1. Experimental Measurement of VoIP Capacity in IEEE 802.11 WLANs Sangho Shin Henning Schulzrinne Department of Computer Science Columbia University

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

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 123N123N ……. MAC Theoretical capacity parametersvalue Voice codec64 kb/s Packet size160B Packetization interval20ms Transport layerUDP PHY data rate11 Mb/s RTS/CTSNo Capacity (calls) Packetization Interval (ms) = 15 calls PLCP = Physical Layer Convergence Procedure PLCPMACIPUDPVoiceACKPLCP backoff DIFSSIFS TtTt TbTb RTP

6. Simulation setup WIFI Ethernet-Wireless parametersvalue Voice codec G.711 (64 kb/s) Packet size160B Packetization interval20ms Transport layerUDP PHY data rate11Mb/s RTS/CTSNo WIFI IEEE 802.11b QualNet simulator v3.9

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

8. Experiments NJ Rutgers University

9. Experiments 80 ft 70 ft Atheros Intel

10. Experimental setup parametersvalue Voice codec G.711 (64 kb/s) Packet size160B Packetization interval20ms Transport layerUDP PHY data rate11Mb/s RTS/CTSNo client clientsclientAPclient IEEE 802.11b Atheros chipset MadWifi-0.9.3

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

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. 90 th percentile delay (ms) Fixed rate ARF (AMRR) Threshold for capacity 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 8% of frames were transmitted with lower rates AMRR= Adaptive Multi-Rate Retry

14. 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 LongShort Preamble size144 us72 us Header size (us)48 us24 us Total size (us)192 us96 us Fraction in a VoIP (size)9%6% Fraction in a VoIP (time)53%36% PLCP = Physical Layer Convergence Procedure 90 th percentile delay (ms) Short Long

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

16. Offset of VoIP traffic start time 1234 Packetization interval 1234 Application layer Offset MAC layer databackoff SIFS ACK DIFS data VoIP source 1 VoIP source 2 VoIP source 3 VoIP source 4 1 2 3 4 1 2 3 4 MAC layer 12341234 collisions

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

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. Scanning APs 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 APclient Probe request (broadcast) Probe response (unicast) (for 100 s)

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. 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 Maximum queuing delay Buffer size Packet size Average service rate µ = 1/500 D = 60ms S = 200B B min = 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!