INTRODUCTION OF 802.11AC
Millions of new wireless devices are activated every day. The volume of application traffic
generated by these devices on wireless networks will overtake the total traffic on wired
networks by 2014.The emerging 802.11ac standard will enhance these wireless networks by
delivering much greater performance than today‘s 802.11n standard.
Wireless has become the preferred and often the only form of access. But while the 802.11ac
standard is a significant improvement in performance it is not enough to provide end users
with the application performance and user experience they have come to expect from their
1GB and 10GB wired- Ethernet networks. With the adoption of 802.11ac, theoretical
throughput for Wi-Fi will go from 450Mbps with 802.11n to 1.3Gbps. However, in the real
world with protocol overhead and interference, actual throughput will be closer to 433Mbps.
Figure 1: Wired and Wireless access
Wired networks have moved from shared hubs to dedicated multi-port switches, but the same
architectural shift has not happened for wireless. Wi-Fi network capacity must play catch up to
replace wired systems. Doing so will require utilizing not only 802.11ac but also multi-radio
Wireless has become the primary access to the network. Many consumers are now carrying
two, three, or four devices. That ratio of wireless devices will continue to explode as it reaches
a 7:1 by 2016. Over 1 million smart phones are activated daily with tablets forecasted to
outsell desktops by late 2013. Unlike wired users, mobile users expect to connect anytime,
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anywhere – at home, in the office, in classrooms, and at conferences. Dependency on mobile
applications and cloud-access are now business-critical. To help meet the exploding need for
more wireless performance, the industry is about to release the next Wi-Fi standard: 802.11ac.
Existing 802.11n networks will eventually be replaced by 802.11ac. A key characteristic of
802.11ac is that it operates only in the 5GHz band. This band provides seven times the
channel bandwidth and has less interference compared to the previously more popular 2.4GHz
band. To simplify network migration, 802.11ac will maintain backward compatibility with
existing 802.11n equipment. The following table summarizes the key characteristics of
Table-1: Key characteristics of 802.11ac
Ratification of the IEEE 802.11ac standard and testing by the Wi-Fi Alliance is expected in
2013. Many leading consumer-grade Wi-Fi vendors have begun shipping products based on
this new standard, showing confidence in the imminent passage of the standard.
To achieve higher performance, 802.11ac uses several technology advancements to triple (and
more) the throughput compared to 802.11n:
Enhanced modulation techniques push beyond 64QAM (used by 802.11n) to
256QAM, making spectrum usage 30% more efficient.
Increased channel bandwidth from 20MHz or 40MHz (used by 802.11n) to 80MHz
widens the channel, increasing capacity.
Support for up to 8 spatial streams and 160MHz channels in future versions of
802.11ac takes the maximum data rates up to 6.9Gbps.
Moving from MIMO (multiple-input and multiple-output) technology in 802.11n to
advanced Multi-User MIMO (or MU-MIMO) will add the ability for multiple devices
to connect to each other simultaneously on the same radio.
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IS 802.11AC ENOUGH?
The increase in performance offered by 802.11ac is critical to the success of wireless.
However, is it enough to make wireless the primary connection to the network?
Today, most Wi-Fi networks provide a fraction of the capacity enjoyed by wired users. Most
wired networks run Gigabit Ethernet, delivering close to 100Mbps of bandwidth to the user
(accounting for uplink aggregation). In contrast, most Wi-Fi networks provision 5Mbps or less
per user based on design guidelines by vendors and industry experts. This is unacceptable if
wireless is truly expected to be the primary access technology. See the following table for an
Table-2: Comparison of wired and wireless network capacity
Why should today‘s wireless users accept 5Mbps as good enough? Either we are over
provisioning the wired or we are under-provisioning the wireless. Wired gigabit infrastructure
has enabled networked medical imaging, HD video services, life-size video telepresence, and
much more. The same high-capacity performance is needed on our wireless networks to
deliver these services.
802.11ac brings us another step up in performance but by itself is not enough. 433Mbps of
achievable throughput on an 802.11ac radio shared between 30 users is still only about
15Mbps per user. Such performance will not effectively replace wired networks.
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A BRIEF HISTORY OF THE 802.11 PROTOCOL
2.1 EVOLUTION OF IEEE 802.11X STANDARDS
Originally, equipment based on the IEEE 802.11 wireless-local-area-networking (WLAN)
standards (first IEEE 802.11a and b and 802.11g in 2003) was used as a means of delivering
simple web browsing and e-mail connectivity ―on the road‖—in airports, hotels, Internet cafes,
and shopping malls. Since then, such equipment has moved firmly into the home and homeoffice environment. Multiple devices now operate in connection to each other: computers,
smartphones, tablets, printers, game consoles, media servers, scanners, and more. In addition,
we want access to all of our stored material—data, pictures, whatever—from devices as small
as a smartphone or as large as the screen in an auditorium—and to be able to share it with
friends and colleagues instantly. We also want speeds that match a wired gigabit LAN
connection, without the cables. The IEEE 802.11x standards are steadily evolving to meet all of
these desires and demands as in Figure 2.
Figure2: Overview of the IEEE 802.11x standards family
IEEE 802.11n was introduced in 2009, improving the maximum single-channel data rate from
the 54 Mb/s of IEEE 802.11g to more than 100 Mb/s. It also introduced multiple-input
multiple-output (MIMO), or ―spatial streaming,‖ communications. As many as four separate
physical transmit and receive antennas carry independent data, which is aggregated in the
modulation/demodulation process. Projections call for even higher data throughput
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requirements in future home and office applications. To cater to these, two new IEEE project
groups have been formed with the goal of providing ―very high throughput‖ (VHT).
Working Group TGac hopes to specify IEEE 802.11ac as an extension of 802.11n, providing a
single-link minimum of 500-Mbps and 1-Gbps overall in the 5-GHz band. Meanwhile,
Working Group TGad and the Wireless Gigabit Alliance (WiGig) jointly proposed IEEE
802.11ad, which is targeting short-range speeds to 7Gbps in approximately 2 GHz of spectrum
at 60 GHz. It was officially accepted as a standard at the end of 2012.
With the huge number of existing client devices, backward compatibility with current
standards using the same frequency range is vital. The goal is for all IEEE 802.11 standards to
be backward compatible and for 802.11ac and 802.11ad to be compatible at the mediumaccess-control (MAC) or data-link layer. They should differ only in physical-layer (PHY)
characteristics. Devices could then have three radios: 2.4 GHz for general use (which may
suffer from interference), 5 GHz for more robust and higher-speed applications and 60 GHz
for ultra-high-speed operation within a room—as well as support session switching amongst
A developer’s challenge
Not only does IEEE 802.11ac need to work with ten years‘ worth of previous releases, it also
uses advanced technologies that are substantially more complex and demanding than its
predecessors. This requires a rethinking of how the technology is developed and tested to
include a much more holistic view through the product development lifecycle. One of the most
difficult challenges is that it can be extremely difficult to identify the root cause of
development problems. For example, when an application performs poorly, it is often times
hard to determine if it is due to an environmental, client, or network issue. The various devices
in an 802.11 network are highly correlated so an issue in one area quickly ripples through to
many other areas. Developers have lacked an effective means to assess the total picture from
the RF to the application layer.
Ensuring testing equipment matches 802.11ac’s capabilities
802.11ac brings the promise of turning Wi-Fi into a trusted and capable communication
protocol and will require equipment and rigor to match. Traditionally, the RF section is
verified using one set of equipment, and then the upper layer functions are tested using a
second set of tools. But the overall technical complexity and the introduction of new
technologies demand coordination and control between the different layers of the protocol
stack. Without this coordination, it would be difficult to utilize these functions and to quickly
pinpoint performance issues.
The new generation of testing should be able to decode every frame in real-time and determine
each frame‘s RF characteristics, as well as their frame-level performance, and generate every
frame without limitation in real-time to adequately test receiver performance. Previous
approaches use a digitized data record approach for both generation and analysis, creating or
capturing what are known as I/Q files, and equipment typically adapted from the generalpurpose RF domain. This result in equipment being capable of a single spatial stream and able
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to generate or capture a small fraction of the frames required to perform testing. To meet the
need, the approach needs to be able to generate and analyse all frames in real-time to the limit
of the specification, tightly integrate RF and MAC functionality in 802.11ac, and include
integral, real-time channel emulation.
Figure 3: Distance vs Data Rate of 11n and 11ac
Benefits for all markets
802.11ac is positioned to overcome the digital content challenge on wireless networks in
residential, enterprise, carrier and large venue markets. Residential video streaming, data
syncing between mobile devices, and data backup will be some of the first applications for
802.11ac‘s faster speeds. Consumers and enterprises will be able to stream digital media
between devices faster, and simultaneously connect more wireless devices. Carriers will
deploy the new technology to offload traffic from congested 3G and 4G-LTE cellular
networks, and in dense operator hotspots 802.11ac will supply better performance to more
But chip and hardware developers must navigate some significant technical challenges to fully
experience the performance and density promise of 802.11ac, as detailed in this article.
Providing backward compatibility and delivering high performance, while at the same time
gracefully migrating from existing deployed solutions will be some of their toughest
challenges. Developers must maintain high performance to multiple clients under the channel
conditions that will exist in real deployments. At a time when IT managers report that network
users are now averaging more than one Wi-Fi connected device per person, solutions to handle
the rapid growth of devices are at a premium. With video consumption rapidly increasing,
developers need to ensure that key application traffic can be delivered with quality, while
simultaneously providing the high reliability and feature robustness to enable enterprise and
carrier-grade 802.11 adoption.
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DIFFERENCE BETWEEN 802.11N AND 802.11AC
Until now, all 802.11 revisions have focused on increasing transport speeds. This leads to
higher traffic delivery rates and ultimately to faster response times as experienced by the end
user. The introduction of 802.11n brought advances of MIMO (multiple-in, multiple-out) to
deliver traffic over multiple spatial streams, and packet aggregation. MIMO delivered marked
improvements in physical transport rates, enabling more bits per second to be transmitted than
ever before over Wi-Fi. Packet aggregation delivered equally impressive improvements in
transport experience, allowing devices to send more data once they had gained access to the
wireless media. The new 802.11ac has not only preserved these aggregation techniques, but it
has also advanced the physical transport rates yet again, and introduced the concept of parallel
transport into Wi-Fi through a technique known as Multi-User MIMO(MU-MIMO), where
multiple client devices are receiving packets concurrently.
Figure 4: Generations of Wi-Fi
Now, directed traffic can be delivered to multiple client devices at the same time – a first for
Wi-Fi‘s history. This ability has a significant impact on delivery of content to any location
with multiple users, especially where content is revenue-generating or critical. This will be
especially important for large venues, hotspots, enterprises, and even home video delivery, all
of which stand to experience improved per-user experience.
Fortunately, adoption of 802.11 continually experiences healthy growth, despite that there
have been four major revisions to the base protocol and numerous additions since inception.
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This is because the designers of 802.11 sided with end users by making the protocol backward
compatible. This way, consumers aren‘t forced to immediately upgrade their network each
time a new solution is released. However, any engineer will tell you that the fastest, most
reliable way to deliver a new technology is to eliminate any requirement to interoperate with
previous technologies. This creates several technical challenges for 802.11 developers.
802.11n versus 802.11ac
At first glance, 802.11ac appears to be an exercise intended to make Claude Shannon nervous
by packing more bits into each slice of spectrum and time. Conceptually, 802.11ac is an
evolution from 802.11n and not a revolutionary departure. Many of the techniques used to
increase speed in 802.11ac are familiar after the introduction of MIMO. Unlike 802.11n,
which developed major new MAC features to improve efficiency, 802.11ac uses familiar
techniques and takes them to a new level, with one exception. Rather than using MIMO only
to increase the number of data streams sent to a single client, 802.11ac is pioneering a multiuser form of MIMO that enables an access point (AP) to send to multiple clients at the same
As mentioned, 802.11ac PHY is based on the well known OFDM PHY used for 802.11n, with
some important modifications necessary to meet the 802.11ac‘s goals. Some of the key
technical specifications that distinguish 802.11ac from 802.11n are summarized in Table-3,
and discussed below. As will be discussed, some differences have a significant impact on the
requirements for the test equipment needed to verify the functionality of 802.11ac-enabled
Table-3: Comparison of 802.11ac and 802.11n Technical Specifications
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802.11ac introduces two new channel sizes: 80 MHz and 160 MHz. Just as with 802.11n,
wider channels increase speed. In some areas, 160 MHz of contiguous spectrum will be hard
to find, so 802.11ac introduces two forms of 160 MHz channels: a single 160 MHz block, and
an ―80+80 MHz‖ channel that combines two 80 MHz channels and gives the same capability.
Like previous 802.11 amendments, 802.11ac transmits a series of symbols, each of which
represents a bit pattern. Prior to 802.11ac, wireless LAN devices transmitted six bits in a
symbol period. By using a more complex modulation that supports more data bits, it is
possible to send eight bits per symbol period, a gain of 30%. The extent to which 256-QAM
can be used reliably in real-world deployments is an open question for 802.11ac at this time.
802.11ac radically simplifies the beamforming specifications to one preferred technical
method. Beamforming in 802.11n required two devices to implement mutually agreeable
beamforming functions from the available menu of options. Very few vendors implemented
the same options, and as a result, there was almost no cross-vendor beamforming
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compatibility. With the key features of 802.11ac depending on beamforming, however, a
simplification was required to enable the core technology.
More spatial streams and multi-user MIMO (MU-MIMO)
802.11ac specifies up to eight spatial streams, compared to 802.11n‘s four spatial streams, at
the AP. The extra spatial streams can be used to transmit to multiple clients at the same time.
With the ability to transmit at high speeds to multiple clients simultaneously, 802.11ac will
speed up networks even more than might be apparent from simply looking at the data rate.
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802.11AC TECHNICAL DETAILS
Within the frame structure, the preamble and training fields make it possible for the receiver to
auto-detect the physical layer standard being used. 802.11n and 802.11ac preamble frames are
shown in Figure 5.
Figure 5: Comparison of 802.11n and 802.11ac frame formats
The first 4 fields in both preambles are intended to be received by non-HT and non-VHT
stations for backwards compatibility. The initial Legacy Short and Long Training Fields (LSTF and L-LTF) and signal field (L-SIG) are similar to the same fields in 802.11a/b/g, while
the difference in the 4th field (symbols 6 and 7) identifies the frame as either 802.11n or
Examining the VHT preamble in more detail, for channels wider than 20 MHz, the legacy
fields are duplicated over each 20 MHz sub-band with appropriate phase rotation. Subcarriers
are rotated by 90 or 180 degrees in certain sub-bands in order to reduce the peak-to-average
power ratio (PAPR). To signal VHT transmission and enable auto-detection, the first symbol
of the VHT-SIG-A is BPSK, while the second symbol is BPSK with 90 degrees rotation
(QBPSK). This differs from the HT-SIG for 802.11n where both symbols use QBPSK
modulation. The VHT-SIG-A field contains the information required to interpret VHT packets
— bandwidth, number of streams, guard interval, coding, MCS and beamforming.
The remaining fields in the preamble are intended only for VHT devices. The VHT-STF is
used to improve automatic gain control estimation in Multiple Input Multiple Output (MIMO)
transmission. Next there are the long training sequences that provide a means for the receiver
to estimate the MIMO channel between the transmit and receive antennas. There may be 1, 2,
4, 6 or 8 VHT-LTFs depending on the total number of space-time streams. The mapping
matrix for 1, 2 or 4 VHT-LTFs is the same as in 802.11n, with new ones added for 6 or 8
VHTLTFs. The VHT-SIG-B field describes the length of the data and the modulation and
coding scheme (MCS) for single or multi-user modes.
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802.11AC MAC FORMAT
Most of the work in the 802.11ac MAC is evolutionary. In contrast with the major efficiency
enhancements introduced in 802.11n, most of the MAC work in 802.11ac consists of
supporting new physical layer features. Frames are bigger, but the aggregation framework in
place handles those larger frames without significant change. One of the few protocol features
to see large changes was around sharing radio resources between channels of different sizes.
For the most part, 802.11ac maintains the frame format used by its predecessors. There are
two major changes, shown in Figure 6. First, 802.11ac extends the maximum frame size from
almost 8,000 bytes to over 11,000 bytes, further increasing the ability to aggregate frames
from higher layers. Second, it reuses the HT Control field from 11n, but does so by defining a
new form of the Control field. When the HT Control field begins with a 0, the format is
identical to 802.11n and the HT Control field is of the HT-variant type. When the HT Control
field begins with a 1, the HT Control field is of the VHT-variant type. Figure 5 shows the
format of the VHT-variant HT Control field. It is composed of fields that are used to
communicate MCS feedback, a seldom-implemented procedure that enables two devices to
exchange information on how well transmissions are received to find the best data rate for the
Figure 6: 802.11ac MAC frame format
Frame Size and Aggregation
Frame aggregation was introduced in 802.11n to improve network efficiency. As with many
network protocols, one of the biggest sources of overhead in 802.11 are acquiring the channel
for the right to transmit. Aggregation works to decrease the relative amount of overhead by
allowing a device to obtain access to the radio channel and then using that opportunity to
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transmit multiple frames. 802.11 standards are not prescriptive and define only the aggregate
frame format. Implementing aggregation requires that a device look ahead through its transmit
queue to find frames to coalesce into a single aggregate frame, and each vendor‘s
implementation may be slightly different.
802.11ac, however, adds an interesting new take on aggregation: all frames transmitted use the
aggregate MPDU (A-MPDU) format. Even a single frame transmitted in one shot is
transmitted as an aggregate frame. Moving to an all-aggregate, all-the-time transmission
model means that the 802.11ac MAC must take over all the framing responsibility, and the
physical layer works only with the total length of what it transports.
All 802.11ac data frames are sent in an A-MPDU, even if the A-MPDU has only one frame in
It might seem at first glance that transmitting every frame as an A-MPDU, regardless of the
content of the data, would not be efficient. However, due to the potentially high speeds in
802.11ac, simply describing the length of the frame requires a large number of bits. The
maximum transmission length is defined by time, and is a little less than 5.5 microseconds. At
the highest data rates for 802.11ac, an aggregate frame can hold almost four and a half
megabytes of data. Rather than represent such a large number of bytes in the PLCP header,
which is transmitted at the lowest possible data rate, 802.11ac shifts the length indication to
the MPDU delimiters that are transmitted as part of the high-data-rate payload.
Management frames signal that they are capable of building an 802.11ac network or
participating in an 802.11ac network by including the VHT Capabilities Information element.
This element is placed in Probe Request and Probe Response frames to enable client devices
to match their capabilities to those offered by a wireless network.
The VHT Operation Information element
All 802.11 physical layers have an information element (IE) that describes their operation, and
the VHT PHY is no exception. The VHT Operation IE, shown in Figure 7, describes the
channel information and the basic rates supported by the transmitter. Basic rates are those
rates that are supported by all clients attached to an AP, and therefore are safe to use for
frames that are destined for a group of multiple stations. Rate support, which is found in the
second field of the IE, is transmitted identically to the rate support in the VHT Capabilities IE.
The first part of the information element describes the channels used by the transmitter
through the following fields:
Channel Width (1 byte)
For either 20 MHz or 40 MHz operation, the Channel Width field is set to 0. 80 MHz
operation sets this value to 1. Because it is necessary to distinguish the 160 MHz channel
width (a value of 2) from the 80+80 MHz channel structure (a value of 3), they receive
separate values. All other values of this field are reserved.
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Channel Center Frequency 0 (1 byte)
These fields are used only with 80 and 160 MHz operation, to transmit the center channel
frequency of the BSS. In 80+80 MHz operation, it is the center channel frequency of the lower
Channel Center Frequency 1 (1 byte)
This field is used only with 80+80 MHz operation, and is used to transmit the center channel
frequency of the second segment.
Figure 7: VHT Operation Information element
Other management frame changes
In addition to communicating capabilities and operating status, some other minor changes
were made to management frames and management protocols in 802.11ac:
The Transmit Power Envelope element enables APs to communicate transmission power
limits for each of the available channel bandwidths.
The Channel Switch Wrapper element extends the existing channel-switch announcements by
enabling a channel switch announcement frame to not only direct devices to a new channel,
but also state the channel bandwidth.
The Extended BSS Load element enables an AP to describe the amount of time spent
transmitting on each channel bandwidth so that a receiver can see how much time is spent on
20, 40, 80, and 160 MHz operations.
The Operating Mode Notification element describes the current channel width and number of
spatial streams active.
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MANDATORY 5 GHZ OPERATION
Previous Wi-Fi standards such as 802.11a/b/g/n operate in the 2.4 GHz band with 802.11n
optionally supporting the 5 GHz band. The 802.11ac standard mandates operation only in the
5 GHz band. 2.4 GHz band is susceptible to greater interference from crowded legacy Wi-Fi
devices as well as many household devices. The 5 GHz band has relatively reduced
interference and there are a greater number of non-overlapping channels available (25 nonoverlapping channels in US) compared to the 2.4 GHz band (3 non-overlapping channels in
the US). 802.11ac is therefore expected to leverage the reduced interference and greater
flexibility of multiple available channels in the 5 GHz band for increased performance.
802.11ac introduces 80 MHz and 160 MHz channel bandwidths in addition to the 20 MHz and
40 MHz specified in 802.11n. The 802.11ac standard requires all devices to support 20, 40,
and 80 MHz channel bandwidths in the 5GHz band, with support of 160 MHz channel
bandwidth being optional. 80 MHz channels can only be formed by combining two adjacent,
non-overlapping 40 MHz channels. The optional 160 MHz channel can be formed by
combining two adjacent or two non-adjacent 80 MHz channels. The wider channel bandwidths
of 802.11ac are shown in Figure 8.
Figure 8: Wider Channel Bandwidths in 802.11ac (5 GHz Band)
Wider bandwidth allows higher data rates to be achieved. In 160 MHz mode, the maximum
data rate that can be achieved using eight spatial streams, 256 QAM modulation, and code rate
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5/6 is 6.933 Gbps. However, initial 802.11ac products will typically use 80 MHz bandwidth
and be implemented in single-stream configuration for handsets and tablets, and up to three
spatial streams in routers for a maximum achievable PHY data rate of 1.3 Gbps, slightly more
than double the maximum data rate of 802.11n using four spatial streams. Table 1 shows the
maximum possible data rates in 802.11ac depending on the highest possible modulation and
coding scheme for a given bandwidth and given number of spatial streams. With gigabit
throughputs easily achievable, 802.11ac promises to deliver high performance and address the
capacity requirements of in-home HD content distribution.
Table-4: 802.11ac Maximum Achievable PHY Data Rates
HIGHER ORDER MODULATION
In 802.11n the highest order modulation is 64-QAM (Quadrature Amplitude Modulation).
Figure 9: Higher Order Modulation (256-QAM) in 802.11ac
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Six bits of coded information can be represented in a 64-QAM constellation. 802.11ac
increased the constellation configuration to 256-QAM that provides an incremental increase in
data rates by 33% over 11n. This increase is achieved by representing eight coded bits per
symbol instead of six. It should be noted however that higher signal-to-noise ratio (SNR) is
required for 256-QAM compared to 64-QAM because the constellation symbols are closer to
each other, as a result making them more susceptible to noise.
Figure 9 shows the impact of 256-QAM on performance. 600 Mbps is the maximum
achievable PHY data rate in 802.11n using four spatial streams and 40 MHz bandwidth.
However, for the same configuration, 802.11ac achieves 800 Mbps. These data rates assume
the short guard interval option.
HIGHER ORDER MIMO
802.11n was the first standard that introduced single-user multiple-input, multiple-output
(MIMO) transmissions. 802.11n allowed a maximum of four MIMO streams that could be
sent to a single device at a time. This increased throughput over the previous standards such as
802.11a/b/g. 802.11ac further increased the maximum number of MIMO streams from four to
eight. An 802.11ac STA can now receive up to eight spatial streams, to effectively double the
total network throughput of 802.11ac compared to 802.11n.
MULTI-USER MIMO (MU-MIMO)
802.11ac is the first Wi-Fi standard that introduces multi-user MIMO (MU-MIMO). In MUMIMO, the AP can serve multiple STAs simultaneously. This technique allows the AP
transmitter to send multiple packets simultaneously to multiple STAs. This is achieved using
up to eight spatial streams that can be divided among up to four STAs. These STAs can
employ different numbers of spatial streams. Each individual STA can have a maximum of
four spatial streams in an MU-MIMO transmission. For example, a configuration can include
four STAs each with two antennas, and an eight antenna AP forms a four-way MU-MIMO to
serve all four STAs with data on two spatial streams each. Without MU-MIMO, the AP would
have to multiplex the four STAs and serve them one at a time, thus effectively reducing their
throughputs by a factor of four.
Single user MIMO (SU-MIMO) has to time-division multiplex the data to support multiple
STAs and requires more antennas for reception, increasing the device cost. As shown in
Figure 10 for example, if an AP has three antennas and there are three
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Figure 10: Multi-User MIMO (MU-MIMO)
STAs in an SU-MIMO system, each STA requires three antennas to receive data from the AP
one-third of the time to achieve the same throughput as that of an MU-MIMO STA with a
single antenna. In an MU-MIMO scenario with the same three-antenna AP, the three STAs
require only a single antenna to receive the same throughput as that of the SU-MIMO case. All
three STAs in MU-MIMO case simultaneously receive a single spatial stream from the AP
100% of the time.
MU-MIMO is especially beneficial with the proliferation of smart phones. With faster
throughput, power consumption can be reduced and with only a single antenna required
instead of three, cost and space requirements are reduced.
DYNAMIC BANDWIDTH MANAGEMENT
Bandwidth management is an important aspect of any Wi-Fi standard. 802.11ac has several
bandwidth combinations allowed from 20 MHz to 160 MHz wide channels. With this increase
in available channel bandwidth comes greater flexibility. However, with this greater
flexibility, comes the challenge of optimizing the use of wider bandwidth in an efficient
manner. Each 802.11ac network includes a 20 MHz primary channel. This primary channel is
accessed using carrier sensing to make sure the channel is free from interference from other
networks. Another use for the primary channel is co-existence and backwards compatibility
with older Wi-Fi standards. Once the access to the primary channel is obtained, additional
secondary channels for wider bandwidth may be added.
Only 20, 40, 80, and 160 MHz bandwidths are allowed in 802.11ac. 60 and 120 MHz
bandwidths are not allowed. To manage a set of primary and secondary channels in a wide
bandwidth configuration, 802.11ac introduced a new concept of dynamic bandwidth
management. 802.11n did not properly define a handshake (ready to send/clear to send, or
CTS/RTS) mechanism for bandwidth management. This sometimes caused the bandwidth of
the receiver to not strictly relate to the actual bandwidth available at the receiver. This is
shown in Figure 11. In this case the sender has no interference in both the primary and
secondary channels; however, the receiver has interference in the secondary channel. The
sender sends the handshake signal called ready to send (RTS) on both channels. The receiver
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cannot acknowledge its respective handshake signal called clear to send (CTS) on either
primary or secondary channel as that is the protocol in static mode. As a result, no
transmission occurs in this case.
On the other hand, in dynamic mode defined in 802.11ac, channel interference is measured per
channel, and the receiver can send CTS signals per channel to indicate which channels are
interference-free. In this case transmission is allowed to take place on the primary channel,
which improves overall bandwidth utilization and network performance.
Figure 11: Static and Dynamic Bandwidth Management
In 802.11ac the interference detection threshold has also improved. Wi-Fi APs use
interference detection to reduce overlap and collisions with other APs operating on secondary
channels. The standard defines a sensitivity threshold for the signal strength on the secondary
channel that an AP must measure in order to determine if that secondary channel is busy.
Shown in Figure 12 is an example to illustrate this concept. 802.11n uses -62 dBm as the
sensitivity threshold for inferring 802.11n signals, whereas 802.11ac improved this to -72
dBm, which means that 802.11ac networks have improved sensitivity towards collision
avoidance and overlap detection.
Figure 12: Improved Sensitivity of CCA Threshold in 802.11ac
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Dynamic bandwidth management and increased sensitivity of the clear channel assessment
(CCA) threshold are additional features that improve the performance of 802.11ac and help
meet the requirements of in-home HD content distribution.
SINGLE-METHOD CLOSED LOOP TRANSMIT BEAMFORMING
In MIMO channels, beamforming is a technique that focuses the APs transmit energy of the
MIMO spatial streams towards the target STAs. This is achieved by using channel estimation
to precode the transmission in such a manner that when the transmitted streams reach the STA
they optimally combine their energies thus producing a stronger signal as seen by the STA. In
802.11n, several methods of beamforming were defined, however none of them was mandated
for certification and as a result, chipset vendors implemented a variety of non-interoperable
techniques. Lack of a single method prevented this feature of 802.11n from providing the
intended range enhancements across end-products, and becoming mainstream.
The 802.11ac standard defines a single closed-loop method for transmit beamforming. In this
method, the AP transmits a special sounding signal to all STAs who estimate the channel and
report their beamforming matrices back to the AP. This feedback from STAs is standardized
so that APs and STAs from different vendors would still interoperate correctly as long as they
are certified to be standard compliant.
Transmit beamforming (TxBF) is an optional feature in the 802.11ac specification and it will
be an optional-tested item in the Wi-Fi certification testing. TxBF is a technique that can
enable a higher modulation and coding scheme (MCS) at a given range. TxBF does not extend
the maximum range or increase the maximum rate. In the 5 GHz band, regulatory
requirements limit the transmit power that reduces the TxBF gain. The reduction in power
depends on the array gain which is a quantity proportional to the number of antennas used at
802.11ac is an IEEE standard amendment to 802.11n specification. It is required to be fully
compatible with 802.11n and 802.11a. 802.11ac only applies to 5 GHz band because there are
no 80 MHz and 160 MHz channels available in the 2.4 GHz band. 802.11ac standard enables
coexistence with 802.11n/a devices by requiring a backwards compatible preamble that has a
section which is understandable by 802.11n/a devices. This would allow legacy devices to
operate as intended.
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ADVANTAGES AND LIMITATIONS OF 802.11AC
IEEE 802.11ac will be the next wireless (Wi-Fi) technology standard after 802.11n. A few
consumer access points are already available with 802.11ac radios, but the currently available
802.11ac standard is only Draft 2.1. The final standard is expected to be released by late 2013.
The first enterprise-grade Wi-Fi devices with 802.11ac radios are expected from early 2013.
While 802.11ac products will eventually support specifications like 8 spatial streams, 8
antennas and 160 MHz channel width, the early (enterprise) 802.11ac products may most
probably support 4 spatial streams, 4 antennas and 80 MHz channel width.
The higher speeds/bandwidth supported by 802.11ac standard is expected to be useful for HD
Video Streaming, Blue-Ray TV‘s, HD Video Conferencing/Surveillance, Cloud
Synchronization, Densely populated Wi-Fi networks and other such highly demanding Wi-Fi
applications. Let us have a look at some advantages and disadvantages/limitations of
802.11ac/Gigabit Wi-Fi, in this article.
ADVANTAGES OF 802.11AC/GIGABIT WI-FI
The amount of bandwidth/speeds supported by 802.11ac will depend mainly on factors
like channel bandwidth, no. of spatial streams and guard intervals. The maximum
speeds/bandwidth supported by 802.11ac will vary from 433 Mbps to More than 7Gbps
(theoretical maximum). Hence, 802.11ac will support much more bandwidth when
compared to 802.11n which supports a theoretical maximum of 600 Mbps.
A single 802.11ac radio can accommodate more users/Wi-Fi devices than previous
standards (for similar bandwidth requirements). Mostly they can even operate at a higher
Since data rates decreases with increasing distances from the access point, it is important
to note that 802.11ac will support higher data rate over longer distances. This is partly due
to the more aggressive error correction codes supported by 802.11ac.
802.11ac products will use a concept called ‗beam-forming‘, which strengthens the signal
of the connection in one direction (where it is strong), instead of sending (weaker) signals
in all directions. This results in better wireless throughput performance.
802.11ac will most probably use 256-QAM (Quadrature Amplitude Modulation) that
provides almost 33% increase in throughput over 64-QAM used in 802.11n.
While 802.11n uses MIMO technique, 802.11ac will use MU-MIMO (Multi User –
MIMO) that will allow access points to transmit single/multiple streams to multiple
clients at the same time. This enables better efficiencies for Wi-Fi networks where a large
number of low-configuration Wi-Fi devices (like mobile phones) need to connect.
Performance of wireless clients in dense Wi-Fi networks will be better with 802.11ac
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While 802.11n supports a maximum configuration of 4×4:4; 802.11ac supports a
maximum configuration of 8×8:8. This is of the format – TxR:S, where T – Transmit
Antennas, R – Receive Antennas, S – Spatial Streams.
The maximum speed/bandwidth supported by a 1×1:1 802.11n device is 150 Mbps. But
the maximum speed supported by a 1×1:1 802.11ac device is 450 Mbps.
Since 802.11ac operates in the 5 GHz spectrum, there is lower interference from other
wireless devices. There are more non-overlapping channels (23) in 5 GHz and this
provides a lot of design flexibility which results in improved operational efficiencies.
802.11ac technology can be used to create short, but high-speed/high-capacity wireless
back-haul links for extending the network.
LIMITATIONS OF 802.11AC/GIGABIT WI-FI
Older 802.11n access points/client adapters cannot be upgraded to 802.11ac. That means,
all the access points and client adapters in the organization needs to be changed (hardware
replacement) to 802.11ac compliant products, if you want high-speed gigabit Wi-Fi in
your premises. Hardware replacements might be required even if you want to upgrade
from older version of 802.11ac to a newer version of 802.11ac, later on.
Since the total bandwidth consumed by 802.11ac devices will be higher, the controller,
edge switches, POE adapters, backbone network, etc. may need to be replaced, as well.
The total cost of an 802.11ac upgrade project will most probably be much higher than an
802.11ac Wi-Fi adapters and access points will be more expensive (at least initially) as
more antennas and better electronics/processors are required.
802.11ac can be implemented only in 5 GHz. All the older technologies (including
802.11n) that run predominantly in the 2.4 GHz band may need a separate radio/separate
access points supporting 2.4 GHz, to connect to the network.
In practical situations, 802.11ac network might be planned as an overlay to the existing
802.11n network. Exclusive 802.11ac network will remain a dream, in most cases.
802.11ac client adapters maybe released soon, but it will take a long time for users to get
it in their laptops/tablets, etc.
Like 802.11n, 802.11ac is the generic name of the technology. Individual 802.11ac
products, however, will come in various flavours and configurations (No. of spatial
streams, antennas supported, etc). The speed/performance of such devices varies
according to these factors. There is a good chance that vendors will just advertise
802.11ac compliance and sell the lowest possible configuration. Buyers need to be aware
of this and check what configuration they are going to get, before buying any products
that are advertised as 802.11ac compliant.
To obtain larger channel widths (80 MHz, 160 MHz), existing channels need to be
combined. Hence, the total number of non-overlapping channels will be reduced when
larger channel widths are employed. Avoiding interference from neighbouring 802.11ac
Wi-Fi devices in a dense network can create challenges in such cases.
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The single-link and multi-station enhancements supported by 802.11ac can improve the
performance and user experience in well-known WLAN applications, and enable several new
ones, particularly in the following markets:
Figure 13: 802.11ac applications in a digital home environment
Access Points (APs) will use the enhanced MIMO capabilities of 802.11ac to effectively
increase the capacity of any home or business WLAN network. With its enhanced multiplestream technology and new MU-MIMO capabilities, 802.11ac promises to increase the
network capacity substantially, and more effectively support the need of the APs to connect
with a growing number and variety of wireless-enabled devices used inside the home. APs
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were some of the very first MIMO 802.11ac-enabled products to hit the market in the second
half of 2012.
802.11ac can be used in TVs, Set-Top Boxes, and Networked Game Consoles to enable inhome distribution of HDTV and other content, including simultaneous streaming of HD video
to multiple clients throughout the home. These devices and applications suffer less from the
space and power constraints typical of mobile devices, and are some of the best candidates for
the successful application of the 802.11ac newly enhanced MIMO techniques (up to 4x4
MIMO - and beyond).
The increased data rate and higher energy efficiency of 802.11ac are ideal for applications in
mobile entertainment devices such as music players, handheld gaming devices, and wirelessenabled cameras and camcorders. Without adding too much to the limited power consumption
availability of these devices, 802.11ac can enable, for example, the rapid synchronization and
backup of large data files between these devices and a personal computer (PC) or tablet. Due
to constraints in term of both power consumption and physical space, it is likely that first
generations of these devices will use only SISO or 2x2 MIMO 802.11ac chipsets.
Portable computing devices such as PCs, laptops, slates and tablets, are obvious ideal
candidates for 802.11ac. The growing number of wireless applications supported by these
devices, and the increasing demand for faster connectivity by their users, will be the key
drivers for adoption of 802.11ac. Portable computing devices will use 802.11ac, as mentioned,
for rapid synchronization and backup of large data files with other 802.11ac-enabled devices,
or for the streaming of HD video and other content.
The enhanced throughput of 802.11ac might enable, in the future, replacing with a wireless
link the wired connection between portable computing devices and their peripheral devices. A
promising application is, for example, wireless displays for laptops. To enable the very high
throughput necessary for these applications will require device manufacturers to overcome the
challenges of implementing MIMO 802.11ac techniques in the form factors of these devices.
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Mobile PhonesMobile phones, specifically smart phones, can use 802.11ac to communicate with mobile
entertainment devices and portable computing devices for the rapid synchronization of large
data file, and more in general to support the growing demand of faster data transfer by their
users. These devices need high throughput but are also generally small and conscious about
power consumption, hence, 802.11ac will probably be implemented single stream only. The
first 802.11ac-enabled smart phones were introduced in early 2013.
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802.11ac represents the fifth generation of IEEE 802.11 WLAN standards and 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. 802.11ac will be backward-compatible with 802.11n
networks operating in the 5GHz range and is expected to offer dramatic improvements in WiFi reliability, throughput and range. 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. Enterprise-class products will ship with variations on the
number of antennas, spatial streams and channel widths, and the most important education for
enterprises to seek at this time is how wider channels, but fewer of them, will affect the
organization‘s channel plan. 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 that have been specified by 802.11ac standards.
802.11ac holds the promise of gigabit+ performance that will enable much broader adoption in
key target markets such as enterprise, residential video, and carrier hot spots.
The IEEE 802.11ac standard leverages new technologies to achieve Gigabit per second
throughputs. Smoother in-home HD video content distribution can be achieved using 802.11ac
over a robust high throughput, low latency Wi-Fi network. Improvements in performance
come from wider bandwidth, more spatial streams, higher order modulation, dynamic
bandwidth management, and multi-user MIMO. In addition, 5 GHz operation allows for
reduced interference. Small form factor devices such as smart phones sensitive to space and
cost can enjoy higher throughputs with single antennas.
Simulation results show 802.11ac in 40 MHz bandwidth using eight antenna AP can easily
support two HD video streams at 24 Mbps each and two Blu-Ray video streams at 54 Mbps
each. The additional benefit is the cost reduction at the client that only requires two antennas.
Thus 802.11ac is a promising technology that can robustly deliver simultaneous HD video
streams throughout the home.
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