1. Isolating Physical PER for Smart Rate Selection in 802.11 Malik Ahmad Yar Khan and Darryl Veitch ARC Special Centre for Ultra-Broadband Information Networks (CUBIN) The University of Melbourne, Australia IEEE Infocom 2009 (acceptance rate = 19.65%)

2. Outline Introduction Introduction The core idea The core idea RCA modification RCA modification –Fragmentation implementation –RTS/CTS implementation Performance Performance Conclusion Conclusion

3. Introduction Rate selection algorithms in use in 802.11 suffer from a fundamental problem Rate selection algorithms in use in 802.11 suffer from a fundamental problem –Implicity or explicity make decisions bases on total Packet Error Rate (PER) False rate decision lowers the system throughput False rate decision lowers the system throughput

4. Introduction Total PER = PPER + CPER Total PER = PPER + CPER –PPER: Physical PER –CPER: contention PER It is often the case CPER dominates PPER It is often the case CPER dominates PPER –Poor latency, loss and throughput

5. PER under a ideal environment

6. Goal of the paper Providing a novel way to isolate the physical error Providing a novel way to isolate the physical error Providing a modular way of isolating the contention error which any rate adaptation algorithm can be enhanced Providing a modular way of isolating the contention error which any rate adaptation algorithm can be enhanced

7. Fragmentation Only consider a packet is fragmented into 2 frames Only consider a packet is fragmented into 2 frames The fragment 2 is sent out only after an SIFS after the final ACK The fragment 2 is sent out only after an SIFS after the final ACK –Provided there are no hidden terminals

8. Fragmentation If the first fragment is lost, for nay reason, the second will not even be sent If the first fragment is lost, for nay reason, the second will not even be sent Observation: the loss of a second fragment on the first transmission attempt Observation: the loss of a second fragment on the first transmission attempt –Physical error

9. RTS/CTS Once the RTS-CTS exchange is completed, the data packet will have the channel reserved, and will only encounter error for physical reasons Once the RTS-CTS exchange is completed, the data packet will have the channel reserved, and will only encounter error for physical reasons

10. Block reservation in 802.11e An option in IEEE 802.11e An option in IEEE 802.11e The second data packet of such blocks could be used to directly measure PPER The second data packet of such blocks could be used to directly measure PPER

11. Main concept Not to use these methods all the time in practice Not to use these methods all the time in practice Only trigger their use often enough to gather good statistics on PPER Only trigger their use often enough to gather good statistics on PPER

12. Scatterplots of PER with no cross traffic

13. Scatterplots of PER with CT and fragmentation

14. Scatterplots of PER with CT and fragmentation (RTS/CTS on)

15. Rate dependence of IP throughput

17. Fragmentation implementation Only descriptors of second fragments are passed to the RCAs Only descriptors of second fragments are passed to the RCAs A mechanism is needed to trigger the fragmentation of selected packets A mechanism is needed to trigger the fragmentation of selected packets –Maintains a simple ON/OFF variable

18. RTS/CTS implementation RTS/CTS protection is switched on by a per-packet decision RTS/CTS protection is switched on by a per-packet decision Use Long Retry Count (LRC) to record the statistic of RTS/CST transmission Use Long Retry Count (LRC) to record the statistic of RTS/CST transmission

19. Performance Two main metrics Two main metrics –Weighted mean rate summaries –IP throughput All experiments are run in IEEE 802.11a in testbed All experiments are run in IEEE 802.11a in testbed –Full rate set:{6,9,12,18,24,36,48,54} –Packet size: 1500 bytes –10 stations if no further explanation

20. The wireless testbed St[1-13] are stations, Sniff[1-2] are sniffers St[1-13] are stations, Sniff[1-2] are sniffers

21. Two implementation protocols SampleRate SampleRate –A MIT master thesis –Implemented on MadWifi AMRR AMRR –A multi-rate algorithm also implemented on MadWifi

22. Single user with different RCAs

23. TPER for single user case for AMRR

24. SampleRate with different packet sizes (1472B)

25. SampleRate with different packet sizes (716B)

26. UDP upstream system throughput

27. TCP upstream system throughput

28. Conclusion The paper investigates two ways to isolate Physical PER The paper investigates two ways to isolate Physical PER –RTS/CTS –MAC level packet fragmentation Higher channel rates were used when PPER was in fact low, even when contention is present Higher channel rates were used when PPER was in fact low, even when contention is present

29. Thank you!!