A Hybrid Approach to Effort-Limited Fair (ELF) Scheduling for 802.11

1. A Hybrid Approach to Effort-Limited Fair (ELF) Scheduling for 802.11 By David Matsumoto June 20th, 2003

2. Agenda • Introduction • Effort-Limited Fair (ELF) Scheduling • The Thesis • Integrate ELF into 802.11 • Hybridize ELF • Simulation setup & results • Possible future research & recommendations • Conclusion

3. application transport network data link physical application transport network data link physical application transport network data link physical Introduction • Applications on a network • Different requirements (bandwidth, timing, reliability) • Network stack • Underlying protocols (TCP, UDP, IP, MAC, etc…) • Providing quality of service (QoS) • Wired versus wireless • Wireless: subject to capacity loss • “Effort” spent does not always equal “outcome”

4. Earlier Work: Effort-Limited Fair (ELF) Scheduling • Motivation • Applications have poor performance due to link errors • Link errors reduce useful link throughput • Solutions introduced: Swapping time slots, Adaptive forward error control • Capacity loss is a fundamental reality for wireless schedulers. • Questions • Can we support reservations amidst capacity loss? • How should we redistribute the remaining capacity among flows?

5. Example: 50% Packet Loss • Wireless cell capacity: 800 kilobits/second • WFQ scheduler serves two guaranteed flows & two best-effort flows

6. Challenges in QoS • Capacity loss should result in throughput loss according to administrative controls • What about location dependent errors? • How should we regulate air-time? • Conclusion: • We must have a way to balance fidelity with efficiency.

7. ELF Scheduling – Design Principles • Wireless scheduler should be equivalent to wireline scheduler in error-free environment • Capacity loss suffered per flow should be administratively configurable • Administratively bound capacity lost due to location-dependent errors • Unless configured otherwise, flows with the same error rate should experience the same capacity loss • Capacity unused by one flow should be distributed “fairly” among other flows

8. The ELF Scheduler User Requests Administrative Controls Application Protocols Admission Control Power factors Packet Scheduler weights “next packet” Effort/outcome Link Layer

9. ELF Power factor • Main Idea: • Extend transmission time in a controlled fashion • Allot a flow extra effort to meet its reservations, up to some administrative bound  Power factor • Adjusted Weight (Ai) • Ai = min (Wi/1 – Ei , Pi X Wi) • Effective throughput (Ti) • Ti = ((Ai/∑jAj) X B) x (1 – Ei) • This effectively balances fidelity with efficiency.

10. The Thesis It is possible to integrate an Effort-Limited Fair scheduler (ELF) into a standard 802.11 implementation, and thus ensure sensible outcomes for flows in response to unrecoverable capacity loss. Moreover, it is possible to improve on the efficiency of ELF within an 802.11 MAC by using a hybrid approach to regulate stations within both a DCF & a PCF.

11. IEEE 802.11 • Standard for wireless Local Area Networks (LAN) • Distributed Coordinator Function (DCF) • Uses Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) • Point Coordinator Function (PCF) • Centralized, polling-based mechanism requiring a base station (Point Coordinator) • DCF & PCF can co-exist • Contention period (CP) under control of DCF • Contention-free period (CFP) under control of PCF • Both methods use explicit Acks • Useful for PC tracking outcome

12. 802.11 PCF/DCF Alternation

13. Integration of ELF scheduling into 802.11 • Core ELF algorithm remains the same • Choose “most deserving” flow • Flow is allocated effort as a function of its power factor • Must have “effort” allocated to be chosen • Separate best-effort flows from guaranteed flows • Update each flow per clock “tick” • ELF scheduling becomes the policy for the PCF • Departs from existing protocol specifications • Records “outcome” via returned Acks (or NULL frames) • Charge a station for “effort” each time it is polled. • Station charged even if packets are corrupted.

14. Extending ELF for DCF • Motivation • DCF is more efficient in general, when traffic load isn’t high. • Ideal: PCF could interject only when necessary to bring flows into conformance with reservations. • Design Principles • DCF must remain completely distributed • ELF-DCF should adhere to original ELF design principles. • Integration of ELF-DCF/PCF should be seamless

15. ELF-DCF: Implementation Design • Continue using both “deserve” & “effort” to (self) regulate flows • “Deserve” carries over from CFP • “Effort” allocated based on past CP history • Cannot force station to expend effort • Make use of power factor • “Effort” is conserved (unused effort plugged into CFP) • No distinction between best-effort versus guaranteed flows • Reduce overhead in beacon ELF Scheduler (PC) Collect statistics/Start CFP Send Beacon with ELF data Mobile Nodes

16. Implementation for ns-2 • Build on top of CMU Monarch group’s work • Basic mobile node functionality with 802.11 DCF • Add semi-complete implementation of PCF. • Beacon timer • Bi-directional polling • Poll list – integrated with ELF • Implement Weighted Round Robin (WRR) instead of WFQ • Measure throughput in frame slots (done for simplicity) • Not concerned with complex based service characterizations (e.g. delay, jitter guarantees) • Administratively configure breakdown of superframe • I.E. Percentage CFP/CP • Assume admissions control module exists that can set appropriate per-flow power factors.

17. Experimental setup • One flow per station • Two CBR flows & two FTP flows (each with 25% weight) • Flows chosen so aggregate throughput consumes the entire link ~ 1.4 Mbit/s • CBR flow ~ 450 Kbit/s • FTP flow ~ 250 Kbits/s (no errors) • Relevant variables: • Percentage CFP/CP • ELF-DCF Boolean • Power factor • Error rate • Error models • Uniform, Markov, real-world Traces WLAN

18. Uniform – 50% (CBR-1 Error prone) • CBR-1 performs poorly while other stations operate normally • No ELF intervention here.

19. Uniform – 50% (CBR-1 Error prone) • ELF-PCF effectively restores throughput to CBR-1 at the expense • of best-effort flows

20. Uniform – 50% (CBR-1 Error prone) • ELF-PCF is only partially effective at restoring throughput

21. Uniform – 50% (CBR-1 Error prone) • ELF-DCF effectively works with ELF-PCF to meet ELF design principles

22. Walls trace – (CBR-1 Error prone) • No ELF intervention – other flows gain throughput as CBR-1 decreases

23. Walls trace – (CBR-1 Error prone) • ELF-PCF/DCF work together • Other flows share throughput loss, up to administrative bound • Capping (“deserve”) mechanism needed

24. Future Work & Recommendations • Future work • Provide a capping mechanism for flows that accumulate large “deserve” values • Can we “encourage” stations to spend effort during a DCF (rather than just limiting the effort allotted)? • Recommendations • Incorporate a “policing” mechanism into beacons • Change 802.11 specifications on how stations are polled • Allow polling of individual flows

25. Conclusions • ELF scheduling can be implemented as the policy for a PCF in 802.11 • ELF-DCF/PCF provides a hybrid mechanism through we can achieve ELF’s design principles

26. Acknowledgements • Peter Steenkiste – Advisor • David Eckhardt – Reader • INI Staff & Professors

27. Questions