Experimental Measurement of VoIP Capacity in IEEE 802.11 WLANs

1. Experimental Measurement of VoIP Capacity in IEEE 802.11 WLANs Sangho Shin Henning Schulzrinne

2. Motivation and Goal • Check the VoIP capacity using wireless cards and compare it with theoretical and simulation results • Identify all factors that affect the VoIP capacity in experiments and simulations

3. Outline • Theoretical capacity for VoIP traffic • VoIP capacity via simulations • VoIP capacity via experiments • ‘Hidden factors’ that affect experiments and simulations • Conclusion

4. Packetization interval ……. ……. 1 2 3 N 1 2 3 N MAC backoff PLCP MAC IP UDP Voice PLCP ACK Tb DIFS SIFS Tt Packetization Interval (ms) Capacity (calls) Theoretical Capacity = 15 calls PLCP = Physical Layer Convergence Procedure

5. WIFI WIFI WIFI WIFI WIFI Simulation setup QualNet simulator v3.9 IEEE 802.11b Ethernet-Wireless

6. Simulation results Delay  90th percentile Retry rate  average Number of VoIP sources Capacity

7. Experiments NJ Rutgers University

8. Experiments 80 ft Atheros Intel 70 ft

9. Experimental setup IEEE 802.11b client client client client client AP clients client client client client client client client client client

10. Experimental results Capacity

11. Comparisons

12. Factors • ARF (Auto Rate Fallback) • Preamble size • PHY data rate of ACK frames • Offset of VoIP traffic start time • Signal strength • Scanning APs • Retry limit • Network buffer size

13. ARF • ARF (Auto Rate Fallback) • PHY data rate are automatically changes • Transmission failure decrease rate • Successful transmission  restore the rate • When frame loss is caused by bad link quality, it helps • When frame loss is caused by congestion, it makes worse • No way to tell the reason for frame losses • Problems • The effect varies according to algorithms • Turned off in simulations • Turned on in wireless cards • The algorithms are mostly implemented in drivers

14. ARF ARF=AMRR (Adaptive Multi-Rate Retry)

15. Long preamble PLCP Preamble PLCP Header 144us 48bits = 48us 192us Short preamble PLCP Preamble PLCP Header 72us 48bits = 24us 96us Preamble size PLCP MAC IP UDP Voice • IEEE 802.11b : long and short preamble • QualNet, NS-2  Long preamble • Atheros + MadWifi driver  Short preamble • Theoretical capacity with the long preamble = 12 calls PLCP = Physical Layer Convergence Procedure

16. Preamble size

17. 2Mb/s 152 us = 57% of a VoIP packet 11Mb/s106 us = 39% of a VoIP packet PHY data rate for ACK frames • ACK frames • Required for ARQ (Automatic Repeat-reQuest) • Theoretical VoIP Capacity using 11 Mb/s for ACK frames  16 calls PLCP MAC Type : 01 Subtype 1101 14B

18. PHY data rate of ACK frames

19. SIFS DIFS data ACK backoff data MAC layer VoIP source 1 1 1 VoIP source 2 2 2 VoIP source 3 3 3 VoIP source 4 4 4 MAC layer 1 2 3 4 1 2 3 4 collisions Offset of VoIP traffic startime Packetization interval 1 2 3 4 1 2 3 4 Application layer Offset

20. Offset of VoIP traffic start time Uplink retry rate ≈ 20 ms / (15 * 2) 650us > 31 x 20us Offset of traffic start time (us) Simulation results with 15 VoIP sources

21. Key Factors • ARF (Auto Rate Fallback) • Preamble size • PHY data rate of ACK frames • Offset of VoIP traffic start time • Signal strength • Scanning APs • Retry limit • Network buffer size Fixed Short 2Mb/s Randomized

22. Signal strength

23. Scanning APs • Scanning APs • When signal strength decreases below a threshold • When the retransmission rate increases above a threshold • Regularly (e.g. once per 30 seconds) • Hard to determine the algorithms • Problems • Management frames have a higher priority than data frames  causes delay • Increases the traffic  make channels congested • 1 probe request and 1 ~ 2 probe responses per channel Probe request (broadcast) client AP Probe response (unicaset)

24. Scanning APs

25. Retry limit • Wireless nodes retransmit frames until the number of retransmission reaches the retry limit • Long retry limit • For the packets whose size is bigger than the RTS threshold • Short retry limit • For the packets whose size is smaller than or equal to the RTS threshold • Effect • More retransmissions • Might reduces packet loss, but increases congestion • Less retransmissions • Increases the packet loss (4) (7)

26. Retry limit

27. Buffer size (B) Average transmission time of a packet u = 2ms D = 60ms S = 200B B = 5.8KB Packet size Maximum queuing delay (ms) < 10KB MadWifi Network buffer size • Packet loss happens mostly because of the buffer overflow at the AP • Small buffer  increase the packet loss • Bigger buffer  reduces packet loss, but increase the delay • Buffer size needs to be big enough to avoid the effect • Simple static queuing analysis

28. Conclusion • Need to consider the following factors when measuring the VoIP capacity experimentally • ARF • Preamble size • PHY data rate of ACK frames • Offset of VoIP traffic start time • Scanning APs • Retry limit • Network buffer size • By adjusting all the factors, we can achieve the same experimental, simulation, theoretical capacity

29. Thank you!