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Understanding 802.11e Voice Behaviour via Testbed Measurements and Modeling Ian Dangerfield, David Malone, Doug Leith. 20 April 2007 1 Voice over WiFi Behaviour of voice over WiFi. Infrastructure mode 802.11(e). AP known to

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Understanding 802.11e Voice Behaviour via Testbed Measurements and Modeling

Ian Dangerfield, David Malone, Doug Leith. 20 April 2007

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Voice over WiFi

Behaviour of voice over WiFi.
Infrastructure mode 802.11(e).
AP known to constrain capacity.
Modeling/Simulation suggests solution.
Buffering also a question.
Simple on-off traffic model.
Use test-bed to understand problem and solution.

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Problem: When busy MAC is per-packet ‘fair’

1 2 3 4 5 6 7 8 9 10 0.2 0.4 0.6 0.8 1 1.2 wired link bandwidth (Mbs) ratio of throughputs

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Delay Measurement

Transmission not complete until MAC ACK.
Hardware supports interrupt after ACK.

ACK received Interface TX Queue Driver Queue

1. Driver notes

enqueue time.

2. Hardware

contends until ACK received Hardware Driver TX Discriptor

3. Hardware

interrupts driver.

4. Driver notes

completion time. Packet transmitted

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0.005 0.01 0.015 0.02 0.025 900 1000 1100 1200 1300 1400 1500 1600 Fraction of Packets Delay (seconds x 10-6)

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5 10 15 20 25 30 35 2 4 6 8 10 12 14 16 18 Throughput (kbits/second) Number of stations (n) Throughput at AP, downlink (queue = 1, 3, 4 and 30) Mean Offered Load Model: Q=1 Model: Q=inf Q=3 Q=4 Q=30

Figure 1: Measured and modelled throughput at the AP. Buffer size at the AP and the STAs is the same. Analytic model also shown.

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5 10 15 20 25 30 35 2 4 6 8 10 12 14 16 18 Throughput (kbits/second) Number of stations (n) Throughput at one STA (queue = 1, 3, 4 and 30) Mean Offered Load Model Q=1 Model Q=inf Q=3 Q=4 Q=30

Figure 2: Measured and modelled throughput at a single

STA. The buffer size at the AP and the STAs is the same.

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5 10 15 20 25 30 35 2 4 6 8 10 12 14 16 18 Throughput (kbits/second) Number of stations (n) Throughput at AP, downlink (mixed buffer sizes) Mean Offered Load STA Q=4 AP Q=4 STA Q=4 AP Q=15 STA Q=30 AP Q=30 STA Q=4 AP Q=399

Figure 3: Measured throughput at the AP. Results are shown for various combinations of buffer sizes at the STAs and the AP.

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5 10 15 20 25 30 35 2 4 6 8 10 12 14 16 18 Throughput (kbits/second) Number of stations (n) Throughput at one STA (mixed buffer sizes) Mean Offered Load STA Q=4 AP Q=4 STA Q=4 AP Q=15 STA Q=30 AP Q=30 STA Q=4 AP Q=399

Figure 4: Measured throughput at a single STA. Results are shown for various combinations of buffer sizes at the STAs and the AP.

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10 20 30 40 50 60 70 80 90 100 2000 4000 6000 8000 10000 Percentage of packets MAC Delay (seconds x 10-6) CDF AP TXOP 1, CWmin 31 (for 5, 10 and 15 STAs) 5 STAs, Q=4 5 STAs, Q=30 10 STAs, Q=4 10 STAs, Q=30 15 STAs, Q=4 15 STAs, Q=30

Figure 5: Cumulative distribution for delays at the access point when there are 5, 10 and 15 stations with standard MAC settings.

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10 20 30 40 50 60 70 80 90 100 2000 4000 6000 8000 10000 Percentage of packets MAC Delay (seconds x 10-6) CDF STA TXOP 1, CWmin 31 (for 5, 10 and 15 STAs) 5 STAs, Q=4 5 STAs, Q=30 10 STAs, Q=4 10 STAs, Q=30 15 STAs, Q=4 15 STAs, Q=30

Figure 6: Cumulative distribution for delays at a station when there are 5, 10 and 15 stations with standard MAC settings.

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Solution: 802.11e?

802.11e makes parameters tunable.
CWmin: base range for backoff.
TXOP: transmission duration.
Simple solution: TXOP = n packets.
Better solution: CWmin = 16, TXOP = n/2 packets?

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0.002 0.004 0.006 0.008 0.01 0.012 900 1000 1100 1200 1300 1400 1500 1600 Fraction of total Packets delay (seconds x 10-6) 1 Sta, CWmin 31 ’1_sta_cw31_1000_col_per’ u 2:1

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5 10 15 20 25 30 35 2 4 6 8 10 12 14 16 18 Throughput (kbits/second) Number of stations (n) Throughput at AP, downlink (queue = 3, 4, n and 30) Mean Offered Load Q=3, TXOP n, CWmin 31 Q=4, TXOP n, CWmin 31 Model: Q=n, TXOP n, CWmin 31 Model: Q=inf, TXOP n, CWmin 31 Q=30, TXOP n, CWmin 31

Figure 7: Throughput at the AP for prioritised voice, with TXOP = n packets.

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5 10 15 20 25 30 35 2 4 6 8 10 12 14 16 18 Throughput (kbits/second) Number of stations (n) Throughput at AP, downlink (queue = 1, 3, 4 and 30) Mean Offered Load Q=3, TXOP n/2, CWmin 15 Q=4, TXOP n/2, CWmin 15 Model: QSTA=1, QAP=n/2, TXOP n/2, CWmin 15 Model: Q=inf, TXOP n/2, CWmin 15 Q=30, TXOP n/2, CWmin 15

Figure 8: Throughput at the AP for prioritised voice, TXOP = n/2, CWmin = 15.

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5 10 15 20 25 30 35 40 2 4 6 8 10 12 14 16 18 Throughput (kbits/second) Number of stations (n) Throughput at one STA (queue = 1, 3, 4 and 30) Mean Offered Load Q=3, TXOP n, CWmin 31 Q=4, TXOP n, CWmin 31 Model: QSTA=1, QAP=n, TXOP n, CWmin 31 Model: Q=inf, TXOP n, CWmin 31 Q=30, TXOP n, CWmin 31

Figure 9: Throughput at a STA for prioritised voice, with TXOP = n packets at the AP.

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200 400 600 800 1000 1200 1400 1600 1800 2 4 6 8 10 12 14 16 Mean delay (seconds x 10-6) n, number of voice stations Mean delays at AP, queue = 3, 4 and 30 Q=30, TXOP 1, CWmin 31 Q=4, TXOP 1, CWmin 31 Q=3, TXOP 1, CWmin 31 Q=4, TXOP n, CWmin 31 Q=3, TXOP n, CWmin 31 Q=30, TXOP n, CWmin 31 Q=3, TXOP n/2, CWmin 15 Q=4,TXOP n/2, CWmin 15 Q=30, TXOP n/2, CWmin 15

Figure 10: Mean MAC delay at the AP. The group with longer mean delays correspond to the experiments in which the AP is not prioritised.

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1000 2000 3000 4000 5000 6000 7000 8000 2 4 6 8 10 12 14 16 Mean delay (seconds x 10-6) n, number of voice stations Mean delay at voice station, queue = 3, 4 and 30 Q=30, TXOP 1, CWmin 31 Q=4, TXOP 1, CWmin 31 Q=3, TXOP 1, CWmin 31 Q=3, TXOP n, CWmin 31 Q=4, TXOP n, CWmin 31 Q=30, TXOP n, CWmin 31 Q=3, TXOP n/2, CWmin 15 Q=4, TXOP n/2, CWmin 15 Q=30, TXOP n/2, CWmin 15

Figure 11: Mean MAC delay a station. The mean inter- packet arrival time at a STA is 10000µs.

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10 20 30 40 50 60 70 80 90 100 2000 4000 6000 8000 10000 Percentage of packets MAC Delay (seconds x 10-6) CDF AP TXOP n/2, CWmin 15 (for 5, 10 and 15 STAs) 5 STAs, Q=4, TXOP n/2, CWmin 15 5 STAs, Q=30, TXOP n/2, CWmin 15 10 STAs, Q=4, TXOP n/2, CWmin 15 10 STAs, Q=30, TXOP n/2, CWmin 15 15 STAs, Q=4, TXOP n/2, CWmin 15 15 STAs, Q=30, TXOP n/2, CWmin 15

Figure 12: Cumulative distribution for delays at the AP when there are 5, 10 and 15 stations and the AP is priori- tised using TXOP = n/2 and CWmin = 15.

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Figure 13: Cumulative distribution for delays at a station when there are 5, 10 and 15 stations with standard MAC settings.

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10 20 30 40 50 60 70 80 90 100 2000 4000 6000 8000 10000 Percentage of packets MAC Delay (seconds x 10-6) CDF STA TXOP n, CWmin 31 (for 5, 10 and 15 STAs) 5 STAs, Q=4, TXOP n, CWmin 31 5 STAs, Q=30, TXOP n, CWmin 31 10 STAs, Q=4, TXOP n, CWmin 31 10 STAs, Q=30, TXOP n, CWmin 31 15 STAs, Q=4, TXOP n, CWmin 31 15 STAs, Q=30, TXOP n, CWmin 31

Figure 14: Cumulative distribution for delays at a station when there are 5, 10 and 15 stations and the AP is priori- tised using TXOP = n.

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10 20 30 40 50 60 70 80 90 100 2000 4000 6000 8000 10000 Percentage of packets MAC Delay (seconds x 10-6) CDF STA TXOP n/2, CWmin 15 (for 5, 10 and 15 STAs) 5 STAs, Q=4, TXOP n/2, CWmin 15 5 STAs, Q=30, TXOP n/2, CWmin 15 10 STAs, Q=4, TXOP n/2, CWmin 15 10 STAs, Q=30, TXOP n/2, CWmin 15 15 STAs, Q=4, TXOP n/2, CWmin 15 15 STAs, Q=30, TXOP n/2, CWmin 15

Figure 15: Cumulative distribution for delays at a station when there are 5, 10 and 15 stations when the AP is pri-

ritised using TXOP = n/2 and CWmin = 15.

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Conclusion

Reproduced capacity problem.
Buffering helps, TXOP helps more.
Models are producing useful predictions.
Burstiness for TXOP seems OK.
Some interesting MAC/buffer tradeoffs.