high-density 802.11ac wi-fi design and deployment for large public venues

#ATM15 |

Very High Density 802.11ac Networks

Chuck Lukaszewski, CWNE #112

@ArubaNetworks

CONFIDENTIAL © Copyright 2015. Aruba Networks, Inc. All rights reserved2#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


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


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/

http://instagram.com/p/W2BuMLQLRB/


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


80-MHz & 160-MHz

Channels


individual STAs




faster


within ~5m of AP


6.9Gbps



DL MU-MIMO Transmit to 4 STAs at the

“same” time

TBD until Wave2 in 2016.

Sounding overhead offsets gains


80-MHz & 160-MHz

Channels


individual STAs




faster


within ~5m of AP


6.9Gbps




80-MHz & 160-MHz

Channels


individual STAs




faster


within ~5m of AP


80-MHz & 160-MHz

Channels


individual STAs




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


Broad-based DFS

channel support


domains)



collisions faster




count environments

Clients converge to 2SS Increase per-STA burst rate

even in narrow channels

Get clients off the air faster


Broad-based DFS

channel support


domains)



collisions faster




count environments


Broad-based DFS

channel support


domains)



collisions faster




Broad-based DFS

channel support


domains)

Add up to 800 Mbps of capacity

@ 50Mbps/channel

9#ATM15 |

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


Why 20-MHz Channels – Reuse Distance

• More channels = more distance between same-channel APs


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


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


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


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


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.


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


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


System vs. Channel vs. Device Throughput

Channel 1

Throughput

Channel 2

Throughput

Channel X

Throughput

Per-Device

Throughput

2.4 GHz 5 GHzTotal

System

Throughput

+

+

+

+


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).

𝑻𝑺𝑻 = 𝑪𝒉𝒂𝒏𝒏𝒆𝒍𝒔 ∗ 𝑨𝒗𝒆𝒓𝒂𝒈𝒆 𝑪𝒉𝒂𝒏𝒏𝒆𝒍 𝑻𝒉𝒓𝒐𝒖𝒈𝒉𝒑𝒖𝒕 ∗ 𝑹𝒆𝒖𝒔𝒆 𝑭𝒂𝒄𝒕𝒐𝒓


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


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


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


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


Step 5 – Calculate TST By Band

VHD Venue TypeSuggested

5-GHz **Suggested

2.4-GHz

Lecture Hall 5% 10%

Convention Center 10% 25%

Airport 15% 25%

Casino 10% 25%

Stadium / Arena 25% 50%

SpatialStream Mix

50Concurrent

75 Concurrent

100 Concurrent

1SS Device 50 Mbps 38 Mbps 31 Mbps



ChannelType

USA 5-GHzCount

Non-DFS 9

DFS 11

Total 20

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

# Description ChannelsUnimpaired Throughput

Impaired 5-GHz TP

Impaired2.4-GHz TP Reuse 5-GHz TST 2.4-GHz TST

1Non-DFS Lecture

Hall9 + 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


1Non-DFS Lecture


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


1Non-DFS Lecture



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

𝑨𝑷𝑫𝑻 =𝑻𝒐𝒕𝒂𝒍 𝑺𝒚𝒔𝒕𝒆𝒎 𝑻𝒉𝒓𝒐𝒖𝒈𝒉𝒑𝒖𝒕

𝑨𝒔𝒔𝒐𝒄𝒊𝒂𝒕𝒆𝒅 𝑫𝒆𝒗𝒊𝒄𝒆 𝑪𝒂𝒑𝒂𝒄𝒊𝒕𝒚 ∗ 𝑫𝒆𝒗𝒊𝒄𝒆 𝑫𝒖𝒕𝒚 𝑪𝒚𝒄𝒍𝒆

It is generally impossible to guarantee a

specific per-device value in a VHD system.


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.


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


Examples

# Description SeatsTake 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-DeviceGoodput

2.4-GHz Per-DeviceGoodput

𝟖𝟓𝟓 𝑴𝒃𝒑𝒔

𝟒𝟔𝟗 ∗ 𝟐𝟎%= 𝟗𝑴𝒃𝒑𝒔

𝟐𝟕𝟎 𝑴𝒃𝒑𝒔

𝟒𝟔𝟗 ∗ 𝟐𝟎%= 𝟐. 𝟗 𝑴𝒃𝒑𝒔



𝟖𝟎𝟎 𝑴𝒃𝒑𝒔

𝟕𝟓𝟎𝟎 ∗ 𝟏𝟎%= 𝟏𝑴𝒃𝒑𝒔

𝟔𝟎 𝑴𝒃𝒑𝒔

𝟐𝟓𝟎𝟎 ∗ 𝟏𝟎%= 𝟐𝟒𝟎 𝑲𝒃𝒑𝒔



𝟑. 𝟐 𝑮𝒃𝒑𝒔

𝟐𝟐. 𝟓𝑲 ∗ 𝟏𝟎%= 𝟏𝑴𝒃𝒑𝒔

𝟐𝟒𝟎 𝑴𝒃𝒑𝒔

𝟕. 𝟓𝑲 ∗ 𝟏𝟎%= 𝟑𝟐𝟎 𝑲𝒃𝒑𝒔

Band Split

DutyCycle

5-GHzTST

2.4-GHzTST

50/50 20% 855 Mbps 270 Mbps

Band Split

DutyCycle

5-GHzTST

2.4-GHzTST

75/25 10% 800 Mbps 60 Mbps

Band Split

DutyCycle

5-GHzTST

2.4-GHzTST

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


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


Aruba Very High Density Lab


VHD Lab Testbed Topology


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


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


Straw Poll #1

Is anyone surprised that capacity

is inversely proportional to client count?


Straw Poll #2

What is the primary explanation for the drop?

1. Collisions & retries

2. Rate adaptation

3. MAC layer operation

4. Higgs boson


MIMO Works!

0%

20%

40%

60%

80%

100%

120%

1 5 10 25 50 75 100

Th

rou

gh

pu

t (%

)

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


Contention Premium

0%

20%

40%

60%

80%

100%

120%

1 5 10 25 50 75 100

Th

rou

gh

pu

t (%

)

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.


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?


Retries Are Not A Major Cause

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

1 25 50 75 100Retry 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 100Retry 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


Control Frame Growth Is Key Factor

28K 48K 70K 74K 84K3K6K

9K 15K 18K

431K420K 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

37K65K 75K 84K 101K

385K355K 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)


Power Save Activity Over 3 ms

16 STAs attempt PS

state in 3.1msec;

13 succeed


Average Frame Size Decreases With Load

0K

50K

100K

150K

200K

250K

300K

350K

400K

450K

500K

1 25 50 75 100

Fra

mes (

#)

>=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

Fra

mes (

#)

>=2347

2048-2346

1024-2047

512-1023

256-511

128-255

64-127

<64


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 NDPNDP NDPRTS

RTS

RTS

RTSRTS

CTS

CTS

CTS

CTS

CTS

BA

BA

BABA

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-MHz3X 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)


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


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.


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


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!


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.


TXOP Time Breakdown (173.3Mbps Rate)

MAC UnitPayload

BytesPayload

BitsData Rate

(Mbps) Usec% Airtimew/CSMA

%AirtimeTXOP 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%


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%


BA 32 256 18 14 3.9% 7.7%

Total Airtime including CSMA 1280 368 100.0% 100.0%

Airtime for TXOP only 186

Effective TXOP Rate (Mbps) including CSMA 3.5

Effective TXOP Rate (Mbps) for TXOP only 6.9

MAC UnitPayload

BytesPayload

BitsData Rate

(Mbps) Usec% Airtimew/CSMA

%AirtimeTXOP Only

AIFS[BE] 43 8.6%

CW[BE] 140 27.8%


RTS 20 160 18 9 1.8% 2.8%

SIFS 16 3.2% 5.0%


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%


BA 32 256 18 14 2.8% 4.4%

Total Airtime including CSMA 24560 503 100.0% 100.0%

Airtime for TXOP only 320

Effective TXOP Rate (Mbps) including CSMA 48.9

Effective 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!!


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+CTS87us

VHT Preamble &A-MPDU2,707us

----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


Typical Frame Size – Office Environment

30 minute capture, 6 Channels >80% of

frames

under 256B


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


Typical Frame Sizes – Football Stadium



Typical Frame Types – Football Stadium


60% are

control

frames

23% are

data frames


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


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


Effect of Beacon Rates in High-BSSID Facilities

1 2 3

1 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 SSIDs

1 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

Number of SSIDs

1 2 3

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

Number of SSIDs

1 2 3

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

http://www.revolutionwifi.net/capacity-planner

62#ATM15 |

Collision Domains & RF Spectrum Reuse


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.


How We Normally Draw Collision Domains

Typical cell diagram showing radius of cell edge


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


Preamble Interference Radius Is HUGE

67#ATM15 |

Summary & Review


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


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2016

THANK YOU

70#ATM15 | @ArubaNetworks

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