Table Of Content802.11ac: A Survival Guide
Matthew S. Gast
802.11ac: A Survival Guide
by Matthew S. Gast
Copyright © 2013 Matthew S. Gast. All rights reserved.
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ISBN: 978-1-449-34314-9
[LSI]
For L.,
who reminds me it’s okay to have my head in the clouds sometimes.
And for the NCSA instruction team who made me into a pilot so I can get there:
Mike, Terence, Larry, Mike, John, Buzz, and John.
Table of Contents
Foreword. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
Preface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi
1. Introduction to 802.11ac. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
History 1
The Core Technology of 802.11ac 4
Beamforming and Multi-User MIMO (MU-MIMO) 6
Operating Frequency Band for 802.11ac 8
802.11ac Product Development Plans 9
2. The PHY. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Extended MIMO Operations 11
Radio Channels in 802.11ac 12
Radio Channel Layout 12
Available Channel Map 15
Transmission: Modulation, Coding, and Guard Interval 17
Modulation and Coding Set (MCS) 17
Guard Interval 20
Error-Correcting Codes 21
PHY-Level Framing 21
The VHT Signal Fields 23
The Data Field 28
The Transmission and Reception Process 29
802.11ac Data Rates 32
802.11ac Data Rate Matrix 32
Comparison of 802.11ac Data Rates to Other 802.11 PHYs 35
Mandatory PHY Features 36
v
3. The MAC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
Framing 37
Frame Size and Aggregation 38
Management Frames 40
Medium Access Procedures 44
Clear-Channel Assessment (CCA) 45
Protection and Coexistence of 802.11ac with Older 802.11 Devices 48
Dynamic Bandwidth Operation (RTS/CTS) 50
Security 54
Mandatory MAC Features 56
4. Beamforming in 802.11ac. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
Beamforming Basics 60
Null Data Packet (NDP) Beamforming in 802.11ac 65
Single-User (SU) Beamforming 69
Channel Calibration for Single-User Beamforming 70
Multi-User (MU) Beamforming 74
Channel Calibration for Multi-User Beamforming 74
Multi-User MIMO Transmission 78
MU-MIMO Implementation 82
5. 802.11ac Planning. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
Getting Ready for 802.11ac 88
Catching the 802.11ac Technology Wave 89
Client Device Mix 91
Application Planning 93
Physical Network Connections 95
Security 98
Additional Planning Considerations 100
802.11ac Radio Planning 101
Available Radio Channels 101
Coverage and Capacity Estimates 101
Equipment Selection 106
Network Architecture for 802.11ac 109
Hardware Considerations 115
Building an 802.11ac Network 118
Channel Selection 118
Network Tuning and Optimization 120
Checklist 124
Glossary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
vi | Table of Contents
Foreword
Today, it’s easy to take Wi-Fi and its magical benefits for granted. Wi-Fi is a fundamental
part of our Internet ecosystem—it’s hard to imagine a world without it. In fact, the world
without Wi-Fi wouldn’t be the world we have; we’d be missing out on vast elements of
the Internet’s potential.
But the invention of Wi-Fi wasn’t inevitable. The technological innovation we call Wi-Fi
required a major innovation in U.S. government spectrum policy.
Wi-Fi is a use of spectrum on an unlicensed basis, and the Federal Communications
Commission (the U.S. government agency created more than 75 years ago to manage
communications, including those using electromagnetic spectrum) didn’t allow that
type of use until 1985. Spectrum frequencies were assigned only on an exclusive li‐
censed basis. These exclusive licenses—granted to launch radio, TV, satellite, and back‐
haul transmissions—helped create tremendous economic and social value, so maybe it
wasn’t a surprise that the FCC hadn’t authorized spectrum bands for unlicensed use.
But then, along came an idea: there were some bands of spectrum that were lying largely
fallow—at 900 MHz, 2.4 GHz, and 5.8 GHz. Nobody could figure out what they could
be licensed for. The bands were surrounded by other commercial uses, and transmis‐
sions at high or even moderate power levels or distances would cause interference. These
became known as the “garbage” or “junk bands,” and they sat there.
That is, until a brilliant policy innovator named Michael Marcus, an FCC staff engineer,
suggested that this spectrum be made available for use without a license and on a shared
basis, as long as the transmissions were at low power levels and they didn’t interfere with
neighboring licensed uses.
The bet was that innovators would figure out how to weave value out of that spectrum.
Although it wasn’t framed this way at the time, the idea was simple, forward-looking,
and in retrospect, obviously consistent with the great arc of American invention: provide
a platform for innovation, and innovators will come.
vii
So on May 9, 1985, the FCC adopted a little-noticed Order on “spread spectrum tech‐
nology” that opened up the junk bands. And innovators got to work.
Before long, someone had invented garage-door openers using unlicensed spectrum;
then wireless microphones, cordless phones, Bluetooth, and eventually Wi-Fi.
Wi-Fi has had staggering success: from a standing start, it’s now been adopted in roughly
200 million households worldwide. There are more than 750,000 Wi-Fi hot spots glob‐
ally, and over 800 million Wi-Fi-enabled devices are sold every year. And all of these
metrics are growing.
Devices and services built on unlicensed spectrum are an essential part of the U.S.
economy: studies estimate that unlicensed spectrum generates as much as $37 billion
annually for the U.S. economy. Wi-Fi hot spots in the United States increase the value
of licensed broadband service by an estimated $25 billion a year.
And the benefits have dovetailed into other key sectors: 80% of wireless healthcare
innovations, for example, are now on done on unlicensed spectrum, according to one
report. Unlicensed spectrum is transforming our homes, with amazing products already
in the market offering entirely new and exciting ways to enjoy music and video, and
other products to drive energy efficiency. Wi-Fi is a key basis of machine-to-machine
communications—or the Internet of Things—a swiftly emerging market with potential
to transform any number of sectors; we’ve had a 300% increase in connected M2M
devices using unlicensed spectrum in the past five years, and that’s just the beginning.
In other words, unlicensed spectrum is a boon for the American economy, and it con‐
tinues today to provide start-ups and innovators access to a test bed for spectrum that
is used by millions, helping bring new technologies to consumers in a rapid fashion.
Wi-Fi hasn’t been the only major spectrum policy innovation in the last three decades.
The FCC pioneered spectrum auctions for the world in the 1990s—an alternative to the
less-efficient, case-by-case administration of licenses through lotteries and comparative
hearings—and has since conducted over 80 auctions, granting more than 30,000 licen‐
ses. These auctions have generated over $50 billion for the U.S. Treasury and, even more
important, over $500 billion in value for the U.S. economy, according to expert
economists.
The FCC also, quite consequentially, began to grant spectrum licenses for flexible use,
rather than strictly circumscribing use to particular purposes. Flexible spectrum rights
help ensure spectrum moves to uses valued most highly by markets and consumers,
and the FCC has been hard at work the past few years to maximize flexibility and remove
outdated use rules and restrictions.
viii | Foreword
