802.11ac basics - surf | home · pdf file© 2011 aerohive networks confidential matthew...

© 2011 Aerohive Networks CONFIDENTIAL

Matthew Gast, Director of Advanced Technology & chair of the 802.11-2012 revision

SURFnet WLPC, June 2, 2014

802.11AC BASICS


Who are you? (and why should I care?)

•  IEEE 802.11 working group ›  Past chair, IEEE 802.11 revision

task group ›  Past officer of 802.11u task

group (core of HotSpot 2.0)

• Wi-Fi Alliance ›  Chair, Security marketing &

technical task groups

•  Industry experience ›  Emerging technology

designer & investigator for Aerohive

›  I make stuff

• Noted wearer of sunglasses ›  And avoider of ties

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Agenda

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• 802.11ac history & key features

• Wider channels › Dynamic bandwidth sharing › Channel planning

• Spatial stream expansion

• 256-QAM

• Beamforming & MU-MIMO


802.11ac standards timeline (as of June 2013)

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2007 2008 2009 2010 2011 2012 2013 2014

May 2007 VHT study group

approved

September 2008 802.11ac PAR

approved

May 2011 802.11ac draft 1.0

January 2012 802.11ac draft 2.0

(basis of Wi-Fi cert program)

January 2013 802.11ac draft 5.0

June 2013 Wi-Fi Alliance 11ac

certification program launches

February 2014 802.11ac ratified

Official timeline: http://grouper.ieee.org/groups/802/11/Reports/802.11_Timelines.htm

December 2013 802.11ac ratified


Key 802.11ac features

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1.   80 & 160 MHz channels ›  Higher throughput than with today’s 802.11n channels ›  802.11ac will require more spectrum than 802.11n

2.   Up to 8 spatial streams ›  It may take some time before products are available with more

than 4

3.   256-QAM ›  Adds additional speed at short ranges where RF interference is

low ›  High-performance radios are required to take maximum

advantage, and benefits of 256-QAM will not cross a building’s wall

4.   Transmit beamforming & multi-user MIMO (MU-MIMO) ›  MU-MIMO is the driving feature of 802.11ac, but is not yet

present in shipping products (or even chips)


Major features of 802.11ac

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802.11ac protocol feature

Description First-wave gain over 802.11n

Second-wave gain over 802.11n

80 & 160 MHz channels

802.11n supports only 40 MHz channels; wider channels support higher data rates

~2x (80 MHz)

~4x (160 MHz)

Up to 8 spatial streams

802.11n is largely 3 SS; currently planned 802.11ac chips only support up to 4 SS

~1x (3 SS) ~1.33x (4 SS)

256-QAM 64-QAM is the maximum in 802.11n

~1.33x ~1.33x

Multi-user MIMO Beamforming was not widely supported in 802.11n

n/a ~2x?

Total ~2.5x ~15x


The short version: 802.11ac speeds

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802.11n (2-stream)

802.11n (3-stream)

802.11ac (3-stream)

Single-stream mobile device (e.g. smartphone)

20 MHz 75 75 87 40 MHz 150 150 200 80 MHz - - 433

Two-stream device (e.g. tablet or low-end notebook)

20 MHz 150 150 173 40 MHz 300 300 400 80 MHz - - 867

Three-stream device (e.g. high-end notebook)

20 MHz 150 225 289 40 MHz 300 450 600 80 MHz - - 1300

Note: These are the data rates for individual frames; maximum throughput will likely be at least 33% lower (as measured by TCP throughput)


80 & 160 MHz channels

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• Wider channels support higher data rates › Double the channel width from 40 MHz to 80 MHz and the data

rate doubles ›  160 MHz channels may be contiguous (one block) or non-

contiguous (two 80 MHz channels)

•  First-wave products support 80 MHz channels ›  Required by 802.11ac standard ›  Five available channels (3 require DFS) will provide coverage

throughout most buildings

• Channel plan now fixes channels so wide channels do not overlap with each other ›  Improvement from 802.11n’s “40-above” and “40-below”

modes that allowed unintentional overlap


Bandwidth sharing

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• Networks have primary channels and secondary channels ›  Primary is the half of the channel you use, secondary is the half

you don’t

• Channel size is allowed to switch per-frame

Figure from 802.11ac: A Survival Guide by Matthew Gast


Time slicing and dynamic bandwidth usage

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Two networks have distinct primary 20 and 40 MHz networks, but share the same 80 MHz channel

20#MHz#primary#

20#MHz#primary#

40#MHz#primary#

40#MHz#primary#

64#

60#

56#

52#

80#MHz#(shared)#

64#

60#

56#

52#

20#

40#

40#

20#

20#

80#

40#

20#

80#

20#

40#

40#



36 40 44 48 52 56 60 64


Channel Planning

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First AP chooses channel 40

Second AP chooses different 80 MHz channel

Third AP chooses unoccupied 40 MHz channel

Fourth AP chooses unoccupied 40 MHz channel

Fifth AP chooses unoccupied 20 MHz channel


Spatial streams

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• 802.11ac defines up to 8 spatial streams (SS) › Compares to a maximum of 4 in 802.11n (and only 3 SS was

widely implemented) ›  Each SS requires an antenna, so 8 SS modes require both AP

and device to have 8 antennas

• Additional streams increase throughput › All streams use the same data rate, so throughput scales

linearly with each SS

• Multi-user MIMO (MU-MIMO) enables 8 SS to be divided among four devices › Can transmit to multiple single-stream devices (e.g. phone,

tablet) at the same instant


256-QAM

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• A constellation point encodes an amplitude/phase shift combination ›  Pack more points into the constellation to transmit more bits

• Additionally, more aggressive error code means more of the bits are data and fewer are overhead

• 802.11ac devices can still fall back to 802.11n rates if radio link is not good enough


And on to that fourth feature

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•  The first three features are pretty easy, and are widely available in shipping products › Well, not 160 MHz yet

•  That leaves beamforming and (downlink) multi-user MIMO › AP-to-device direction only

•  Three out of four ain’t bad, but that last one is a doozy Duesy


Wired Networking Can Teach Us Something

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Netgear EN-104TP Actually a hub! Kalpana EtherSwitch

$1500 per port (& that was cheap!) Not wire speed

Aerohive AP 370 Just like a hub

Hang 10 with 802.11ac wave 2!


Beamforming in 802.11ac

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• Objective of beamforming is to steer the energy towards particular receivers ›  By increasing SNR, we can increase

data rates

• Beamforming may be explicit or implicit ›  Implicit measures by inference ›  Explicit is an actual channel measurement

All figures from 802.11ac: A Survival Guide (and used with permission!)


Multi-user MIMO

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•  The stuff everybody knows: › Multi-user transmissions go to multiple receivers at the same

time (up to 4 in the spec) ›  Spatial streams can be divided up between the four receivers

•  The less obvious stuff: ›  Explicit channel measurement needs to happen more often –

maybe 10x as frequently (10 ms instead of 100ms?) › MU-MIMO trades peak speed to a single client for total system

throughput › Much better for single-stream clients – viva la BYOD!`


Matrix Math!

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• Doing beamforming requires knowing what the path is like between each {transmitter, receiver} pair

• Practically speaking, the only way to do this is matrix math, where each matrix entry describes on of the many paths

• Several matrices are used ›  H is the channel matrix that describes the

path between transmitter & receiver › Q is the steering matrix that alters the

distribution of energy along a path ›  V is the feedback matrix, sent as part

of the measurement process to derive Q


Focusing energy

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• Omnidirectional transmission sprays energy everywhere

• Application of a steering matrix will send energy preferentially in one direction


Null steering

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• MU-MIMO clients may interference with each other › A new “inter-something” interference: inter-user interference

•  Clients must be separated “enough” to avoid interfering with each other

•  Ideal situation: the steering matrix creates a “null” for all other users


Unanswered Questions in MU-MIMO

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• Beamforming implementation unknowns › Magnitude of inter-client interference ›  Required (angular?) separation between clients ›  Performance improvements in different physical settings

(closed offices, cubicles, open space) ›  How much does increased beamforming range increase

required channel separation? ›  Effectiveness of null steering – will this practically require that

the number of streams be less than the number of radio chains – i.e., can you use all N streams in an N-stream device

• Potential major changes to QoS queuing because MU-MIMO may transmit different priority frames simultaneously


What does this all mean? Designing for 802.11ac Today

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•  Lots of things are the same as 802.11n › One wire to the AP is ok today › Multipath is your friend › Nope, WEP is really dead

• 802.11ac is about capacity, not coverage › New installs should use a second

(cat 6) cable for future capacity ›  Improved capacity reduces

average latency

• Changes in capacity change the way we build networks ›  Track application demands


100% discount today!

• Many presentations include a discount code to purchase the book

•  Today, Aerohive sponsored books

•  If you’d like a signature, ask at the break ›  But I won’t make it out to

“eBay”


Blue Skies and Tailwinds for Wi-Fi

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Not my plane, but I’ve flown the type…

It is my flight trace, though!

Questions to: mgast (at) aerohive (dot) com


DANK U WEL VOOR HET LUISTEREN

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