ieee 802.11ac standard

Registration Number: CUPB\MTECH-CS\SET\CST\2013-14\01 CBS.503

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].


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


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


• 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


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.