@troymart
An analysis of Wi-Fi Efficiency
#WLPC_EU #WiFiwithPurpose
#WLPC_EU
“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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#WLPC_EU#WLPC_EU #WiFiwithPurpose