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 constrain capacity. • Modeling/Simulation suggests solution. • Buffering also a question. • Simple on-off traffic model. • Use test-bed to understand problem and solution. 2
Problem: When busy MAC is per-packet ‘fair’ 1.2 1 0.8 ratio of throughputs 0.6 0.4 0.2 0 1 2 3 4 5 6 7 8 9 10 wired link bandwidth (Mbs) 3
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Delay Measurement • Transmission not complete until MAC ACK. • Hardware supports interrupt after ACK. Interface TX Queue 1. Driver notes Driver enqueue time. 4. Driver notes completion time. Driver TX Discriptor Queue 3. Hardware 2. Hardware interrupts Hardware contends until driver. ACK received Packet transmitted ACK received 5
0.025 0.02 Fraction of Packets 0.015 0.01 0.005 0 900 1000 1100 1200 1300 1400 1500 1600 Delay (seconds x 10-6) 6
Throughput at AP, downlink (queue = 1, 3, 4 and 30) 35 30 Throughput (kbits/second) 25 20 15 10 Mean Offered Load Model: Q=1 Model: Q=inf 5 Q=3 Q=4 Q=30 0 0 2 4 6 8 10 12 14 16 18 Number of stations (n) Figure 1: Measured and modelled throughput at the AP. Buffer size at the AP and the STAs is the same. Analytic model also shown. 7
Throughput at one STA (queue = 1, 3, 4 and 30) 35 30 Throughput (kbits/second) 25 20 15 10 Mean Offered Load Model Q=1 Model Q=inf 5 Q=3 Q=4 Q=30 0 0 2 4 6 8 10 12 14 16 18 Number of stations (n) Figure 2: Measured and modelled throughput at a single STA. The buffer size at the AP and the STAs is the same. 8
Throughput at AP, downlink (mixed buffer sizes) 35 30 Throughput (kbits/second) 25 20 15 10 Mean Offered Load STA Q=4 AP Q=4 5 STA Q=4 AP Q=15 STA Q=30 AP Q=30 STA Q=4 AP Q=399 0 0 2 4 6 8 10 12 14 16 18 Number of stations (n) Figure 3: Measured throughput at the AP. Results are shown for various combinations of buffer sizes at the STAs and the AP. 9
Throughput at one STA (mixed buffer sizes) 35 30 Throughput (kbits/second) 25 20 15 10 Mean Offered Load STA Q=4 AP Q=4 5 STA Q=4 AP Q=15 STA Q=30 AP Q=30 STA Q=4 AP Q=399 0 0 2 4 6 8 10 12 14 16 18 Number of stations (n) Figure 4: Measured throughput at a single STA. Results are shown for various combinations of buffer sizes at the STAs and the AP. 10
CDF AP TXOP 1, CWmin 31 (for 5, 10 and 15 STAs) 100 90 80 70 Percentage of packets 60 50 40 30 5 STAs, Q=4 5 STAs, Q=30 20 10 STAs, Q=4 10 STAs, Q=30 10 15 STAs, Q=4 15 STAs, Q=30 0 0 2000 4000 6000 8000 10000 MAC Delay (seconds x 10-6) Figure 5: Cumulative distribution for delays at the access point when there are 5, 10 and 15 stations with standard MAC settings. 11
CDF STA TXOP 1, CWmin 31 (for 5, 10 and 15 STAs) 100 90 80 70 Percentage of packets 60 50 40 30 5 STAs, Q=4 5 STAs, Q=30 20 10 STAs, Q=4 10 STAs, Q=30 10 15 STAs, Q=4 15 STAs, Q=30 0 0 2000 4000 6000 8000 10000 MAC Delay (seconds x 10-6) Figure 6: Cumulative distribution for delays at a station when there are 5, 10 and 15 stations with standard MAC settings. 12
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? 13
1 Sta, CWmin 31 0.012 ’1_sta_cw31_1000_col_per’ u 2:1 0.01 0.008 Fraction of total Packets 0.006 0.004 0.002 0 900 1000 1100 1200 1300 1400 1500 1600 delay (seconds x 10-6) 14
Throughput at AP, downlink (queue = 3, 4, n and 30) 35 30 Throughput (kbits/second) 25 20 15 10 Mean Offered Load Q=3, TXOP n, CWmin 31 Q=4, TXOP n, CWmin 31 5 Model: Q=n, TXOP n, CWmin 31 Model: Q=inf, TXOP n, CWmin 31 Q=30, TXOP n, CWmin 31 0 0 2 4 6 8 10 12 14 16 18 Number of stations (n) Figure 7: Throughput at the AP for prioritised voice, with TXOP = n packets. 15
Throughput at AP, downlink (queue = 1, 3, 4 and 30) 35 30 Throughput (kbits/second) 25 20 15 10 Mean Offered Load Q=3, TXOP n/2, CWmin 15 Q=4, TXOP n/2, CWmin 15 5 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 0 0 2 4 6 8 10 12 14 16 18 Number of stations (n) Figure 8: Throughput at the AP for prioritised voice, TXOP = n/ 2, CW min = 15. 16
Throughput at one STA (queue = 1, 3, 4 and 30) 40 35 30 Throughput (kbits/second) 25 20 15 Mean Offered Load 10 Q=3, TXOP n, CWmin 31 Q=4, TXOP n, CWmin 31 Model: QSTA=1, QAP=n, TXOP n, CWmin 31 5 Model: Q=inf, TXOP n, CWmin 31 Q=30, TXOP n, CWmin 31 0 0 2 4 6 8 10 12 14 16 18 Number of stations (n) Figure 9: Throughput at a STA for prioritised voice, with TXOP = n packets at the AP. 17
Mean delays at AP, queue = 3, 4 and 30 1800 Q=30, TXOP 1, CWmin 31 Q=4, TXOP 1, CWmin 31 1600 Q=3, TXOP 1, CWmin 31 Q=4, TXOP n, CWmin 31 Q=3, TXOP n, CWmin 31 1400 Mean delay (seconds x 10-6) Q=30, TXOP n, CWmin 31 Q=3, TXOP n/2, CWmin 15 1200 Q=4,TXOP n/2, CWmin 15 Q=30, TXOP n/2, CWmin 15 1000 800 600 400 200 0 0 2 4 6 8 10 12 14 16 n, number of voice stations 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. 18
Mean delay at voice station, queue = 3, 4 and 30 8000 Q=30, TXOP 1, CWmin 31 Q=4, TXOP 1, CWmin 31 7000 Q=3, TXOP 1, CWmin 31 Q=3, TXOP n, CWmin 31 Q=4, TXOP n, CWmin 31 Mean delay (seconds x 10-6) 6000 Q=30, TXOP n, CWmin 31 Q=3, TXOP n/2, CWmin 15 Q=4, TXOP n/2, CWmin 15 5000 Q=30, TXOP n/2, CWmin 15 4000 3000 2000 1000 0 0 2 4 6 8 10 12 14 16 n, number of voice stations Figure 11: Mean MAC delay a station. The mean inter- packet arrival time at a STA is 10000 µs . 19
CDF AP TXOP n/2, CWmin 15 (for 5, 10 and 15 STAs) 100 90 80 70 Percentage of packets 60 50 40 30 5 STAs, Q=4, TXOP n/2, CWmin 15 5 STAs, Q=30, TXOP n/2, CWmin 15 20 10 STAs, Q=4, TXOP n/2, CWmin 15 10 STAs, Q=30, TXOP n/2, CWmin 15 10 15 STAs, Q=4, TXOP n/2, CWmin 15 15 STAs, Q=30, TXOP n/2, CWmin 15 0 0 2000 4000 6000 8000 10000 MAC Delay (seconds x 10-6) 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 CW min = 15. 20
CDF STA TXOP 1, CWmin 31 (for 5, 10 and 15 STAs) 100 90 80 70 Percentage of packets 60 50 40 30 5 STAs, Q=4 5 STAs, Q=30 20 10 STAs, Q=4 10 STAs, Q=30 10 15 STAs, Q=4 15 STAs, Q=30 0 0 2000 4000 6000 8000 10000 MAC Delay (seconds x 10-6) Figure 13: Cumulative distribution for delays at a station when there are 5, 10 and 15 stations with standard MAC settings. 21
CDF STA TXOP n, CWmin 31 (for 5, 10 and 15 STAs) 100 90 80 70 Percentage of packets 60 50 40 30 5 STAs, Q=4, TXOP n, CWmin 31 5 STAs, Q=30, TXOP n, CWmin 31 20 10 STAs, Q=4, TXOP n, CWmin 31 10 STAs, Q=30, TXOP n, CWmin 31 10 15 STAs, Q=4, TXOP n, CWmin 31 15 STAs, Q=30, TXOP n, CWmin 31 0 0 2000 4000 6000 8000 10000 MAC Delay (seconds x 10-6) 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 . 22
CDF STA TXOP n/2, CWmin 15 (for 5, 10 and 15 STAs) 100 90 80 70 Percentage of packets 60 50 40 30 5 STAs, Q=4, TXOP n/2, CWmin 15 5 STAs, Q=30, TXOP n/2, CWmin 15 20 10 STAs, Q=4, TXOP n/2, CWmin 15 10 STAs, Q=30, TXOP n/2, CWmin 15 10 15 STAs, Q=4, TXOP n/2, CWmin 15 15 STAs, Q=30, TXOP n/2, CWmin 15 0 0 2000 4000 6000 8000 10000 MAC Delay (seconds x 10-6) Figure 15: Cumulative distribution for delays at a station when there are 5, 10 and 15 stations when the AP is pri- oritised using TXOP = n/ 2 and CW min = 15. 23
Conclusion • Reproduced capacity problem. • Buffering helps, TXOP helps more. • Models are producing useful predictions. • Burstiness for TXOP seems OK. • Some interesting MAC/buffer tradeoffs. 24
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