Experimental Measurement of the Capacity for VoIP Traffic in IEEE 802.11 WLANs

1. Experimental Measurement of the Capacity for VoIP Traffic in IEEE 802.11 WLANs Authors : Sangho Shin, Henning Schulzrinne [INFOCOM 2007] Reporter : 林緯彥

2. Motivation • Measure the capacity for VoIP traffic in 802.11b test-bed and compare it with the theoretical capacity and simulation results • Identify factors that have been commonly overlooked in past studies but affect experiments and simulations.

3. Outline • Methodology • Theoretical capacity for VoIP traffic • VoIP capacity via simulations • VoIP capacity via experiments • “Hidden factors” that affect experiments and simulations • Conclusion

4. Methodology • Theoretical capacity for VoIP traffic • Capacity for VoIP traffic via simulation • Capacity for VoIP traffic via experiments • Each method includes two types of VoIP traffic • Constant Bit Rate (CBR) • Variable Bit Rate (VBR)

5. Methodology • Theoretical capacity for VoIP traffic • Capacity for VoIP traffic via simulation • Capacity for VoIP traffic via experiments

6. Packetization interval ……. ……. 1 2 3 N 1 2 3 N MAC Theoretical method • CBR backoff PLCP MAC IP UDP RTP Voice PLCP ACK Tb DIFS SIFS G.711 Tt Packetization Interval (ms) = 15 calls Capacity (calls) PLCP = Physical Layer Convergence Procedure

7. Theoretical method (cont.) • VBR • The VBR VoIP traffic is characterized by talking and silence periods, which determine the activity ratio. • The activity ratio is defined as the ratio of talking-periods at the whole conversation time. • The activity ratio in the conversation model described in ITU-T P.59 is about 0.39 • (CBR capacity / activity ratio) = 15 / 0.39 = 38 calls

8. Methodology • Theoretical capacity for VoIP traffic • Capacity for VoIP traffic via simulation • Capacity for VoIP traffic via experiments

9. WIFI WIFI WIFI WIFI WIFI Simulation method Ethernet QualNet simulator v3.9 IEEE 802.11b Wireless

10. Simulation method (cont.) • CBR

11. Simulation method (cont.) • VBR

12. Methodology • Theoretical capacity for VoIP traffic • Capacity for VoIP traffic via simulation • Capacity for VoIP traffic via experiments

13. Experiment method 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

14. Experiment method (cont.) • CBR

15. Experiment method (cont.) • VBR

16. summary

17. Factors • Preamble size • Rate control • VoIP packet generation intervals among VoIP sources • Other minor factors

18. Preamble size • Preamble default： • QualNet, NS-2  Long preamble • Experiment  Short preamble • Theoretical capacity with the long preamble = 12 calls long short

19. Rate control • 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 8% of frames were transmitted with lower rates

20. SIFS DIFS data ACK backoff data MAC layer Offset of VoIP traffic start time Packetization interval 1 2 3 4 1 2 3 4 Application layer Offset 650 μs = the optimal offset

21. Other minor factors • Scanning APs • Network buffer size and packet loss

22. Scanning APs Probe request (broadcast) AP client Probe response (unicast) Probe request and response frames increase the delay of VoIP packet transmission due to traffic increase

23. Network buffer size and packet loss • 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

24. Conclusion • Need to consider the following factors when measuring the VoIP capacity experimentally • Rate control • Preamble size • Offset of VoIP traffic start time • The study can be used in 802.11 experiments and the analysis and comparison

25. Thank you!