1. Service differentiation mechanisms for IEEE 802.11-based wireless networks § Srikant Kuppa & Ravi Prakash Distributed Systems Laboratory The University of Texas at Dallas Email: {ksrikant, ravip}@utdallas.edu § Work supported in part by NSF CAREER grant no. CCR-0093411.

2. Srikant Kuppa & Ravi Prakash 2March 23, 2004 Outline  IEEE 802.11 DCF Scheme  QoS-aware MAC sub-layer policies  Schedule After Backoff (SAB) policy  Schedule Before Backoff (SBB) policy  Performance evaluation  SAB vs SBB-based MAC Schemes  Conclusions

3. Srikant Kuppa & Ravi Prakash 3March 23, 2004 IEEE 802.11 DCF Scheme  Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA).  Binary exponential backoff. Above figure is a copy of Fig. 49, p. 74 of ANSI/IEEE Std 802.11, 1999 Edition.

4. Srikant Kuppa & Ravi Prakash 4March 23, 2004 DCF Scheme (Contd.) Select head-of-line frame to transmit Transmit frame Backoff initiation and backoff procedure Audio Video Video-probe Data Single Transmission Queue Frame Selection Step Channel Access Operations Single Backoff Instance

5. Srikant Kuppa & Ravi Prakash 5March 23, 2004 QoS-aware MAC sub-layer policies  DCF provides only best-effort service.  Channel access independent of frame priorities.  A single FIFO transmission queue.  Service differentiation at MAC sub-layer  Offers greater flexibility.  Compatibility issues.

6. Srikant Kuppa & Ravi Prakash 6March 23, 2004 Schedule After Backoff (SAB)  Access Category (AC) as a virtual node.  Multiple ACs within a node operate independently.  Each AC uses:  AIFS[AC] instead of DIFS,  CW min [AC], CW max [AC] instead of CW min, CW max.  Virtual and real collisions.  Need for a frame scheduler.

7. Srikant Kuppa & Ravi Prakash 7March 23, 2004 SAB (contd.) Select head-of-line frame to transmit Transmit frame Backoff initiation and Backoff procedure Select head-of-line frame to transmit Backoff initiation and Backoff procedure Select head-of-line frame to transmit Backoff initiation and Backoff procedure Select head-of-line frame to transmit Backoff initiation and Backoff procedure Virtual Collision Handler AC 3AC 2AC 1AC 0 Transmit frame Multiple Transmission Queues Prioritized Channel Access Operations Frame Selection Step Multiple Backoff Instances

8. Srikant Kuppa & Ravi Prakash 8March 23, 2004 SAB (contd.) AC 3AC 2AC 1AC 0 Transmit frame Virtual Collision Transmit frame

9. Srikant Kuppa & Ravi Prakash 9March 23, 2004 SAB (contd.)  4 ACs within a node  (4×n) virtual contending nodes, where n is the total number of nodes.  Example: IEEE 802.11e Enhanced DCF (EDCF).  High collision rate and starvation in EDCF under heavy loads of high-priority traffic. Frame Priority ACTraffic Class AIFS (in slots) CW min (in slots) CW max (in slots) 6, 7 3AudioDIFS8 16 4, 5 2VideoDIFS+11632 31VProbeDIFS+2321024 0, 1, 20DataDIFS+2321024

10. Srikant Kuppa & Ravi Prakash 10March 23, 2004 Schedule Before Backoff (SBB)  Multiple transmission queues (like EDCF).  Focus on a single frame (like DCF).  Frame selection step independent of channel access operations.  Frame selection: Round-Robin (RR), Weighted Round-Robin (WRR), etc.  Channel access operations: Prioritized/Non- prioritized.

11. Srikant Kuppa & Ravi Prakash 11March 23, 2004 SBB (contd.) Select head-of-line frame to transmit Transmit frame Backoff initiation and Backoff procedure AC 3AC 2AC 1AC 0 Multiple Transmission Queues Channel Access Operations Frame Selection Step Single Backoff Instance

12. Srikant Kuppa & Ravi Prakash 12March 23, 2004 SBB (contd.) AC 3AC 2AC 1AC 0 Transmit frame

13. Srikant Kuppa & Ravi Prakash 13March 23, 2004 SBB (contd.)  In WRR, an AC k frame is chosen with probability, p k such that p 3 > p 2 > p 1 > p 0.  In our case, p 3 = 0.45, p 2 = 0.25, p 1 = 0.15 and p 0 = 0.15.  Some SBB-based schemes  RR.  WRR.  RR with Varying CW (RR with VCW).  WRR with Varying CW (WRR with VCW).

14. Srikant Kuppa & Ravi Prakash 14March 23, 2004 Performance evaluation: SAB vs SBB-based MAC Schemes  Simulation set-up  Network scenario  One-hop ad hoc network with n=10 stationary nodes.  Single wireless channel with ideal characteristics.  Available bandwidth = 36 Mbps.  Two traffic scenarios  Scenario A under saturation conditions  Scenario B under more realistic scenarios.

15. Srikant Kuppa & Ravi Prakash 15March 23, 2004 Performance evaluation (contd.)  Traffic scenario A  All transmission queues within a node remained non-empty at all times.  Total traffic flows = 4×n.  All ACs generated data frames of same size.

16. Srikant Kuppa & Ravi Prakash 16March 23, 2004 Performance evaluation (contd.)  Traffic scenario B  Some transmission queues within a node remained empty during the entire simulation run.  Total traffic flows = 2×n.  AC 3, AC 2, AC 1, AC 0 traffic generated in the ratio 4:5:4:7.  Data frames generated by ACs were different in size.

17. Srikant Kuppa & Ravi Prakash 17March 23, 2004 Performance metrics  B andwidth share per AC.  Mean latency per AC.  Standard deviation per AC: Measure of jitter.  t idle : Period of time channel was idle.  t succ : Period of time channel was used for successful transmissions.  t col : Period of time collisions occurred on the channel.  Total throughput.

18. Srikant Kuppa & Ravi Prakash 18March 23, 2004 SAB vs SBB (contd.)

19. Srikant Kuppa & Ravi Prakash 19March 23, 2004 SAB vs SBB (contd.)

20. Srikant Kuppa & Ravi Prakash 20March 23, 2004 SAB vs SBB (contd.)

21. Srikant Kuppa & Ravi Prakash 21March 23, 2004 SAB vs SBB (contd.)

22. Srikant Kuppa & Ravi Prakash 22March 23, 2004 Conclusions  Two possible QoS-aware MAC policies  SAB as in IEEE 802.11e EDCF scheme.  SBB with single backoff instance as in DCF.  Under high loads,  EDCF suffers from high collision rate and starves low-priority traffic.  SBB-based MAC schemes avoid starvation.  Under low loads,  SBB-based schemes perform at par with EDCF.

23. Srikant Kuppa & Ravi Prakash 23March 23, 2004 Conclusions (contd.)  Multiple backoff instances not necessary.  Different frame sizes  Weighted Fair Queuing (WFQ) instead of WRR.  Future work:  What factors lead to starvation in EDCF?  Is EDCF over-engineered?