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Gigabit Wireless Networking with IEEE
802.11ac:Technical Overview and Challenges
Farhan Siddiqui
School of Information Systems and Technology, Walden University, USA
Email: farhan.siddiqui@waldenu.edu
Sherali Zeadally1 and Khaled Salah2
1College of Communication and Information, University of Kentucky, USA
Email: szeadally@uky.edu
2Electrical and Computer Engineering, Khalifa University of Science and Technology, UAE
Email: khaled.salah@kustar.ac.ae
Abstract— The ever-growing proliferation of wireless devices and
concurrent deployments of bandwidth intensive applications has
been having a significant impact on user experience in high-
density wireless areas. IEEE 802.11ac is a recently ratified
Wireless Local Area Network (WLAN) standard that promises to
improve wireless user experience by delivering gigabit speed to
end-user applications. 802.11ac utilizes new technologies such as
Channel Bonding, Beamforming, and Multi-User Multiple-Input
Multiple-Output (MU-MIMO) to improve wireless performance.
This article reviews recent technological advances made in the
field of WLANs and then focuses on the recent IEEE 802.11ac
standard. We present actual data rates attained by currently
available 802.11ac hardware, and we also discuss foreseen
technical challenges that still need to be addressed to enable
efficient and seamless gigabit wireless networking.
Index Terms— wireless, MIMO, WLAN, standards,
Beamforming
I. INTRODUCTION
The history of Wireless Local Area Network (WLAN)
technology dates back to the year 1997 [9], when the first
802.11 standard was ratified by the Institute of Electrical and
Electronics Engineers (IEEE). Since then, WLAN
technologies have been continuously evolving, resulting in
improvement of transmission rates, coverage areas, security,
Quality of Service (QoS), and mobility. At the same time,
WLAN usage and applications have undergone remarkable
changes. Recent years have also witnessed an exponential
growth in the use of mobile devices and a continuous demand
from consumers for faster and more robust wireless
connectivity. Figure 1 illustrates the growth in the number of
wireless devices, increase in wireless data usage and the rising
number of households adopting wireless-only services over
the last few years [15]. The recently ratified 802.11ac standard
promises to deliver gigabit rates to wireless clients. IEEE
802.11ac, also known as Gigabit Wi-Fi and 5G Wi-Fi, is built
upon 802.11n and takes advantage of technological advances
in communication systems, coding techniques, signal
processing, and processing power. 802.11ac offers significant
enhancements in data transmission rates, reliability, and
Quality of Service (QoS). In this article we review the
802.11ac standard with respect to its new technological
features, consumer demands, benefits, and technical
performance challenges.
Figure 1. Wireless Statistics [15]
The rest of this paper is organized as follows. In Section 2, we
present the evolution of WLAN focusing on the various
wireless standards that have been developed to enhance
wireless speeds. Section 3 presents some of the main drivers
behind the demand for faster and more reliable wireless
connections. In Section 4, we describe the modifications made
to the Physical (PHY) and Medium Access Control (MAC)
layers of the 802.11 protocol stack in order to provide support
for gigabit transmission rates. In Section 5, we discuss
technical challenges that still need to be addressed to deliver
gigabit wireless throughput to end-users. Finally, in Section 6,
we present our concluding comments.
II. EVOLUTION OF WLAN STANDARDS
The original IEEE 802.11 standard defines two underlying
layers for WLANs: the Physical (PHY) and the Medium
Access Control (MAC). The PHY layer is responsible for
modulating and transmitting data. The MAC layer is in charge
of controlling transmissions among WLAN clients within a
single coverage area known as a Basic Service Set (BSS). The
PHY layer of the original 802.11 version standardized three
transmission techniques: Infrared (IR), Frequency Hopping
Spread Spectrum (FHSS), and Direct Sequence Spread
Spectrum (DSSS). Frequency hopping is the process of
transmitting on a given frequency for a short interval and
switching to another frequency according to a pre-defined
frequency-hopping pattern known to both, transmitter and
receiver. DSSS systems spread transmissions across a
relatively wide band by artificially increasing the used
bandwidth. A DSSS transmitter converts an incoming data
stream into a symbol stream where each symbol represents a
group of 1, 2, or more bits. DSSS transmitter modulates or
multiplies each symbol with a pseudorandom sequence, which
is called a "chip" sequence. The multiplication operation in a
DSSS transmitter artificially increases the used bandwidth
based on the length of the chip sequence. This modulation
technique is called Quadrature Phase Shift Keying (QPSK).
IEEE defines the use of an 11- chip Barker sequence that is
actually a sequence of 11 “-1” and “+1” values. The spread
signal can undergo as many as 11 phase changes per symbol
period where a non-spreaded QPSK signal would undergo a
maximum of one phase change per symbol period. The
receiver correlates the received signal with the 11-chip
sequence to obtain the originally sent data [3]. The Barker
modulation technique provides data rates of 1 or 2 Mbps [5].
TABLE I. IEEE 802.11 Standards
Wireless
Generation
1G
2G
3G
4G
5G
IEEE WLAN
Standard
802.11
802.11b
802.11a
802.11g
802.11n
802.11ac
Wave1
802.11ac
Wave2
802.11ad
Date Ratified
1997
1999
1999
2003
2009
2012
2013
2012
Max. Theoretical
Data Rate
2 Mbps
11 Mbps
54 Mbps
54 Mbps
600 Mbps
1.3 Gbps
6.7 Gbps
7 Gbps
Typically
Achieved Data
Rate
1 Mbps
6.5 Mbps
25 Mbps
25 Mbps
200 Mbps
400 – 700
Mbps
Yet to be tested
Yet to be tested
Frequency band
2.4 GHz
2.4 GHz
5 GHz
2.4 GHz
2.4/ 5 GHz
5 GHz
5 GHz
60 GHz
Max. Spatial
Streams
1
1
1
1
4
3
8
-
Backward
Compatibility
-
-
-
802.11b
802.11a/g,
802.11b
802.11n
802.11n
-
Coverage Area
-
30 m
30 m
30 m
50 m
70m
80m
Short range
Channel
Bandwidth (MHz)
20
20
20
20
20,40
20, 40, 80
20, 40, 80, 160
2160
Modulation
Scheme
FHSS, DSSS
HR-DSSS
OFDM
DSSS,OFDM
OFDM, 64-
QAM
OFDM (256-
QAM)
OFDM(256-QAM)
SC OFDM
Radio
Architecture
SISO
SISO
SISO
SISO
MIMO
MIMO
MIMO
-
Typical
Usage
Basic Wireless
connectivity
Wireless
connectivity in
homes/offices
Web
browsing,
email, data
transfer
Web
browsing,
email, data
transfer
Video
transmission,
gaming
Real-time
video/ audio,
high density
WLAN zones
Real-time video/
audio, high density
WLAN zones,
connectivity to
multiple devices
High Definition
Video,
connectivity to
multiple
devices
Advanced
Antenna
Technologies
-
-
-
-
MIMO, up to 4
spatial streams
SU-MIMO
MU-MIMO,
Transmit
Beamforming
Beam-forming
FHSS: Frequency Hopping Spread Spectrum; DSSS: Direct-Sequence Spread Spectrum; HR-DSSSS: High-Rate Direct-Sequence Spread Spectrum; OFDM: Orthogonal Frequency-
Division Multiplexing; QAM: Quadrature Amplitude Modulation; SC-OFDM: Single Carrier Orthogonal Frequency-Division Multiplexing; SISO: Single-Input Single-Output;
MIMO: Multiple-Input Multiple-Output; SU-MIMO: Single-User Multiple-Input Multiple-Output; MU-MIMO: Multi-User Multiple-Input Multiple-Output;
Table 1 presents a summary of various WLAN standards
developed to date, and their main characteristics. The
summary focuses on the main technological changes that
were introduced in each WLAN standard to achieve
higher wireless rates:
802.11b: IEEE used the Complementary Code Keying
(CCK) standard with the DSSS technology to achieve
5.5Mbps and 11Mbps rates. CCK uses a set of 64 eight-
bit unique complex code words called polyphase codes.
Using CCK, up to 6 bits can be represented by any code
word (instead of the 1 bit represented by a Barker
symbol). As a set, these code words have unique
mathematical properties that allow them to be correctly
distinguished from one another by a receiver, even in the
presence of significant noise (such as interference due to
multiple radio reflections indoors). The CCK modulation
used by 802.11b transmits data in symbols of eight chips,
where each chip is a complex QPSK bit-pair at a chip rate
of 11 Mchip/s. The 5.5 Mbps rate uses CCK to encode 4
bits per carrier, while the 11 Mbps rate encodes 8 bits per
carrier [5].
802.11a: IEEE 802.11a utilizes the Orthogonal
Frequency Division Multiplexing (OFDM) modulation
method and operates in the 5 GHz band. OFDM
improves efficiency and reduces interference between
signals by splitting the radio signal into several sub
signals before they reach a receiver. This method uses
multiple sub-carriers to transport information between
users. In the OFDM method, a high-speed signal is split
into multiple lower-speed sub-signals that are
transmitted in parallel at varying frequencies. The
parallel transmission over multiple sub-carriers allows
OFDM WLANs to achieve a higher data rate (about 54
Mbps). The OFDM technique has a lower multi-path
distortion as well as lower multi-path delays.
802.11g: IEEE 802.11g [6] employs DSSS, OFDM, or
both at the 2.4 GHz frequency band to provide high data
rates of up to 54 Mbps. The combined use of both DSSS
and OFDM is made possible through the establishment
of four different physical layers known as Extended Rate
Physicals (ERP). ERPs coexist during a frame exchange,
so the sender and the receiver have the option to select
and use one of the four layers. The first layer uses DSSS
technology with CCK modulation and provides data
rates similar to 802.11b. The second layer (new to
802.11g) uses OFDM at the 2.4 GHz band. The third
layer uses DSSS with a coding algorithm called Packet
Binary Convolutional Coding (PBCC) and provides 22
and 33 Mbps. The fourth layer uses a combination of
DSSS (for transmitting packet header) and OFDM (for
transmitting packet payload).
802.11n: IEEE 802.11n [8] brought in significant
improvements to the application throughput by
introducing new PHY and MAC layer features. IEEE
802.11n provided an approximate transmission rate of
130 Mbps. 802.11n adopted the OFDM modulation with
52 data sub-carriers in 20-MHz channel (instead of 48
sub-carriers used in IEEE 802. 11g). The higher data sub-
carriers helped to improve the highest data rate per stream
to 65 Mbps as compared to 54 Mbps supported in IEEE
802.11g. 802.11n utilizes the Multiple-Input Multiple-
Output (MIMO) technology. A MIMO system
(represented by N x M ) has N transmitters and M
receivers. MIMO utilizes Spatial Multiplexing to transmit
two or more parallel data streams in the same frequency
channel. Using MIMO and Spatial Multiplexing 802.11n
doubles its transmission capacity (130 Mbps). The
transmission capability was improved by transmitting and
receiving two parallel spatial data streams over two
transmitters at the same time.
IEEE 802.11n also implemented the efficient frame
aggregation and block acknowledgement mechanisms to
improve throughput. 802.11n introduced frame
aggregation mechanisms called Aggregated MAC Service
Data Unit (A-MSDU) and Aggregated Multi-Protocol
Data Unit (A-MPDU) to reduce the overhead of IEEE
802.11n packets. Frame aggregation aims at combining
payloads of various PHY/MAC frames such that header
size is considerably less than payload size. A-MSDU
aggregates MAC frame payloads. IEEE 802.11n also
provides optional features that can be used to further
improve the performance in the PHY layer. For example,
a two adjacent 20 MHz channels can be bonded and a 40
MHz channel may be used (instead of 20 MHz) if
desired. IEEE 802.11n also expands the number of sub-
carriers in 40-MHz channel to 108 sub-carriers.
802.11ac: The 802.11ac standard operates only in the 5
GHz band. Theoretically, 802.11ac proposes data rates
over 1 Gbps. The new specifications are based on the
802.11n standard, by expanding the channel bandwidth to
80 MHz and adding optional 160 MHz channels. In
addition, 802.11ac utilizes MIMO with up to 8 spatial
streams and a higher order modulation scheme called
256-Quadrature Amplitude Modulation (256-QAM). The
standard also provides support for other advanced
features such as beamforming and Low Density Parity
Check (LDPC). In the beamforming process, an access
point utilizes more than one antenna to transmit a signal.
The multiple signals are sent to client devices to receive
feedback about the best transmission path to the client.
Beamforming is used to create a directional Radio
Frequency (RF) beam while LDPC is utilized by
802.11ac to improve the Signal-to-Noise Ratio (SNR).
We discuss further details about beamforming in section
4.
802.11ad: The 802.11ad standard operates in the
unlicensed 60 GHz band. The standard proposes a
theoretical data rate of 7 Gbps with low power
consumption. 802.11ad is expected to employ wide
channels of 2.16 GHz. This standard is intended to
support high-performance wireless implementations such
as high- definition video.
III. DRIVERS OF GIGABIT WIRELESS SPEEDS
802.11ac Access Point
Laptop
Tablet
Smart Phone
Wireless Bridge
Game Console
Blue-Ray DVD Player
High-Definition Television (HDTV)
Figure 2. A Typical WLAN Deployment Scenario using IEEE 802.11ac
Technology
The rapid proliferation of wireless devices at homes and
work places has given rise to huge demands for higher
wireless speeds and wider coverage areas. The ever-
increasing need for mobile connectivity and high
throughput requirements of present-day wireless devices
and applications drive the need for deploying wireless
technology with significant enhancements in terms of
speed, reliability and Quality of Service (QoS). IEEE
802.11ac has been developed to meet the requirements of
new mobile devices such as smartphones, tablets, laptops,
and smart TVs. 802.11ac has also been established to
fulfill the demands of bandwidth-intensive and latency-
sensitive applications. Figure 2 illustrates a typical
scenario of how the 802.11ac technology can be
employed in homes. The main drivers behind the demand
for gigabit wireless speeds can be summarized as follows:
Increased Usage of Video Streaming Applications: the
use of video applications has increased exponentially in
the last decade. Many video streaming applications are
being run by wireless users. Typical video usage includes
gaming applications, digital home entertainment, videos
for patient care, instructional videos in education, video
presentations in corporate environments, etc. Demand for
the ever- increasing, bandwidth-intensive applications has
given rise to the need for gigabit wireless speeds.
High-density WLANs: the rise in the usage of all kinds of
wireless devices such as smartphones and tablets has
resulted in increased traffic in wireless networks. The
heavy wireless traffic plays a major role in driving the
demand for gigabit speed technology. Furthermore, with
the latest trend of corporations to switch from having
employees to bring single computing device to BYOD
(Bring Your Own Device) paradigm, networks are
experiencing increased load due to each person
connecting multiple gadgets to the network. In order to
provide high WLAN capacity, corporations are looking
forward to the deployment of gigabit WLANs.
Popular Use of Latency-Sensitive Applications: various
such as Voice over IP (VoIP), audio and video streaming,
real-time videoconferencing, etc. require stringent low
network latencies. With newer wireless technology that
can offer higher bandwidths, these latency-sensitive
applications can run more efficiently and provide
enhanced user experience. Hence, there is a demand for
improved transmission techniques that can support
increasing network loads and reduced transmission
delays.
Increased Usage of WLAN-enabled devices in Education:
the heavy usage of WLANs at schools and university
campuses is yet another driving force behind the
deployment of gigabit wireless technologies. With the
advent of mobile devices, students now carry multiple
WLAN enabled devices laptops, tablets, smartphones on
campus and frequently run bandwidth-intensive
applications such as streaming videos, making
audio/video calls, or playing live or pre-recorded lecture
materials and videos. The high consumption of network
bandwidth calls for implementing high-speed wireless
access technologies that can deliver higher network
throughput in the future. !
Use of WLAN-enabled devices in Healthcare: many
healthcare applications heavily depend on reliable and
high-speed wireless connectivity include cardiac and
radiology imaging, telemedicine, health scanners, etc.
Running these healthcare applications efficiently requires
high network capacity, sustained performance, and
ubiquitous wireless coverage. The availability of gigabit
wireless speeds will allow medical applications to stream
multimedia content smoothly to physician’s wireless
devices. Doctors, nurses and caregivers want to use
gigabit wireless technology more often because it
improves efficiency and provides faster access to
healthcare services.
IV. TECHNIQUES FOR ENHANCING WIRELESS SPEEDS IN
IEEE 802.11AC
In this section, we describe briefly the advances and
improvements made to IEEE 802.11ac PHY and MAC
Layers so that gigabit wireless speed can be achieved.
4.1. PHY Layer Enhancements
The PHY layer changes implemented in IEEE 802.11ac
are closely aligned with 802.11n, with further
improvements. The following are the main enhancements
in the PHY layer of 802.11ac [13] [14]:
Improved Channel Width: 802.11ac supports channel
widths of 20, 40, 80, and 160 MHz. The maximum
channel width supported in previous 802.11 standards did
not exceed 40 MHz (as in the case of 802.11n). With
more than a two-fold increase in the channel width,
802.11ac has the capability to offer much higher data
rates.
Denser Modulation Method: 802.11ac adds 256
Quadrature Amplitude Modulation (QAM) to OFDM and
achieves increased throughput as compared to 64-QAM.
Enhancement of Spatial Streams: 802.11ac supports up to
8 spatial streams and therefore increases the cumulative
data speed. For example, an aggregate data rate of
1.3Gbps can be achieved using three spatial streams in
802.11ac.
Improvements to MIMO Technology: Multi-User,
Multiple-Input Multiple-Output (MU-MIMO) [10] is an
enhancement of the MIMO technology that allows
simultaneous downlink transmission of separate streams
to different clients in the same channel. MU-MIMO can
only be used when the access point transmits to the client
devices (downlink). This feature cannot be used when
client devices transmit to the access point (uplink).
Downlink MU-MIMO improves throughput when
transmitting to multiple single-stream clients (e.g.,
smartphones) or multiple multi-stream clients (e.g., PCs).
Simultaneous downlink transmission in MU-MIMO is
illustrated in Figure 3. !
Beamforming: 802.11ac specifies a standard
implementation procedure for the beamforming
technology. Beamforming is expected to improve
bandwidth utilization and increase the range of the
wireless network. Conventional 802.11 access points
used antennas that were omnidirectional. Omnidirectional
antennas keep the radio channel busy in all directions.
Beamforming focuses energy toward a client, as shown in
Figure 4. The colored path shows the area where
beamforming focus increases power, and thereby
contributes to the improvements in the signal-to-noise
ratio and data rate.
Stream 1
4X4 MU-MIMO Access Point
Laptop
Smart Phone
Tablet
MU-MIMO: Multi-User Multiple-Input Multiple-Output
Stream 2
Stream 4
Stream 3
Figure 3. Multi-User Multiple-Input Multiple-Output (MU-MIMO) in
802.11ac
802.11ac Access Point
Client Device
Cleint Device
Conventional 802.11
Access Point
Laptop
PC
Terminal
Client Device
Figure 4. Beamforming (Left) versus Omnidirectional (Right) Wi-Fi
Antennas
4.2 MAC Layer Enhancements
The original MAC layer of WLANs was designed to only
support one-on-one communication. Later and in order to
enable a WLAN Access Point (AP) to transmit
simultaneously to multiple WLAN clients, certain
modifications have been implemented at the MAC layer.
Some of these main changes include the implementation
of Transmit Opportunity (TXOP) Sharing and Enhanced
Aggregation.
4.2.1. TXOP Sharing
The MAC protocol employs a mandatory contention-
based channel access function called Distributed
Coordination Function (DCF), which is based on the
Carrier Sense Multiple Access (CSMA) mechanism.
Mobile stations deliver MAC Service Data Units
(MSDUs) after detecting that there is no other
transmission on the same wireless medium.
The Enhanced Distributed Channel Access Method
(EDCA) is an IEEE 802.11 MAC access method in which
a station with high priority traffic has a lower waiting
time (for transmission) than a station with low-priority
traffic. EDCA provides contention-free access to the
channel for a period called a transmit opportunity
(TXOP) [1]. A TXOP is a bounded time interval during
which a station can send as many frames as possible (as
long as the duration of the transmissions does not extend
beyond the maximum duration of the TXOP). If a frame
is too large to be transmitted in a single TXOP, it is
fragmented into smaller frames.
In the case of the legacy TXOP, only frames belonging to
the same Access Category (AC) are transmitted and
multiple frames belonging to different ACs cannot be
simultaneously transmitted. Access Categories (ACs) are
four QoS classifications (voice, video, best effort and
background) made by Wireless Multimedia Extensions
(WME) [4].
TXOP sharing, also referred to as the Multi-User
Transmit Opportunity (MU-TXOP) mechanism, extends
the legacy 802.11 TXOP concept to support Down-Link
(DL) MU-MIMO. TXOP Sharing defines two types of
Access Categories (primary and secondary), and two
types of destinations (primary and secondary). A Primary
AC is the AC that wins the TXOP for channel access
after both external and internal competitions. There is
only one primary AC at any time. A Secondary AC is an
AC that does not win a TXOP but wants to share the
TXOP obtained by the primary AC for simultaneous
transmissions. There could be multiple secondary ACs at
any time. Primary destinations are those targeted by the
frames belonging to the primary AC. There could be one
or more Primary destinations at any time. Secondary
destinations are those targeted by the frames belonging to
secondary ACs. There could be one or more Secondary
destinations at any time.
For each AC, an enhanced version of the DCF, called
Enhanced Distributed Channel Access function
(EDCAF), contends for TXOPs using a set of EDCA
parameters. Once an EDCAF wins a TXOP it becomes
the owner of the TXOP. Its corresponding AC becomes
the Primary AC, and its corresponding destination(s)
become Primary destination(s). The Primary AC can
share the TXOP with other ACs that did not win the
medium access – the TXOP becomes the MU-TXOP.
These other ACs become Secondary ACs and their
corresponding destinations become Secondary
destinations. The AP then groups the Secondary
destinations together with the Primary destination(s) for
simultaneous transmission (e.g. using GroupID). When
the primary AC allows secondary ACs to share the TXOP
for simultaneous transmissions, it results in Multi-User
TXOP (MU-TXOP).
Access Point
STA-1
Primary Destination
STA-2
Secondary Destination
STA-3
Primary Destination
AC_VI - 1
AC_BE - 1 AC_VO - 2
AC_BE - 2 AC_VO - 1 AC_VI - 2
MU-TXOP
MAC Service Data Unit (MSDU)
AC_VO AC_VI AC_BE AC_BK
11
2
1
2
EDCAF (AC_VO) EDCAF (AC_VI) EDCAF (AC_BE) EDCAF (AC_BK)
2
AC: Access Category
VO: Voice
VI: Video
BE: Best Effort
BK: Background
STA: Station
MU: Multi-User
TXOP: Transmit Opportunity
MAC: Medium Access Control
EDCAF: Enhanced Distributed
Channel Access Function
!
!
Figure 5. Multi-User Transmit Opportunity (MU-TXOP) Sharing
Figure 5 shows an example of MU-TXOP Sharing [7].
Here AC_VI is the primary AC and has 2 MSDU frames
ready for transmission. The first MSDU is destined for
STA-1 and the second MSDU is destined for STA-3.
Since, STA-1 and STA-3 are destinations for a MSDU
belonging to the primary AC, STA-1 and STA-3 become
the Primary destinations. AC_VO and AC_BE are
secondary ACs and STA-2 is the Secondary destination.
Higher priority traffic (AC_VI) is transmitted earlier than
lower priority traffic (AC_VO, AC_BE).
4.2.2 Enhanced Aggregation
Two forms of aggregation were included in IEEE
802.11n. These include the Aggregation MAC Service
Data Unit (A-MSDU) (shown in Figure 6(a)) and the
Aggregation MAC Protocol Data Unit (A-MPDU)
(shown in Figure 6(b)).
PHYHDR MACHDR FCS
sub-frame sub-frame........ ....sub-frame sub-frame
DA SA LMSDU PAD
A-MSDU
Preamble A-MPDU
sub-frame sub-frame........ ....sub-frame sub-frame
MPDU header A-MSDU FCS
DA: Destination Address
SA: Source Addresss
L: Length
MSDU: MAC Service Data Unit
MAC: Medium Access Control
PHYHDR: Physical Header
MACHDR: MAC Header
PHY: Physical
A-MSDU: Aggregated MSDU
FCS: Frame Check Sequence
MPDU: Multi-Protocol Data Unit
A-MPDU: Aggregated MPDU
Figure 6. (a)Aggregated MSDU (A-MSDU), (b)Aggregated MPDU
(A-MPDU)
As shown in Figure 6, several MSDUs with the same
destination address are concatenated to form an A-
MSDU. The A-MSDU is encapsulated within a MPDU.
Several MPDUs are aggregated to form an A-MPDU. For
each MSDU sub-frame in an A-MSDU frame, the MSDU
sub-frame includes the Sub-frame Header, the MSDU
data payload and the Padding field. The Sub-frame
Header includes three fields: the Destination Address
(DA), the Source Address (SA) and Length that indicates
the MSDU data payload. A-MSDU aggregation can only
be done for packets with the same SA and DA. A single
A-MSDU contains multiple MSDU sub-frames. A single
A-MSDU frame is transmitted after adding the Physical
Header, the MAC header and the FCS field. The principle
of A-MPDU is to send multiple MPDU sub-frames with a
unique PHY header so as to reduce the overhead of the
PHY header. For each A-MPDU, every MPDU sub-frame
includes an MPDU frame, the MPDU delimiter and the
padding bytes. Multiple MPDU sub-frames are
concatenated into one larger A-MPDU frame. All the
MPDU sub-frames within an A-MPDU should be
addressed to the same receiver, but the MPDU sub-frame
could have a different source address. The aggregation
process was proposed to improve the MAC efficiency in
802.11n. 802.11ac further improves the MAC efficiency
by implementing Enhanced Aggregation wherein A-
MSDU and A-MPDU have further extensions in their
lengths. In the case of 802.11ac, an A-MSDU can have a
maximum length of 11426 bytes, and an A-MPDU can
have a maximum length of 1048579 bytes [2].
V. CURRENT CHALLENGES IN ACHIEVING GIGABIT
THROUGHPUT WITH 802.11AC
The new characteristics of the 802.11ac are significant
and constitute a major advancement in the development
of WLAN technologies. However, the theoretically
calculated data rate of 1.3 Gbps at the physical layer has
not been practically achieved thus far by the end-user.
Several vendors have tested their 802.11ac products and
identified factors that can vary the performance of
802.11ac.
TABLE 2. OBSERVED THROUGHPUT USING AN 802.11AC ACCESS POINT
WITH DIFFERENT DEVICES
Device
Achieved
Throughput
(Mbps)
No. of Spatial
Streams
supported by
device
MacBook Air Laptop
400
2
Samsung Galaxy S4
Smart Phone
218
1
Wireless Bridge
722
3
Tests conducted by an Aruba Networks [11] showed that
when the wireless network was loaded with 120 clients,
an 802.11ac equipped MacBook Air laptop achieved a
throughput of 400 Mbps. Similarly, results of tests
conducted by Cisco [13] using the 802.11ac access point
are shown in table 2. It can be observed that a Samsung
Galaxy S4 smart phone (with one spatial stream) client
yielded a throughput of about 218 Mbps while a wireless
bridge (with three spatial streams) yielded a throughput
of about 722 Mbps. The observed throughputs show that
increase in throughput can be achieved in devices
supporting more than one spatial stream. The tests also
reveal that although the throughput obtained using
802.11ac is definitely better than that obtained via
802.11n, however the measured throughput using
802.11ac has currently not achieved the theoretical
gigabit rates of greater than or equal to 1Gbps.
The maximum achievable data rates for 802.11ac are
dependent on the following factors:
1. Number of spatial streams used
2. Use of 80 MHz wide channels
3. Use of the 256-QAM modulation scheme
4. Use of the Transmit Beamforming
5. Use of the Low Density Parity Check (LDPC)
There are various reasons why the first generation of
802.11ac devices (referred to as Wave 1) cannot deliver
the theoretical maximum physical data rate of 1.3 Gbps:
Limitations of client devices: to achieve the maximum
speed, all three spatial streams should be utilized.
However a large percentage of client devices are smart
phones and they support only a single spatial stream.
Most 802.11ac access points being deployed currently
support up to three spatial streams. The 802.11ac enabled
client devices support several spatial streams
(simultaneously). For example, the Apple’s MacBook Air
laptops support two streams whereas the Samsung Galaxy
S4 can only support one spatial stream.
Issues in deploying 80 MHz and 160 MHz channel
bonding: WLAN bandwidth can be increased by bonding
multiple channels together. 802.11ac allows the creation
of 20, 40, 80, or 160 MHz wide channels. The 160 MHz
channel can also be a combination of two non-contiguous
80 MHz channels (80+80). From a Radio Frequency (RF)
planning perspective, there are a few obstacles to using
these wider channels. For example, a wider channel is
more susceptible to RF interference from neighboring
wireless networks. Although channel bonding increases
bandwidth, wider channels are more susceptible to signal
interference that may lead to reduced range and poorer
signal quality.
Complexities involved in utilizing 256-QAM
Modulation: to enable higher data rates, 802.11ac has
introduced a novel and efficient modulation technique. In
contrast to the conventional 64-QAM modulation, 256-
QAM enhances efficiency by approximately 33%. 256-
QAM modulation increases modulation complexity by
another order of magnitude, representing 8 bits with each
constellation point. However, 256-QAM modulation can
only be used in scenarios that have high Signal-to-Noise
(SNR) ratios, or in very favorable channel conditions
(low interference) such devices operating close by in a
home. Furthermore, in order to support 256-QAM, the
transmitter and receiver need to be designed such that
transmit and receive Error Vector Magnitude (EVM) is
able to support the higher constellation. Therefore, RF
design of a system supporting 256-QAM remains a
significant challenge [12]. With currently available
products, 256-QAM works only within a very short range
(10-20m). Hence, the data rate gains proposed by
802.11ac depend on the deployment scenario.
Beamforming and Low Density Parity Check (LDPC):
these features in 802.11ac can be used when the client
device is in close range with the 802.11ac access point.
When the client device moves farther away from the
access point, the 802.11ac radios switch to narrower
channels and lower modulations.
In addition to the aforementioned technical limitations
that still need to be overcome to achieve high 802.11ac
throughput, there are also challenges related to security,
capacity planning and deployment, and interoperability.
For security, Galois/Counter Mode Protocol (GCMP) is
the recommended encryption protocol to be used for
802.11ac. GCMP gives much higher performance than its
widely adopted predecessor Counter Mode with CBC-
MAC Protocol CCMP that is used in 802.11n. The
challenge with GCMP is still computational overhead as
the data rate approaches the gigabit rates. This leads to
additional cost in chipset design to accommodate for
newer generation of crypto accelerator processors to
speed up the internal encryption algorithm of GCMP.
More importantly, GCMP protocol is not backward
compatible with existing GCMP protocol, and that poses
a major deployment and migration challenge that needs to
be addressed.
Capacity planning and deployment are also major
challenges to ensure the seamless adoption of 802.11ac.
The wired core network capacity has to be adequate
enough to meet the new multi-gigabit Wi-Fi clients
connected to 802.11ac APs. Proper capacity planning for
the core links, switches, and routers should be at least
upgraded to 10 Gbps. Above all, legacy network
middleboxes of firewalls, web servers, and intrusion
detection should also be upgraded to meet the enormous
expected increase of incoming and outgoing gigabit
traffic to end users; otherwise, user experience will be
relatively poor as these network middleboxes will not
keep up with the multi-gigabit network demand. In
addition, gradual deployment and migration are more
practical than a full 802.11ac rollout to ensure smooth
and seamless transition.
Finally, interoperability in terms of performance and
management features of the different 802.11ac vendor
chipset and product offerings can be a big challenge.
This may have as much impact on the overall system
performance as 802.11ac itself. So, it is advisable to
perform thorough and comprehensive interoperability
testing among different vendor products and solutions
prior to making such products and chipsets available on
the market.
VI. CONCLUSION
In this paper, we have presented recent technological
advancement and developments that have enhanced
wireless speeds and led to the deployment of various
WLAN standards. We have highlighted the primary
drivers that have created the need for gigabit wireless
speeds. Early evaluations of the latest 802.11ac products
(Wave 1) have demonstrated that IEEE 802.11ac
provides better throughput than its predecessor (802.11n).
However, it has been observed that several new features
(beamforming, support for spatial streams, channel
bonding, advanced modulation) designed for 802.11ac
with the intent of maximizing throughout beyond 1 Gbps
are currently not being utilized to their full capability.
Therefore, delivering gigabit user throughout to IEEE
802.11ac users currently remains a challenge. Despite
the present limitations, the 802.11ac standard shows
significant potential to achieve gigabit user throughput in
the right environmental conditions and with the
deployment of mobile devices that support higher number
of spatial streams.
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Farhan Siddiqui received a B.E. (2000) degree in Computer
Science from Osmania University, India, and M.S. (2003) and
Ph.D. (2007) degrees in Computer Science from Wayne State
University, MI, USA. She was a full-time faculty member at
Bradley University, IL, USA during Fall 2008. Since March
2009, she is a faculty member in the School of Information
Systems and Technology at Walden University, MN, USA. Her
current research interests are in Wireless Networks, Mobile
Computing, Internet-of-Things (IoT), and Network Security.
Sherali Zeadally is an Associate Professor at the College of
Communication and Information, University of Kentucky,
Lexington, KY, USA. He received his bachelor and doctorate
degrees in computer science from the University of Cambridge,
Cambridge, England, and the University of Buckingham,
Buckingham, England, respectively. He is a Fellow of the
British Computer Society and a Fellow of the Institution of
Engineering Technology, England.
Khaled Salah is an Associate Professor in the Electrical and
Computer Engineering Department, Khalifa University of
Science, Technology and Research (KUSTAR). He received the
B.S. degree in Computer Engineering with a minor in Computer
Science from Iowa State University, USA, in 1990, the M.S.
degree in Computer Systems Engineering from Illinois Institute
of Technology, USA, in 1994, and the Ph.D. degree in
Computer Science from the same institution in 2000. He has
over 10 years of industrial experience in software and firmware
development. He joined KUSTAR in August 2010. Prior to
joining KUSTAR, Khaled was an associate professor in the
Department of Information and Computer Science, King Fahd
University of Petroleum and Minerals (KFUPM), Dhahran,
Saudi Arabia. Khaled has been teaching graduate and
undergraduate courses and has over 100 publications in the
areas of cloud computing, computer and network security,
operating systems, computer networks, and performance
evaluation. Khaled is an Editorial Board member of a number of
prestigious international journals including IET
Communications, IET Networks, Elsevier JNCA, Wiley IJNM,
Wiley SCN, and J.UCS. Khaled was the recipient of Khalifa
University Outstanding Research Award 2014/2015, KFUPM
University Excellence in Research Award of 2008/09, and
KFUPM Best Research Project Award of 2009/10, and also the
recipient of the departmental awards for Distinguished Research
and Teaching in prior years.
