal strength and
fi d e l i t y .
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.)
Other features include improved beamform-
ing, which will enable the multiple signal emis-
sions to work together, and MAC modifi cations to
support the multiclient changes noted above. The
standard as currently specifi ed 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 theoreti-
cal throughput rates—sometimes embarrass-
ingly so. 802.11ac will be faster than 802.11n,
but probably not as fast as the throughput
rates claim. For example, 802.11ac will prob-
ably 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
fi le synchronization and backup, and may even
permit direct transmission of wireless video
signals.
The chart above describes a series of possible 802.11ac usage scenarios based on device
and network confi gurations.
Scenario
Typical Client Form
Factor PHY Link Rate
Aggregate
Capacity
1-ANTENNA AP, 1-ANTENNA
STA, 80MHz
Handheld 433Mb/s 433Mb/s
2-ANTENNA AP, 2-ANTENNA
STA, 80MHz
Tablet, Laptop 867Mb/s 867Mb/s
1-ANTENNA AP, 1-ANTENNA
STA, 160MHz
Handheld 867Mb/s 867Mb/s
2-ANTENNA AP, 2-ANTENNA
STA, 160MHz
Tablet, Laptop 1.73Gb/s 1.73Gb/s
4-ANTENNA AP, 4 1-ANTENNA
STAs, 160MHz (MU-MIMO)
Handheld 867Mb/s to each STA 3.47Gb/s
8-ANTENNA AP, 160MHz
(MU-MIMO)
—1 4-ANTENNA STA
—1 2-ANTENNA STA
—2 1-ANTENNA STAs
Digital TV, Set-top
Box, Tablet, Laptop,
PC, Handheld
3.47Gb/s to 4-antenna STA
1.73Gb/s to 2-antenna STA
867Mb/s to each 1-antenna STA
6.93Gb/s
8-ANTENNA AP, 4 2-ANTENNA
STAs, 160MHZ (MU-MIMO)
Digital TV, Tablet,
Laptop, PC
1.73Gb/s to each STA 6.93Gb/s