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 Department of Computer Science Columbia University

2. WIFI VoIP over Wireless LANs Internet PBX AP (Access Point)

3. 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

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

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

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

7. Simulation results 90th percentile delay (ms) Downlink delay Uplink delay Number of VoIP sources Capacity

8. Experiments NJ Rutgers University

9. Experiments 80 ft Atheros Intel 70 ft

10. Experimental setup Atheros chipset MadWifi-0.9.3 IEEE 802.11b client client client client client AP clients client client client client client client client client client

11. Experimental results 90th percentile delay (ms) Downlink delay Uplink delay Capacity

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. 8% of frames were transmitted with lower rates 90th percentile delay (ms) ARF (AMRR) Threshold for capacity Fixed rate ARF • ARF (Auto Rate Fallback) • PHY data rate are automatically changes • When frame loss is caused by bad link quality, it helps • When frame loss is caused by congestion, it makes worse • Problems • The effect varies according to algorithms • Turned off in simulations • Turned on in wireless cards • Experimental results AMRR= Adaptive Multi-Rate Retry

14. 90th percentile delay (ms) Long Short Preamble size • 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 • Experimental results PLCP = Physical Layer Convergence Procedure

15. 2Mb/s 152 us = 57% of a VoIP packet 11Mb/s106 us = 39% of a VoIP packet 90th percentile delay (ms) 2 Mb/s 11 Mb/s PHY data rate for ACK frames PLCP MAC Type : 01 Subtype 1101 • ACK frames • Required for ARQ • Theoretical VoIP capacity using 11 Mb/s for ACK frames  16 calls • Experimental results 14B MadWifi2Mb/s QualNet11Mb/s NS-21Mb/s

16. 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 start time Packetization interval 1 2 3 4 1 2 3 4 Application layer Offset

17. Offset of VoIP traffic start time Uplink retry rate 90th percentile delay (ms) 650 μs = the optimal offset (20ms/(15 sources*2)) Offset of traffic start time (μs) Simulation results with 15 VoIP sources

18. 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

19. Signal strength

20. (for 100 s) Scanning APs Probe request (broadcast) • Scan APs based on • signal strength • transmission failure • Regularly (e.g. every min) • 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 client AP Probe response (unicast)

21. Retry limit • Wireless nodes retransmit frames until the number of retransmission reaches the retry limit • Long retry limit - frame size > RTS threshold • Short retry limit - frame size ≤ RTS threshold • Effect • More retransmissions  reduces packet loss, but increases congestion • Less retransmissions  Increases the packet loss • Experimental results (4) (7)

22. Buffer size Average service rate Packet size Maximum queuing delay 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 allow 60ms of delay • Simple static queuing analysis µ = 1/500 D = 60ms S = 200B Bmin = 5.8KB < 10KB MadWifi

23. 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 • Our study can be used in any 802.11 experiments and the analysis and comparison

24. Thank you!