@troymart

An analysis of Wi-Fi Efficiency

#WLPC_EU #WiFiwithPurpose

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“It depends…” -@samuel_clements

About Me

• Troy Martin

• B.A.Sc. Electrical Engineering

• B.Sc. Computer Science

• 1-term away from a degree in Philosophy

• Would rather be cycling or snowboarding

#WiFiwithPurposeCourtesy of @80211eh#WLPC_EU

Ethernet vs Wi-Fi

• CSMA/CD

• Full duplex

• Fixed Data Rate

• Confined Medium

• 5-4-3-2-1 Rule

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Ethernet Frame Analysis

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64 - 1518 bytes12 bytes unused

8

Interframe gapEthernet Frame Preamble

46-1500 bytes 18 bytes

Payload Ethernet Header

4 bytes

.1Q

4 bytes

CRC

Measuring speed of light

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Bulk of traffic is than 256-byte frames.

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Ethernet Frame Efficiency

Inter Frame Gap (bytes)

Preamble (bytes)

Ethernet Header (bytes)

Payload (bytes)

CRC Efficiency

12 8 18 46 4 52.3%

12 8 18 80 4 65.6%

12 8 18 100 4 70.4%

12 8 18 125 4 74.9%

12 8 18 200 4 82.6%

12 8 18 256 4 85.9%

12 8 18 500 4 92.3%

12 8 18 750 4 94.7%

12 8 18 1000 4 96.0%

12 8 18 1250 4 96.7%

12 8 18 1500 4 97.3%

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Not just about a wired protocol!

Ethernet vs Wi-Fi

• CSMA/CD

• Full duplex

• Fixed Data Rate

• Confined Medium

• 5-4-3-2-1 Rule

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• CSMA/CA

• Half duplex

• Variable Data Rate

• Unconfined medium

802.11 Transmission Analysis

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0 – 2,304 bytesDIFS Preamble

802.11 Frame (MPDU)

SIFS14 1 - 1,023 STSIG

ACK CW

16µs4µs0 – 3,072µs 9 –9,207µs16µs34µs 44µs

Based on OFDM (11a @ 6Mbps)

Variable Data Rate

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11Mbps DSSS

5.5Mbps DSSS

2Mbps DSSS

1Mbps DSSS7.2 Mbps

14 Mbps

65 Mbps

Data rate is

dynamic with

802.11 depending

on the

modulation,

coding and

possibly with

number of spatial

steams used.

29 Mbps

Modulation - Clause 17 (OFDM) - .11a

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ModCodingrate (R)

Coded bits per

subcarrier(NBPSC)

Coded bits per OFDM symbol

(NCBPS)

Data bits per OFDM symbol

(NDBPS)

Data rate (Mb/s) (20MHz channel

spacing)

Data rate (Mb/s) (10MHz channel

spacing)

Data rate (Mb/s) (5MHz channel

spacing)

BPSK 1/2 1 48 24 6 3 1.5

BPSK 3/4 1 48 36 9 4.5 2.25

QPSK 1/2 2 96 48 12 6 3

QPSK 3/4 2 96 72 18 9 4.5

16-QAM 1/2 4 192 96 24 12 6

16-QAM 3/4 4 192 144 36 18 9

64-QAM 2/3 6 288 192 48 24 12

64-QAM 3/4 6 288 216 54 27 13.5

IEEE 802.11-2016 Table 17-4

MCS parameters – Clause 19 (HT) – .11n

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MCS Mod R NBPSCS NSD NSP NCBPS NDBPS

Data rate (Mbps)

800ns GI 400ns GI

0 BPSK 1/2 1 52 4 52 26 6.5 7.2

1 QPSK 1/2 2 52 4 104 52 13.0 14.4

2 QPSK 3/4 2 52 4 104 78 19.5 21.7

3 16-QAM 1/2 4 52 4 208 104 26.0 28.9

4 16-QAM 3/4 4 52 4 208 156 39.0 43.3

5 64-QAM 2/3 6 52 4 312 208 52.0 57.8

6 64-QAM 3/4 6 52 4 312 234 58.5 65.0

7 64-QAM 5/6 6 52 4 312 260 65.0 72.2

IEEE 802.11-2016 Table 19-27

VHT-MCSs – Clause 21 (.11ac) for 20MHz, Nss = 3

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MCS Mod R NBPSCS NSD NSP NCBPS NDBPS NES

Data rate (Mbps)

800ns GI 400ns GI

0 BPSK 1/2 1 52 4 156 78 1 19.5 21.7

1 QPSK 1/2 2 52 4 312 156 1 39.0 43.3

2 QPSK 3/4 2 52 4 312 234 1 58.5 65.0

3 16-QAM 1/2 4 52 4 624 312 1 78.0 86.7

4 16-QAM 3/4 4 52 4 624 468 1 117.0 130.0

5 64-QAM 2/3 6 52 4 936 624 1 156.0 173.3

6 64-QAM 3/4 6 52 4 936 702 1 175.5 195.0

7 64-QAM 5/6 6 52 4 936 780 1 195.0 216.7

8 256-QAM 3/4 8 52 4 1248 936 1 234.0 260.0

9 256-QAM 5/6 8 52 4 1248 1040 1 260.0 288.9

IEEE 802.11-2016 Table 21-32

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MCS DCM Modulation R NBPSCS NSD NCBPS NDBPS

Data rate (Mb/s)

0.8µs GI 1.6µs GI 3.2µs GI

01

BPSK1/2

112 12 6 0.4 0.4 0.4

0 1/2 24 24 12 0.9 0.8 0.8

11

QPSK

1/2

2

12 24 12 0.9 0.8 0.8

0 1/2 24 48 24 1.8 1.7 1.5

2 N/A 3/4 24 48 36 2.6 2.5 2.3

31

16-QAM

1/2

4

12 48 24 1.8 1.7 1.5

0 1/2 24 96 48 3.5 3.3 3.0

41 3/4 12 48 36 2.6 2.5 2.3

0 3/4 24 96 72 5.3 5.0 4.5

5

N/A

64-QAM

2/3

6

24

144

96 7.1 6.7 6.0

6 3/4 108 7.9 7.5 6.8

7 5/6 120 8.8 8.3 7.5

8256-QAM

3/48 192

144 10.6 10.0 9.0

9 5/6 160 11.8 11.1 10.0

101024-QAM

3/410 240

180 13.2 12.5 11.3

11 5/6 200 14.7 13.9 12.5

Example of 802.11ax – HE for 26-tone RU, Nss = 1

Slot Times and IFS

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802.11b 802.11g 802.11a802.11n2.4GHz

802.11n5GHz

SIFS 10µs 10µs 16µs 10µs 16µs

Slot Time

20µsLong = 20µsShort = 9µs

9µsLong = 20µsShort = 9µs

9µs

DIFS 50µsLong = 50µsShort = 28µs

34µsLong = 50µsShort = 28µs

34µs

The Game

• Picture of butterflies and dice

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DCA contention window

Queue Type802.11b PHY 802.11 a/g/n PHY

CWmin CWmax CWmin CWmax

Voice 7 15 3 7

Video 15 31 7 15

Best Effort 31 1023 15 1023

Background 31 1023 15 1023

802.11b/g* PHY 802.11 a/g/n PHY

CWmin CWmax CWmin CWmax

All traffic (no-QoS) 31 1023 15 1023*when .11b stations are present

Birthday Problem

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Contention Window Scale

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1023

CWmin 1531

63

127

CWmax

255

511

1023

Attempt1 42 3 85 76

Probability 2 STAs will pick same backoff

value

# of STA’sP (2 STAs will pick same backoff)

CWmax = 4 (%) CWmax = 8 (%)

1 0.00 0.00

2 25.00 12.50

3 62.50 34.38

4 90.63 58.98

5 100.00 79.49

6 100.00 92.31

7 100.00 98.08

8 100.00 99.76

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Probability 2 STAs will pick same backoff

value with CWmax=16

STAs P(%) STAs P(%)

1 0.00 9 93.96

2 6.25 10 97.36

3 17.97 11 99.01

4 33.35 12 99.69

5 50.01 13 99.92

6 65.63 14 99.99

7 78.52 15 100.00

8 87.92 16 100.00

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Probability 2 STAs will pick same backoff

value with CWmax=32

STAs P(%) STAs P(%) STAs P(%) STAs P(%)

1 0.00% 9 71.07% 17 99.48% 25 100.00%

2 3.13% 10 79.21% 18 99.76% 26 100.00%

3 9.18% 11 85.71% 19 99.89% 27 100.00%

4 17.69% 12 90.62% 20 99.96% 28 100.00%

5 27.98% 13 94.14% 21 99.98% 29 100.00%

6 39.24% 14 96.52% 22 99.99% 30 100.00%

7 50.63% 15 98.04% 23 100.00% 31 100.00%

8 61.43% 16 98.96% 24 100.00% 32 100.00%

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Probability 2 STAs will pick same backoff

value with CWmax=64

STAs P(%) STAs P(%) STAs P(%) STAs P(%)

1 0.00% 17 90.32% 33 100.00% 49 100.00%

3 4.64% 19 94.89% 35 100.00% 51 100.00%

5 14.79% 21 97.53% 37 100.00% 53 100.00%

7 28.81% 23 98.91% 39 100.00% 55 100.00%

9 44.52% 25 99.56% 41 100.00% 57 100.00%

11 59.77% 27 99.84% 43 100.00% 59 100.00%

13 72.93% 29 99.95% 45 100.00% 61 100.00%

15 83.15% 31 99.99% 47 100.00% 63 100.00%

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Probability 2 STAs will pick same backoff value

# of STA’sProbability 2 STAs will pick same backoff value

If CWmax = 128 (%) If CWmax = 256 (%) If CWmax = 512 (%) If CWmax = 1024 (%)

1 0.00% 0.00% 0.00% 0.00%

5 7.60% 3.85% 1.94% 0.97%

10 30.27% 16.31% 8.46% 4.31%

15 57.39% 34.17% 18.70% 9.79%

20 79.11% 53.32% 31.33% 17.03%

25 91.87% 70.21% 44.87% 25.57%

30 97.51% 82.94% 57.95% 34.88%

35 99.41% 91.25% 69.55% 44.44%

40 99.89% 95.99% 79.07% 53.78%

45 99.99% 98.36% 86.36% 62.51%

50 100.00% 99.41% 91.57% 70.36%

Interference at Rx matters!

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Design Methodology

1. Primary Coverage

2. Secondary Coverage

3. Co-Channel interference (CCI)

4. Capacityand

Follow good design methodology.

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Wrap-up

1. Effective throughput is 50-70% of data rate, as negotiated between each STA and the AP.

2. Ask the right questions to get answers that are meaningful.

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