very high density 802.11ac networks · 2015-03-27 · • non-wi-fi interferers • higher my-fi...

#ATM15 |

Very High Density 802.11ac Networks

Chuck Lukaszewski, CWNE #112

@ArubaNetworks

CONFIDENTIAL © Copyright 2015. Aruba Networks, Inc. All rights reserved

2 #ATM15 |

Agenda

•  Announcing New Very High Density VRD •  802.11ac vs. 802.11n @VHD •  80-MHz vs. 40-MHz vs. 20-MHz Channels •  DFS Channels •  New VHD Capacity Planning Methodology •  How 802.11 Channels Behave Under High Load •  Understanding 802.11 TXOP Airtime Usage •  Collision Domains & RF Spatial Reuse


3 #ATM15 |

Announcing New Very High Density VRD

•  100% 802.11ac •  End-to-end system

architecture & dimensioning •  New capacity planning

methodology •  Addresses a wide range of

customer use cases •  Available late March


4 #ATM15 |

Modular VRD Structure

Different guides for different audiences IT Leaders Carrier Standards Account Managers

Customer Engineers Partner & Aruba SEs Install Technicians

WLAN Achitects Carrier RF Engineers

All Audiences

Venue Owners

5 #ATM15 |

802.11ac vs. 802.11n @VHD

6 CONFIDENTIAL © Copyright 2015. Aruba Networks, Inc. All rights reserved

#ATM15 |

How Far We’ve Come

•  “Coverage” WLANs came first •  These evolved into “Capacity”

WLANs (with limited HD zones) –  250m2 / 2500 ft2 = 25 devices per cell

•  BYOD made every capacity WLAN a high-density network –  3 devices/person = 75 per cell

•  HD WLANs from 2011 are now very high-density (VHD) –  100+ devices per “cell”. Devices may

be associated to multiple BSS operators in same RF domain.

Waiting for the new Pope in St. Peter’s Square

NBC Today Show, February, 2013, http://instagram.com/p/W2BuMLQLRB/


#ATM15 |

How Do 802.11ac Features Impact VHD Design?

802.11ac Technology Promise VHD Impact

80-MHz & 160-MHz Channels

Increase burst rates for individual STAs

None. Stay with 20-MHz channels for VHD areas

256-QAM New MCS rates up to 33% faster

Minimal. Rates only usable within ~5m of AP

8 Spatial Streams Peak data rates up to 6.9Gbps

None. In future, clients will be mostly capped at 2SS

DL MU-MIMO Transmit to 4 STAs at the “same” time

TBD until Wave2 in 2016. Sounding overhead offsets gains

Frame Aggregation (A-MSDU & A-MPDU)

Enable the MAC to keep up with the 802.11ac PHY

Minimal. Majority of frames in VHD areas are bursty & small









DL MU-MIMO Transmit to 4 STAs at the “same” time

TBD until Wave2 in 2016. Sounding overhead offsets gains




















#ATM15 |

Why 802.11ac Does Matter for VHD

802.11ac Impact Promise VHD Impact

Broad-based DFS channel support

Open up 15 channels (FCC domains)

Add up to 750Mbps of capacity

Faster AP CPUs Process small frames and collisions faster

Increase channel bandwidth closer to theoretical max

More AP memory Larger table sizes

Better handle very high BSSID count environments

Clients converge to 2SS Increase per-STA burst rate even in narrow channels

Get clients off the air faster

TxBF Improve SINRs both directions

Robustness improvement; client-side CSI feedback









Clients converge to 2SS Increase per-STA burst rate even in narrow channels

Get clients off the air faster


















Add up to 800 Mbps of capacity @ 50Mbps/channel

9 #ATM15 |

80-MHz vs. 40-MHz vs. 20-MHz Channel Widths


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Why 20-MHz Channels – Reuse Distance

•  More channels = more distance between same-channel APs


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Why 20-MHz Channels – More RF Reasons

•  Reduced Retries – Bonded channels are more exposed to interference on subchannels •  Using 20-MHz channels allows some channels to get

through even if others are temporarily blocked

•  Higher SINRs – Bonded channels have higher noise floors (3dB for 40-MHz, 6dB for 80-MHz) •  20-MHz channels experience more SINR for the same data

rate, providing extra link margin in both directions


#ATM15 |

Why 20-MHz Channels - Performance

Which Chariot test will deliver higher goodput? Each test uses the exact same 80-MHz bandwidth

Each test uses the exact same number of STAs

Both VHT40 BSS will victimize each other with ACI

All four VHT20 BSS will victimize each other


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VHT20 Beats VHT40 & VHT80 – 1SS Clients

0 Mbps

50 Mbps

100 Mbps

150 Mbps

200 Mbps

250 Mbps

300 Mbps

5 10 25 50 75 100 Clients

VHT20x4 Up

VHT20x4 Bidirect

VHT40x2 Up

VHT40x2 Bidirect

VHT80x1 Up

VHT80x1 Bidirect

Down

Up

Bidirect

VHT80 falls off at 25 STAs

VHT40 falls off at 75 STAs


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VHT20 Beats VHT40 & VHT80 – 2SS Clients

200 Mbps

250 Mbps

300 Mbps

350 Mbps

400 Mbps

450 Mbps

500 Mbps

550 Mbps

600 Mbps

650 Mbps

5 10 25 50 75 100 Clients

VHT20x4 Up

VHT20x4 Bidirect

VHT40x2 Up

VHT40x2 Bidirect

VHT80x1 Up

VHT80x1 Bidirect

Down

Up

Bidirect

Why? What is the mechanism?

15 #ATM15 |

DFS Channels: To Use or Not To Use


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General Rule

Use DFS channels in the USA for VHD areas!!

•  The number of collision domains is the primary constraint on VHD capacity

•  The number of STAs per collision domain is the second major constraint on capacity

•  VHD networks are ultimately about tradeoffs

The benefit of employing DFS channels almost always* outweighs the cost.


#ATM15 |

DFS Usage Exceptions

1.  Client capabilities –  If more than 15-20% of the expected clients are proven to be 5-

GHz capable but unable to use DFS

2.  Proven, recurring radar events –  A DFS survey with the actual APs to be deployed shows regular

events on specific channels. •  Just because certain channels are impacted do not rule out the band

–  Survey should be done at multiple locations & elevations

3.  Avoid channel 144 until >50% of STAs can see it


#ATM15 |

Balancing the Risks & Rewards

•  Client capabilities •  As of 2015, the vast majority of mobile devices shipping in USA

support DFS channels •  Non-DFS clients will be able to connect due to stacking of multiple

channels (although perhaps at lower rates) •  It is easily worth it to provide a reduced connect speed to a an

unpredictable minority of clients, in exchange for higher connect speeds for everyone else all the time

•  Radar events •  It is worth having a small number of clients occasionally interrupted

in exchange for more capacity for everyone all the time

19 #ATM15 |

New Capacity Planning Methodology


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System vs. Channel vs. Device Throughput

Channel 1 Throughput

Channel 2 Throughput

Channel X Throughput

Per-Device Throughput

2.4 GHz 5 GHz Total System Throughput

+

+

+

+


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Total System Throughput Formula

Where: •  Channels = Number of channels in use by the VHD

network •  Average Channel Throughput = Weighted average

goodput achievable in one channel by the expected mix of devices for that specific facility

•  Reuse Factor = Number of RF spatial reuses possible. For all but the most exotic VHD networks, this is equal to 1 (e.g. no reuse).

!"!=#$%&&'()∗*+',%-'##$%&&'(#!$,./-$0/1∗2'/)'#3%41.,


#ATM15 |

Step 1 – Choose Channel Count

•  US allows: •  9 non-DFS channels •  13-16 DFS channels*

•  Deduct: •  Channel 144 •  House channel(s) •  Proven radar channels •  AP-specific channel limitations


#ATM15 |

Step 2 – Choose Unimpaired Channel Throughput

0 Mbps

50 Mbps

100 Mbps

150 Mbps

200 Mbps

250 Mbps

1 5 10 25 50 75 100 Simultaneously Transmitting Clients

GS4 TCP Up

GS4 TCP Bidirect

MBA TCP Up

MBA TCP Bidirect

MBP TCP Up

MBP TCP Bidirect

Down

Up

Bidirect

Choose spatial stream mix that approximates expected device population


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Step 3 – Apply Impairment Factor

VHD Venue Type

Suggested 2.4-GHz

Impairment

Suggested 5-GHz

Impairment** Rationale

Classroom / Lecture Hall 10% 5%

•  Above average duty cycles •  Little or no reuse of channels in the same room •  Structural isolation of same-channel BSS in adjacent rooms •  Minimal My-Fi usage

Convention Center 25% 10%

•  Moderate duty cycles •  Significant numbers of same-channel APs •  Large open areas with direct exposure to interference sources •  Non-Wi-Fi interferers •  Higher My-Fi usage in booth displays, presenters, attendees

Airport 25% 15% •  Minimal duty cycles (except for people streaming videos) •  Structural isolation of same-channel BSS in adjacent rooms •  Heavy My-Fi usage

Casino 25% 10% •  Low duty cycles on casino floor •  Low My-Fi usage

Stadium / Arena 50% 25%

•  Low-to-moderate duty cycles •  Significant numbers of same-channel APs •  Large open areas with direct exposure to interference sources •  Non-Wi-Fi interferers •  High My-Fi usage


#ATM15 |

Step 4 – Choose Reuse Factor

RF spatial reuse must be assumed not to exist unless proven otherwise in VHD facilities of 10,000 seats or less (RF = 1).

•  Reuse factor is the number of devices that can use the same channel at exactly the same time

•  Reusing channel numbers is not the same as reusing RF spectrum


#ATM15 |

Step 5 – Calculate TST By Band

VHD Venue Type Suggested

5-GHz ** Suggested

2.4-GHz Lecture Hall 5% 10% Convention Center 10% 25% Airport 15% 25% Casino 10% 25% Stadium / Arena 25% 50%

Spatial Stream Mix

50 Concurrent

75 Concurrent

100 Concurrent

1SS Device 50 Mbps 38 Mbps 31 Mbps 2SS Device 100 Mbps 72 Mbps 51 Mbps 3SS Device 158 Mbps 118 Mbps 78 Mbps

Channel Type

USA 5-GHz Count

Non-DFS 9 DFS 11 Total 20

Step 1 - Channels Step 2 – Unimpaired Throughput Step 3 – Impairment

# Description Channels Unimpaired Throughput

Impaired 5-GHz TP

Impaired 2.4-GHz TP Reuse 5-GHz TST 2.4-GHz TST

1 Non-DFS Lecture Hall 9 + 3 100Mbps 95Mbps 90Mbps 1 9 * 95Mbps = 855 Mbps 3 * 90Mbps = 270 Mbps

2 DFS Arena 20 + 3 40 Mbps 30 Mbps 20 Mbps 1 20 * 40Mbps = 800 Mbps 3 * 20 Mbps = 60 Mbps

3 DFS Stadium 20 + 3 40 Mbps 30 Mbps 20 Mbps 4 3.2 Gbps 240 Mbps

Step 5 – Calculate TST By Band


Impaired 5-GHz TP



2 DFS Arena 20 + 3 40 Mbps 30 Mbps 20 Mbps 1 20 * 40Mbps = 800 Mbps 3 * 20 Mbps = 60 Mbps


Impaired 5-GHz TP




#ATM15 |

Per-Device Throughput Formula

Where: •  Associated Device Capacity (ADC) = Percentage of

seating capacity with an active Wi-Fi device * average number of Wi-Fi devices per person. Typically computed per band.

•  Device Duty Cycle = Average percent of time that any given user device attempts to transmit

*56!=# !.1%(#"8)1'9#!$,./-$0/1#/*)).4;%1'<#6'+;4'##%0%4;18∗6'+;4'#6/18##84(' 

It is generally impossible to guarantee a specific per-device value in a VHD system.


#ATM15 |

Step 1 – Estimate ADC

•  Start with the seating / standing capacity of the VHD area to be covered

•  Then estimate the take rate (50% is a common minimum)

•  Choose the number of devices expected per person. This varies by venue type. It might be lower in a stadium and higher in a university lecture hall or convention center salon. –  For example, 50% of a 70,000 seat stadium would be 35,000 devices assuming each

user has a single device –  100% of a 1,000 seat lecture hall where every student has an average of 2.5 devices

would have an ADC equal to 2,500

•  More users should be on 5-GHz than 2.4-GHz. ADC should be computed by frequency band. In general you should target a ratio of 75% / 25%.

•  Association demand is assumed to be evenly distributed throughout the coverage space.


#ATM15 |

Step 2 – Choose a Device Duty Cycle •  Subjective decision made by the network

architect, based on expected user applications

•  This duty cycle is %Time the user or device wants to perform this activity. •  It is not the same as the application duty cycle!

Category Duty Cycle User & Device Behavior Examples Background 5% Network keepalive / App phonehome

Checking In 10% Web browsing / Checking email / Social updates

Semi-Focused 25% Streaming scores / Online exam

Working 50% Virtual desktop

Active 100% Video streaming / Voice streaming / Gaming


#ATM15 |

Examples

# Description Seats Take Rate

Devices/ Person ADC

3 DFS Stadium 60K 50% 1 30K


Devices/ Person ADC

1 Lecture Hall 500 75% 2.5 938


Devices/ Person ADC

2 DFS Arena 20K 50% 1 10K

5-GHz Per-Device Goodput

2.4-GHz Per-Device Goodput $%%#>?0)/&'(#∗)*% =(#>?0)

),*#>?0)/&'(#∗)*% =).(#>?0)


2.4-GHz Per-Device Goodput $**#>?0)/,%**#∗.*% =.#>?0)

'*#>?0)/)%**#∗.*% =)&*#@?0)


2.4-GHz Per-Device Goodput /.)#A?0)/)).%@#∗.*% =.#>?0)

)&*#>?0)/,.%@#∗.*% =/)*#@?0)

Band Split

Duty Cycle

5-GHz TST

2.4-GHz TST

50/50 20% 855 Mbps 270 Mbps

Band Split

Duty Cycle

5-GHz TST

2.4-GHz TST

75/25 10% 800 Mbps 60 Mbps

Band Split

Duty Cycle

5-GHz TST

2.4-GHz TST

75/25 10% 3.2 Gbps 240 Mbps

500 * 75% * 2.5 = 938 938 / 2 = 469

20,000 * 50% * 1 = 10,000

60,000 * 50% * 1 = 30,000

10K * 75% = 7,500

30K * 75% = 22,500

If only 1 or 2 reuses is actually achieved, drops by 50-75%

31 #ATM15 |

How 802.11 Channels Work Under High Load


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Section Agenda

•  Introduce the Aruba VHD Lab •  Review client scaling charts out to 100 STAs •  Introduce “contention premium” concept •  Break down contention premium using pcaps


#ATM15 |

Aruba Very High Density Lab


#ATM15 |

VHD Lab Testbed Topology


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Test SSID Configuration

wlan ssid-profile "hdtest100-ssid" essid "HDTest-5" a-basic-rates 24 a-tx-rates 18 24 36 48 54 max-clients 255 wmm wmm-vo-dscp "56" wmm-vi-dscp "40" a-beacon-rate 24 ! wlan ht-ssid-profile "HDtest-htssid-profile" max-tx-a-msdu-count-be 3 ! rf dot11a-radio-profile "hdtest100-11a-pf" channel 100E disable-arm-wids-functions Dynamic !

Minimum recommended VHD control rate

Trim out low TX rates

Minimum recommended VHD beacon rate

Increase A-MSDU

Disable WIDS scanning

Increase client count


#ATM15 |

VHT20 Client Scaling to 100 STAs

0 Mbps

50 Mbps

100 Mbps

150 Mbps

200 Mbps

250 Mbps

1 5 10 25 50 75 100 Clients

GS4 TCP Up

GS4 TCP Bidirect

MBA TCP Up

MBA TCP Bidirect

MBP TCP Up

MBP TCP Bidirect

Down

Up

Bidirect

MacBook Pro 3SS BRCM 43460

MacBook Air 2SS BRCM 4360

Galaxy S4 1SS BRCM 4335

Absolute Scale in Bits-per-Second


#ATM15 |

Straw Poll #1

Is anyone surprised that capacity is inversely proportional to client count?


#ATM15 |

Straw Poll #2

What is the primary explanation for the drop?

1.  Collisions & retries 2.  Rate adaptation 3.  MAC layer operation 4.  Higgs boson


#ATM15 |

MIMO Works!

0%

20%

40%

60%

80%

100%

120%

1 5 10 25 50 75 100

Thro

ughp

ut (%

)

Clients

GS4 TCP Up

GS4 TCP Bidirect

MBA TCP Up

MBA TCP Bidirect

MBP TCP Up

MBP TCP Bidirect

Down

Up

Bidirect

Normalized to 3SS = 1

2SS is about 66% across the range

1SS is about 33% across the range

Relative Scale in Percent


#ATM15 |

Contention Premium

0%

20%

40%

60%

80%

100%

120%

1 5 10 25 50 75 100

Thro

ughp

ut (%

)

Clients

GS4 TCP Up

GS4 TCP Bidirect

MBA TCP Up

MBA TCP Bidirect

MBP TCP Up

MBP TCP Bidirect

Down

Up

Bidirect

5% - 10% contention premium

30% - 50%

50% - 60%

10% - 30%

“Contention premium” is the delta between aggregate goodput for 1 STA as compared with a larger number of STAs.


#ATM15 |

Is It Inevitable?

•  CP is a fundamental property of 802.11 •  Capacity losses are expected with CSMA-based contention

•  Or is it? •  Why should cutting the pie into more slices shrink the pie by nearly

60%?

•  Why would there be significantly higher collisions in a clean test environment with a single BSS and a well ordered channel?

•  And why is the drop so similar for a 3SS laptop that can move over 3X the data in the same airtime as a 1SS smartphone?


#ATM15 |

Retries Are Not A Major Cause

0%#

10%#

20%#

30%#

40%#

50%#

60%#

70%#

80%#

90%#

100%#

1# 25# 50# 75# 100#Retry#Packets# Non:Retry#

MBA 2SS, TCP Up, AP-225, 20-MHz

0%#

10%#

20%#

30%#

40%#

50%#

60%#

70%#

80%#

90%#

100%#

1# 30# 50# 75# 100#Retry#Packets# Non:Retry#

MBA 2SS, TCP Down, AP-135, 40-MHz

Retries are well under 10% of frames They do not

grow

Same behavior with 802.11n AP, different channel width, different direction


#ATM15 |

Control Frame Growth Is Key Factor

28K# 48K# 70K# 74K# 84K#3K#6K#

9K# 15K# 18K#

431K#420K# 359K# 335K# 288K#

0K#

50K#

100K#

150K#

200K#

250K#

300K#

350K#

400K#

450K#

500K#

1# 25# 50# 75# 100#

Fram

es'(#

)'

Mgmt#Packets# Ctrl#Packets# NDP#Packets# CRC#Errors# Data#Packets#

37K# 65K# 75K# 84K# 101K#

385K# 355K# 352K# 338K# 326K#

0K#

50K#

100K#

150K#

200K#

250K#

300K#

350K#

400K#

450K#

500K#

1# 30# 50# 75# 100#

Fram

es'(#

)'

Mgmt#Packets# Ctrl#Packets# CRC#Errors# Data#Packets#

Data frames drop by 34%

MBA 2SS, TCP Up, AP-225, 20-MHz MBA 2SS, TCP Down, AP-135, 40-MHz

Control frames increase by 3X

PS NDP frames increase by 5X

Similar behavior with 802.11n AP, different channel width, different direction (NDP not included in this analysis)


#ATM15 |

Power Save Activity Over 3 ms

16 STAs attempt PS state in 3.1msec; 13 succeed


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Average Frame Size Decreases With Load

0K

50K

100K

150K

200K

250K

300K

350K

400K

450K

500K

1 25 50 75 100

Fram

es (#

)

>=2347

2048-2346

1024-2047

512-1023

256-511

128-255

64-127

<64

MBA 2SS, TCP Up, AP-225, 20-MHz

TCP Data

TCP Ack

Control Frames

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

1 25 50 75 100

Fram

es (#

)

>=2347

2048-2346

1024-2047

512-1023

256-511

128-255

64-127

<64


#ATM15 |

0%#

10%#

20%#

30%#

40%#

50%#

60%#

70%#

80%#

90%#

100%#

1# 25# 50# 75# 100#

What Is Driving The Control Frame Growth?

NDP# NDP# NDP# NDP# NDP#RTS#RTS#

RTS#RTS#

RTS#

CTS#

CTS#

CTS#CTS#

CTS#

BA#

BA#

BA#BA#

BA#

Ack#

Ack#

Ack#Ack#

Ack#

0K#

10K#

20K#

30K#

40K#

50K#

60K#

70K#

80K#

90K#

100K#

1# 25# 50# 75# 100#

Fram

es'(#

)'

MBA 2SS, TCP Up, AP-225, 20-MHz 3X more TXOPs = Poor A-MPDU packing efficiency

ACKs are for NDPs. Combined total grows from 6.4K ! 34.2K

NDP#

RTS#

CTS#

BA#

Ack#

Preceded by SIFS (16usec)

Preceded by full arbitration (43usec AIFS + EDCA CW)


#ATM15 |

Rate Adaptation Is Not A Major Contributor

0K#

50K#

100K#

150K#

200K#

250K#

300K#

350K#

400K#

450K#

500K#

1# 25# 50# 75# 100#

Fram

es'(#

)'173.3#Mbps#

130#Mbps#

117#Mbps#

115.5#Mbps#

104#Mbps#

86.5#Mbps#

78#Mbps#

72#Mbps#

39#Mbps#

24#Mbps#

18#Mbps#

12#Mbps#

6#Mbps#

Rate adaptation on data frames

NDPs @ 18Mbps Acks @ 12Mbps


#ATM15 |

Contention Premium Summary

•  The principal mechanism behind the contention premium is MAC layer overhead growing with STA count. •  Decreased A-MPDU packing efficiency •  More TXOPs due to more STAs contending •  Airtime efficiency of each TXOP decreases •  Power save NDP/Ack growth due to elevated TXOP activity level

For any given number of STAs, it is better to divide them

across more small channels to minimize this effect.


#ATM15 |

Contention Premium Explains the 80/40/20 Result

0 Mbps

50 Mbps

100 Mbps

150 Mbps

200 Mbps

250 Mbps

300 Mbps

5 10 25 50 75 100 Clients

VHT20x4 Up

VHT20x4 Bidirect

VHT40x2 Up

VHT40x2 Bidirect

VHT80x1 Up

VHT80x1 Bidirect

Down

Up

Bidirect Contention premium for 100 STAs/channel is 50-60%

C.P. for 50 STAs per channel is 10-30%

C.P. for 25 STAs per channel is just 5-10%

50 #ATM15 |

Understanding 802.11 TXOP Airtime Usage


#ATM15 |

How we usually think of a TXOP

TXOP Structure

RTS

CTS

LP

A-MPDU

BA

IFS

IFS

IFS

AIF

S

ED

CA

CW

RTS

CTS

A-MPDU

BA

SIF

S

SIF

S

SIF

S

AIF

S

ED

CA

CW

LP

LP VP

6 24

6 24 6 24

6 MCS9

TXOP with preambles included

BPSK!! BPSK!!

BPSK!! BPSK!!

LP

These diagrams have nothing to do with time!


#ATM15 |

TXOP Scaled to Time (173.3 Mbps Rate)

90 byte A-MPDU (TCP ack)

Arbitration 49.6%

Payload 13.2%

RTS/CTS 23.7%

BA 13.5%

3,000 byte A-MPDU (TCP data)

Even with 3K frame, payload is only 27.6%. VHT preamble is another 8.8%

Best case scenario with no retries. Retries & collisions significantly degrade the payload airtime share.


#ATM15 |

TXOP Time Breakdown (173.3Mbps Rate)

MAC'Unit'Payload'Bytes'

Payload'Bits'

Data'Rate'(Mbps)' Usec'

%'Air>me'w/CSMA'

%Air>me'TXOP'Only'

AIFS[BE]# ## ## ## #############43## 11.7%'##CW[BE]# ## ## ## ###########140## 37.9%'##Legacy#Preamble# ## ## ## #############20## 5.4%' 10.8%#RTS# 20# 160# 18################9## 2.4%' 4.8%#SIFS# ## ## ## #############16## 4.3%' 8.6%#Legacy#Preamble# ## ## ## #############20## 5.4%' 10.8%#CTS# 14# 112# 18################6## 1.7%' 3.4%#SIFS# ## ## ## #############16## 4.3%' 8.6%#VHT#Preamble# ## ## ## #############44## 12.0%' 23.7%#1st#A:MPDU#Delimiter# 4# 32# 173.3################0## 0.1%' 0.1%##1st#A:MPDU## 90# 720# 173.3################4## 1.1%' 2.2%##SIFS## ## ## ## #############16## 4.3%' 8.6%#Legacy#Preamble# ## ## ## #############20## 5.4%' 10.8%#BA# 32# 256# 18##############14## 3.9%' 7.7%#Total#AirTme#including#CSMA# 1280### ###########368## 100.0%' 100.0%#AirTme#for#TXOP#only# ## ## ## ###########186##

Effec>ve'TXOP'Rate'(Mbps)'including'CSMA' 3.5'EffecTve#TXOP#Rate#(Mbps)#for#TXOP#only# ## 6.9#

MAC'Unit'Payload'Bytes'

Payload'Bits'

Data'Rate'(Mbps)' Usec'

%'Air>me'w/CSMA'

%Air>me'TXOP'Only'

AIFS[BE]# ## ## ## #############43## 8.6%'##CW[BE]# ## ## ## ###########140## 27.8%'##Legacy#Preamble# ## ## ## #############20## 4.0%' 6.2%#RTS# 20# 160# 18################9## 1.8%' 2.8%#SIFS# ## ## ## #############16## 3.2%' 5.0%#Legacy#Preamble# ## ## ## #############20## 4.0%' 6.2%#CTS# 14# 112# 18################6## 1.2%' 1.9%#SIFS# ## ## ## #############16## 3.2%' 5.0%#VHT#Preamble# ## ## ## #############44## 8.8%' 13.7%#1st#A:MPDU#Delimiter# 4# 32# 173.3################0## 0.0%' 0.1%##1st#A:MPDU## 3000# 24000# 173.3############138## 27.6%' 43.3%##SIFS## ## ## ## #############16## 3.2%' 5.0%#Legacy#Preamble# ## ## ## #############20## 4.0%' 6.2%#BA# 32# 256# 18##############14## 2.8%' 4.4%#Total#AirTme#including#CSMA# 24560### ###########503## 100.0%' 100.0%#AirTme#for#TXOP#only# ## ## ## ###########320##

Effec>ve'TXOP'Rate'(Mbps)'including'CSMA' 48.9'EffecTve#TXOP#Rate#(Mbps)#for#TXOP#only# ## 76.7#

90 Byte A-MPDU 3,000 Byte A-MPDU

Payload is 1.2% without the preamble

Arbitration is 49.6% using default CWmin[BE]

The “effective” data rate for the TXOP is just 3.5Mbps

Payload increases to 27.6%

Faster rates reduce airtime

TXOP effective rate is just 28% of the payload rate!!


#ATM15 |

TXOP Scaled to Time (866.7 Mbps Rate)

- 500 1,000 1,500 2,000 2,500 3,000 3,500

AIFS+ CW[BE] 183us

RTS+ CTS 87us

VHT Preamble & A-MPDU 2,707us

- - - - BA 50us

A-MPDU of 64 x 4,500B MPDUs

Arbitration 6%

Payload 89.4%

RTS/CTS 2.9%

BA 1.7%

By design, the 802.11 MAC only achieves high efficiency with large A-MPDUs


#ATM15 |

Typical Frame Size – Office Environment

30 minute capture, 6 Channels >80% of frames under 256B


#ATM15 |

Typical Frame Size – Office Environment

30 minute capture, 6 Channels >80% of frames under 256B

<5% of frames over 1KB

>80% of frames under 256B

<15% of frames over 512B


#ATM15 |

Typical Frame Sizes – Football Stadium

10 minute capture, 7 Channels


#ATM15 |

Typical Frame Types – Football Stadium

10 minute capture, 7 Channels

60% are control frames

23% are data frames


#ATM15 |

Importance of Maximizing Payload Data Rates

•  VHT preambles consume as much or more airtime for common TCP frame sizes

•  Even large frames at 80-MHz rates barely equal the VHT preamble time

•  Retries are expensive

1SS VHT20 must be >500B to equal preamble time

Preamble time blows away payload time for all small packets

MPDU must be >=3KB to match preamble time


#ATM15 |

Importance of Reducing Control & Mgmt Rates

•  Increasing minimum rate can greatly reduce airtime used by control frames

•  Multiplier effect at high STA counts due to control frame growth


#ATM15 |

Effect of Beacon Rates in High-BSSID Facilities

1 2 31 0.44% 0.89% 1.33%5 2.22% 4.44% 6.67%

10 4.44% 8.89% 13.33%15 6.67% 13.33% 20.00%20 8.89% 17.77% 26.66%25 11.11% 22.22% 33.33%30 13.33% 26.66% 39.99%35 15.55% 31.10% 46.66%40 17.77% 35.55% 53.32%45 20.00% 39.99% 59.99%50 22.22% 44.43% 66.65%

APs per Channel

Number of SSIDs1 2 3

1 0.19% 0.38% 0.57%5 0.95% 1.90% 2.86%

10 1.90% 3.81% 5.71%15 2.86% 5.71% 8.57%20 3.81% 7.62% 11.43%25 4.76% 9.52% 14.28%30 5.71% 11.43% 17.14%35 6.67% 13.33% 20.00%40 7.62% 15.23% 22.85%45 8.57% 17.14% 25.71%50 9.52% 19.04% 28.56%

APs per Channel


1 0.16% 0.32% 0.48%5 0.80% 1.59% 2.39%

10 1.59% 3.18% 4.78%15 2.39% 4.78% 7.16%20 3.18% 6.37% 9.55%25 3.98% 7.96% 11.94%30 4.78% 9.55% 14.33%35 5.57% 11.14% 16.71%40 6.37% 12.73% 19.10%45 7.16% 14.33% 21.49%50 7.96% 15.92% 23.88%

APs per Channel


1 0.13% 0.26% 0.38%5 0.64% 1.28% 1.92%

10 1.28% 2.56% 3.84%15 1.92% 3.84% 5.76%20 2.56% 5.12% 7.68%25 3.20% 6.40% 9.59%30 3.84% 7.68% 11.51%35 4.48% 8.96% 13.43%40 5.12% 10.23% 15.35%45 5.76% 11.51% 17.27%50 6.40% 12.79% 19.19%

APs per Channel

Number of SSIDs

Revolution Wi-Fi Capacity Planner, http://www.revolutionwifi.net/capacity-planner. Reprinted with permission.

6 Mbps Beacon Rate 18 Mbps Beacon Rate 24 Mbps Beacon Rate 36 Mbps Beacon Rate

62 #ATM15 |

Collision Domains & RF Spectrum Reuse


#ATM15 |

What Is A Collision Domain?

•  A physical area in which 802.11 devices attempting to send can decode one another’s frame preambles.

•  A moment in time. •  Two nearby stations on the same channel will not collide if they

send at different times.

•  Dynamic regions that are constantly moving in space and time based on which devices are transmitting

A collision domain is therefore an independent block of capacity in an 802.11 system.


#ATM15 |

How We Normally Draw Collision Domains

Typical cell diagram showing radius of cell edge


#ATM15 |

How Collision Domains Actually Work

Standard rate vs. range curve (MCS rates)

Decode limit for high-rate control frames Preamble

interference range for all frames


#ATM15 |

Preamble Interference Radius Is HUGE

67 #ATM15 |

Summary & Review


#ATM15 |

Summary – Maximizing VHD Capacity

•  Use every possible channel "  20-MHz channel width + DFS

•  Spread the clients evenly across all the channels " RF design / Steering features

•  Maximize rates of repetitive frame types "  Trim low basic & TX rates / Raise beacon rates / unicast conversion

•  Facilitate aggregation "  Jumbo frames / Increase A-MSDU / Increase TCP window sizes

•  Eliminate unnecessary TX/RX " Broadcast-multicast filters / Probe filtering / RX sensitivity tuning

•  Use top-down capacity planning to force a system-level viewpoint


#ATM15 |

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70 #ATM15 | @ArubaNetworks

very high density 802.11ac networks · 2015-03-27 · • non-wi-fi interferers • higher my-fi...

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