1. Dynamic Rate Adaptation in IEEE 802.11 WLANs Bob Kinicki PEDS March 26, 2007 PEDS March 26, 2007

2. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 2 OutlineOutline  Review CSMA/CA DCF basics  Motivate Rate Adaptation Issues over increasingly complex configurations.  List of Rate Adaptation Algorithms  ARF  RBAR  MultiRate Retry  CARA  RRAA  Final Thoughts

3. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 3 Basic CSMA/CA for DCF [N. Kim]

4. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 4 CSMA/CA (RTS/CTS) and Fragmentation [Zhu]

5. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 5 One-to-One Configuration {Ad Hoc} Access Point client

6. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 6 One-to-One Configuration {Ad Hoc} Access Point client

7. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 7 One-to-One Configuration {Ad Hoc} Access Point client Distance Effects : attenuation fading interference

8. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 8 Rate Adaptation attacks Distance [J. Kim]

9. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 9 Physical Layer Overhead [N. Kim]

10. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 10 Physical Layer Overhead [N. Kim]

11. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 11 BER vs SNR [Pavon]

12. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 12 Throughput vs SNR [Pavon]

13. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 13 Single AP multiple clients (homogeneous) Access Point client node contention effects: collisions hidden terminal

14. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 14 Node Contention [N. Kim]

15. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 15 Single AP multiple clients (heterogeneous) Access Point client node effects: collisions link capture hidden terminal performance anomaly AP queue overflow different NIC cards

16. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 16 UnfairnessUnfairness [Choi]

17. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 17 Multiple APs multiple clients (heterogeneous) Access Point client Access Point client

18. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 18 Hidden Terminals [Wong]

19. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 19 Rate Adaptation Algorithms AARF ARFAMRR CARACROARDOFRA Fast-LAHRCLA LD-ARFMiSer MultiRateRetry MPDUOARONOE PERRBARRFT RRAASampleRate SwissRA

20. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 20 Rate Adaptation Algorithms 1997 ARF 199819992000 2001 RBAR 2002 MPDU OAR PER 2003 LA MiSer SwissRA 2004 AARF AMRR HRC MultiRateRetry 2005 Fast-LA LD-ARF RFT SampleRate 2006 CARA CROAR DOFRA RRAA 2007ONOE??

21. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 21 Rate Adaptation Algorithms Use recent history and probes: ARF AARF SampleRate Long interval smoothing to increase: ONOE SampleRate Use multiple rates: MultiRateRetry AMRR RRAA Use RTS/CTS: RBAR OAR CROAR CARA LA :uses RSSI to approximate SNR, each node maintains 12 dynamic RSS thresholds. LD-ARF: puts checksum on header and uses NACK to signal link loss error Table lookup with thresholds:HRC MPDU(len,rSNR,count) Fragmentation: DOFRA RFT PER MiSer SwissRA Fast-LA

22. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 22 “Round up the Usual Suspects”  ARF  RBAR  MultirateRetry

23. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 23 ARF Algorithm  If two consecutive ACK frames are not received correctly, the second retry and subsequent transmissions are done at a lower rate and a timer is started.  When the number of successfully received ACKs reaches 10 or the timer goes off, a probe frame is sent at the next higher rate. However, if an ACK is NOT received for this frame, the rate is lowered back and the timer is restarted.

24. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 24 RBAR Algorithm  {not 802.11 compatible}  Receivers control sender’s transmission rate.  RTS and CTS are modified to contain info on size and rate.  Uses analysis of RTS reception (RSSI?) to estimate SNR and send choice back to sender in CTS.  Receiver picks rate based on apriori SNR thresholds.

25. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 25 MultiRateRetryMultiRateRetry  AR5212 chipset with Atheros driver  Allows 9 FIFO queues with transmission descriptors containing ordered set of 4 pairs or rate and counts (r0/c0, r1/c1, r2/c2, r3/c3).  Algorithm tries each rate and drops down after count errors.

26. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 26 “New Kids on the Block”  CARA - Collision Aware Rate Adaptation  RRAA – Robust Rate Adaptation Algorithm

27. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 27 Collision Aware Rate Adaptation  Two methods for identifying collisions: –RTS probing –CCA detection RTS Probing  Assumes all RTS transmission failures are due to collisions.  Transmission failure after RTS/CTS must be due to channel errors.  RTS probing that enables an RTS/CTS exchange ONLY when a data frame transmission fails.

28. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 28 RTS Probing CARA default:  Data frame transmitted without RTS/CTS.  If the transmission fails, RTS/CTS exchange is activated for the next retransmission. If this retransmission fails, then the rate is lowered.  If retransmission is successful, stay at same rate and send next frame without RTS/CTS.

29. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 29 CCA Detection [J. Kim]

30. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 30 CARA-1 (with RTS Probing) [J. Kim]

31. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 31 CARA-2 (with CCA) [J. Kim]

32. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 32 Robust Rate Adaptation Algorithm  Goals: –Robust against random loss –Responsive to drastic changes  Three components: –Loss estimation –Rate change –Adaptive RTS filter

33. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 33 Loss Estimation  Instead of single probe frame,  Uses a loss estimation window and computes the estimated loss ratio over the window.  Uses upper and lower loss threshold for each rate and estimated loss ratio to decide when to switch rates.

34. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 34 Loss Ratio Thresholds [Wong]

35. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 35 Adaptive RTS Filter  Selective use of RTS/CTS.  Mentions CARA idea and problem of hidden terminals.  RTSwindow (RTSwnd) :: all frames sent RTS on. –Window is increased by one when last frame lost without RTS (potentially due to a collision) –When the last frame was lost with RTS or succeeded without RTS, RTSwnd is halved (assume no collision involved).

36. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 36 RRAA Performance [Wong]

37. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 37 Other Issues  Frame length effects  Using fragmentation  Saturation of the WLAN => AP queuing  Fairness

38. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 38 Random Thoughts  The major issue is designing rate adapatation for more complex scenarios and being able to determine the reasons for a frame loss.  The concern is the speed of the adaptation.  Several cross-layer papers attempt to coordinate rate adaptation with either streaming rate or TCP behavior.

39. Wireless Rate Adaptation Algorithms PEDS March 26, 2007 - Wireless Rate Adaptation Algorithms 39 Questions?Questions? Thank You!