This Document is describing the features of new wireless standard 802.11ac

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IEEE 802.11ac: Wi-Fi Standard
Harsh Kishore Mishra
M.Tech. Cyber Security
Centre for Computer Science & Technology
Abstract - Wi-Fi has become such an amazingly
successful technology because it has continuously
advanced while remaining backwards compatible.
802.11ac can be considered the next step after
802.11n, along the path running from 11b, to
11a/g, then 11n, and now 802.11ac. 802.11ac has
the capability to maintain a higher level of
performance at any range, compared with its
predecessors and it is likely to be introduced along
with related amendments to 802.11 including
video-related improvements in 802.11aa (video
transport streams) and 802.11ad (very high
throughput, short-range at 60 GHz). 802.11ac
solves mobile devices problems by significantly
improving range and providing 3 times the
performance, while preserving the battery life.
The goal is to continue the thrust of 802.11n to
extend rates and throughput. In short, 802.11ac
will have the capability to handle our insatiable
demand for robust, high speed connectivity – from
a wide range of devices. This paper explains the
latest advance in Wi-Fi, i.e. 802.11ac, which
provides the next step forward in performance.
Keywords—Wi-Fi, 802.11ac, IEEE Standards,
Wireless LAN Standard, Wi-Fi Advancements
I. INTRODUCTION
The First Wi-Fi enabled devices were introduced in
1997. Over the years, Wi-Fi has become ubiquitous
on laptop computers, tablets, televisions, video game
consoles, and smart phones. Every few years since
the 802.11b amendment was ratified, the industry has
released successive amendments increasing Wi-Fi
data rates and capabilities, but even the latest Wi-Fi
systems are able to interoperate with 1999 equipment
built to the original standard. Luckily the IEEE
802.11 working group and the Wi-Fi Alliance, the
industry bodies standardizing Wi-Fi are already
working on 802.11ac, the successor standard to
802.11n and its corresponding interoperability
certification program. The IEEE 802.11ac
amendment is expected to achieve final IEEE
ratification at the end of 2013 [1].
802.11ac is an improved version of 802.11n offering
higher speeds over wider bandwidths. 802.11ac will
be backward-compatible with 802.11n networks
operating in the 5GHz range and is expected to offer
dramatic improvements in Wi-Fi reliability,
throughput and range. The increase in speed is
achieved by providing wider frequency bands, faster
processing, and multiple antennas.802.11ac is worth
having when it is available, and especially when the
client mix converges to being dominated by 802.11ac
devices.
II. 802.11ac STANDARD
802.11ac represents the fifth generation of IEEE
802.11 WLAN standards. The IEEE 802.11 standard
refers to the PHY rates of 802.11n as high throughput
(HT) and those of 802.11ac as very high throughput
(VHT) while those prior to 802.11n are non-HT [1].
The important new technologies in 802.11ac should
be considered as extensions of the physical layer
wireless techniques pioneered in 802.11n, notably
using multiple antennas at the transmitter and
receiver to exploit multiple input/multiple output
(MIMO) for parallel delivery of multiple spatial
streams. The 5 GHz channel of 802.11ac is much
cleaner with less interference, with 23 non-
overlapping channels − 8 times more than what is
available in the 2.4 GHz spectrum − which makes it
far more suitable for applications such as video
streaming and gaming, which are very sensitive to
packet loss and delay [4]. It is expected to deliver a
data rate connection of at least three times that of
802.11n. Many of the algorithms of 802.11n are
being reused but enhanced, with 802.11ac, which
should make the technology easy to fold into existing
networks [3].

2. Registration Number: CUPBMTECH-CSSETCST2013-1401 CBS.503
It’s also better equipped to handle the seemingly
boundless growth in the number and type of Wi-Fi
devices (even many appliances are becoming Wi-Fi
equipped), as well as the corresponding traffic that
comes with that growth. 802.11ac will significantly
enhance the user experience by improving the
playback quality to any point throughout the house.
III. 802.11ac TECHNOLOGY
Among the technologies that 802.11n and 802.11ac
have in common [3]:
Channel bonding for wider channels and greater
throughput
Multiple input, multiple output (MIMO) antenna
technology to avoid multipath interference problems
and improve data throughput
Air time fairness to prevent overall network
performance from “falling back” to the transmission
speed of the slowest device on the network. In other
words, just as an 802.11a client joining a 5GHz
802.11n network no longer degrades an 802.11n
client’s performance in the 5GHz band, a 5GHz-band
802.11n client is not expected to degrade the
performance of an 802.11ac client.
This section gives a brief overview of the new
features and technologies in 802.11ac [1].
Wider RF channel bandwidths: it is clear that
doubling the RF channel bandwidth allows twice the
data throughput, representing a significant
improvement. The 40-MHz channel of 802.11n is
extended to 80- and 160-MHz in 802.11ac. There are
practical obstacles to using these wider channels, but
now that they are defined, equipment will be
developed to use them. The details:
• 80-MHz and 160-MHz channel bandwidths are
defined
• 80 MHz mandatory, 160 MHz optional
• 80-MHz channels are two adjacent 40-MHz
channels but with tones (sub channels) in the
middle filled in.
• 160-MHz channels are defined as two 80-MHz
channels. The two 80-MHz channels may be
contiguous or noncontiguous.
More spatial streams: 802.11n defines up to four
spatial streams, although there are to date few chips
and APs using more than three streams. 802.11ac
extends this to eight spatial streams. There will be a
number of consequences. A divergence between
chips and equipment for APs (with four+ antennas)
and clients (typically with < four antennas) will occur
due to cost, physical size and power constraints.
APs will grow by adding antennas, while clients will
become more capable by implementing multiple
spatial streams and beam forming features behind a
smaller number of antennas. This divergence will
create opportunities for multi-user MIMO, where a
high-capacity AP can communicate with multiple,
lower-throughput clients simultaneously.
• Support for up to eight spatial streams (vs. four
as in 11n) in both single-user (SU) and multi-
user (MU) MIMO
• No more than four spatial streams per client in a
MU transmission
• For each user in an MU transmission, all spatial
streams have same MCS
• In single-user transmission, all spatial streams
have same MCS
Beamforming: Another feature that is expected to
boost the reliability of the connection at required
speed and range is the much improved
“beamforming” standard, which provides directional
signal transmission and reception. Previous standards
can only receive and transmit omnidirectional
Figure 1: 802.11ac Beamforming Technology

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signals, which are subject to significant levels of
interference, due to the fact that the signals are
transmitted indiscriminately in every possible
direction. With beamforming, there’s an
understanding of the relative location of the device,
and the signal is correspondingly strengthened in that
direction.
Multi-user MIMO (MU-MIMO): Thus far, all
802.11 communications has been point-to-point (one-
to-one) or broadcast (one-to-all). With 802.11ac, a
new feature allows an AP to transmit different
streams to several targeted clients simultaneously.
This is a good way to make use of the expected
surplus of antennas at APs over clients, and it
requires beam forming techniques to steer signal
maxima over the desired clients while minimizing the
interference caused at other clients.
For example, if an AP wishes to use MU-MIMO for
clients A and B simultaneously, it will beam form the
transmission for A so it presents a maximum at A but
a minimum at B, and vice versa for the transmission
for B. There are some new terms associated with this:
• Space Division Multiple Access (SDMA): A term
for streams not separated by frequency or time,
but instead resolved in space like 802.11n-style
MIMO.
• Downlink MU-MIMO where the AP transmits
simultaneously to multiple receiving devices is
an optional mode.
Modulation and coding: As semiconductor radios
become ever-more accurate, and digital processing
ever-more powerful, 802.11ac continues to exploit
the limits of modulation and coding techniques, this
time with the leap from 64-quadrature amplitude
modulation (QAM) to 256-QAM.
256-QAM, rate 3/4 and 5/6 are added as optional
modes. For the basic case of one spatial stream in a
20 MHz channel, this extends the previous highest
rate of 802.11n from 65 Mbps (long guard interval)
to 78 Mbps and 86.7 Mbps respectively, a 20% and
33% improvement. (Note that 802.11ac does not
offer every rate option for every MIMO
combination).
Below is a summary of additional elements and
features.
• Single sounding and feedback method for beam
forming (vs. multiple in 11n). This should enable
inter-vendor beam forming to work with
802.11ac devices; the diversity of optional
feedback formats in 802.11n resulted in differing
implementations and stifled adoption.
• MAC modifications (mostly to adapt to above
changes)
Figure 2 : Single and Multi User MIMO
Figure 3 : How 802.11ac accelerated 802.11n

4. Registration Number: CUPBMTECH-CSSETCST2013-1401 CBS.503
• Coexistence mechanisms for 20-, 40-, 80- and
160-MHz channels, 11ac and 11a/n devices.
Extensions of 802.11n techniques to ensure that
an 802.11ac device is a good neighbor to older
802.11a/n equipment.
• Non-HT duplicate mode duplicates a 20-MHz
non-HT (non-802.11n) transmission in four
adjacent 20-MHz channels or two sets of four
adjacent 20-MHz channels. Sometimes termed
quadruplicate and octuplicate mode.
IV. ADVANTAGES OF 802.11ac
The main advantages of the standard – speed,
reliability, and quality. But apart from the “cool
factor”, or advancement solely for the sake of
advancement, why should a user consider moving to
the new standard? What needs can it meet better or
more easily than the current standard? The answer is
that the new standard has two main advantages for
the everyday user – it improves current use cases and
paves the way for future use cases.
In addition to meeting today’s growing needs such as
streaming video, the new standard will also enable a
variety of new use cases such as simultaneous HD
video streams to multiple receivers, wireless displays,
and large file wireless transfers. It’s also better
equipped to handle the seemingly boundless growth
in the number and type of Wi-Fi devices (even many
appliances are becoming Wi-Fi equipped), as well as
the corresponding traffic that comes with that growth.
In short, 802.11ac will have the capability to handle
our insatiable demand for robust, high speed
connectivity – from a wide range of devices.
802.11n does include many options with reduced
value. 802.11ac takes a very pragmatic approach to
them. If a “useless” option is used and affects a third-
party device, then typically 802.11ac forbids an
802.11ac device (operating in 802.11ac mode) from
using the option. If a “useless” option has not been
used in 802.11n products or only affects the devices
that activate the option, then the feature is not
updated for 802.11ac but is instead “left to die.”
Another area of concern that will be addressed by
802.11ac is the Wi-Fi performance for mobile
devices like Smartphones and tablets. Dropped
connections, poor quality connections, and limited
mobility are major areas of frustration for users
today. 802.11ac solves these problems by
significantly improving range and providing 3 times
the performance, while preserving the battery life.
Wireless LAN sites will see significant
improvements in the number of clients supported by
an access point (AP), a better experience for each
client, and more available bandwidth for a higher
number of parallel video streams. Even when the
network is not fully loaded, users see a benefit: their
file downloads and email sync happen at low lag
gigabit speeds. Also, device battery life is extended,
since the device’s Wi-Fi interface can wake up,
exchange data with its AP, then revert to dozing that
much more quickly [2] .
Speed is largely irrelevant if the connection lacks
reliability. For example, most users have experienced
the irritating video buffering during video playback,
which causes frozen or jittery screens. By increasing
the bandwidth capacity and improved processing,
802.11ac enables far more bandwidth to be available
for consumption by wireless devices, which helps
avoid interference, and improves the speed for
demanding applications such as hi-definition video
streaming. The result is more effective coverage with
fewer dead zones. The 3X speed improvement
achieved by the new standard means that the 450
Mbps performance from today’s fastest 3 antenna
802.11n device can be achieved by single antenna
802.11ac device – with similar power consumption.
This means that a typical tablet with single antenna
802.11n 150Mbps Wi-Fi can now support 450 Mbps
with 802.11ac − without any increase in power
consumption or decrease in battery life [4].
Table 1: Wireless Performance Comparison
Antenna
Configuration
802.11n 802.11ac
Single Stream 150 Mbps 450 Mbps
Dual Stream 300 Mbps 900 Mbps
Triple Stream 450 Mbps 1.3 Gbps

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Key advantages of 802.11ac over 802.11n [4]:
• Gigabit speed wireless with approximately 3
times the performance of 802.11n.
• Better performance at any range with fewer dead
spots and backward compatibility.
• More reliable connections for media streaming
with beam-forming.
• More Wi-Fi bandwidth on your mobile.
• Only utilizes the 5 GHz Band, which is less
prone to interference.
V. CONCLUSION
802.11ac will be backward-compatible with 802.11n
networks operating in the 5GHz range and is
expected to offer dramatic improvements in Wi-Fi
reliability, throughput and range. There’s a fine
balance between accommodating the high density of
these devices with enough channels to avoid co
channel interference and reaping the aggregate
throughput benefits of the greater channel widths of
80MHz and, eventually, 160MHz, which have been
specified by 802.11ac standards [3].
801.11ac is expected to be ratified by IEEE late 2013.
The earliest, pre-ratified products are expected late
2012 and will likely ship for the home/consumer
market. From there, it’s expected that the rollout of
new IEEE 802.11ac devices will take between one
and three years. By 2015, according to experts, all
new Wi-Fi products coming to market are expected
to be based on 802.11ac technology. 802.11ac-
enabled products are the culmination of efforts at the
IEEE and Wi-Fi Alliance pipelines. IEEE 802.11ac
delivered an approved Draft 2.0 amendment in
January 2012 and a refined Draft 3.0 in May 2012,
with final ratification planned for the end of 2013. In
parallel, the Wi-Fi Alliance is expected to take an
early IEEE draft, most likely Draft 3.0, and use that
as the baseline for an interoperability certification of
first-wave products in early 2013. Later, and more in
line with the ratification date of 802.11ac (that is,
after December 2013), the Wi-Fi Alliance is expected
to refresh its 802.11ac certification to include testing
of the more advanced 802.11ac features [2].
REFERENCES
[1] Aruba Networks, Inc. (2012). 802.11ac In-Depth. Sunnyvale,
California, USA.
[2] Cisco Systems, Inc. (2012, August). 802.11ac: The Fifth
Generation of Wi-Fi. San Jose, CA, USA.
[3] Motorola Solutions. (2012, July). What You need to know
about 802.11ac. USA.
[4] NETGEAR. (2012). Next Generation Gigabit Wi-Fi -
802.11ac. USA.