Wireless Fair Scheduling

1. Wireless Fair Scheduling

2. PAs • PA report should consist of • Problem statement • Solution description • Performance Results • Analysis and Insights • Conclusions • Email your report in .pdf format to TA by 11.59pm EST on deadline (same for video students) • Also, turn in report and associated files into the grpk/SUBMIT/PA1/ directory

3. Wireless Fair Queuing • Wireless channel capacities are scarce • Fair sharing of bandwidth becomes critical • Both short-term and long-term fairness important

4. Wireless FQ & Wireless Environment • Location dependent and bursty errors • For the same wireless channel, a mobile station might experience a clean channel while another might experience high error rates. Why? • In wireline fair queuing, the channel is either usable by all flows or unusable by any of the flows …

5. Wireless Channel Model • Base station performs arbitration • Schedules both uplink and downlink traffic • Neighboring cells use different channels • Every mobile host has access to base-station

6. Wireless Channel Characteristics • Dynamically varying capacity • Location dependent channel errors and bursty errors • Contention • No global state • Scarce resources (battery & processing power)

7. Service Model • Short term fairness • Long term fairness • Short term throughput bounds • Long term throughput bounds • Delay bounds for packets

8. Some terminology … • Error free service • Leading flows • Lagging flows • In sync flows

9. Impact of Location Dependent Errors • Example 1 • 3 flows f1, f2, f3 • Period 1: f3 experiences lossy channel • Flows f1 and f2 receive ½ of channel • Period 2: f3 experiences clear channel • Wireline fair queuing would give a net service of 5/6 to f1 and f2, and 1/3 to f3 – UNFAIR! • Wireline fair queuing does not distinguish between flows that are not backlogged and flows that are backlogged but cannot transmit!

10. Impact (Contd.) • Example 2 • Same scenario • Flow f1 has only 1/3 offered service • Hence, for period 1 f2 receives 2/3 service • If some compensation is given to f3 during period 2, should f1 be penalized for compensating f3?

11. Issues addressed by Wireless Fair Scheduling • Is it acceptable to compromise on separation for f1? • How soon should f3 get its share back? • Should f2 give up service and over what period of time?

12. Generic Wireless FS Model • Error free service • Lead/lag/in-sync • Compensation model • Channel monitoring and prediction

13. Error Free Service • Reference for how much service a flow should receive in an ideal error free channel • Example: WFQ • Each packet stamped with a finish tag based upon the packet’s arrival time and the weight of the flow • Packet with the minimum finish tag transmitted

14. Lead and lag model • Lag • Lead • Two approaches • Lag of flow incremented as long as the flow is backlogged and is unable to transmit. Such a flow will be compensated at a later time. • Lag of flow incremented only if the slot given up by the flow is taken up by another flow (which will have its lead incremented). At a later time, compensation will be given at the expense of a flow with lead.

15. Compensation Model • No explicit compensation • Flow with maximum lag is given preference • Leading and lagging flows swap slots • Bandwidth is reserved for compensation

16. Instantiations • Channel state dependent packet scheduling (CSDPS) • Idealized wireless fair queuing (IWFQ) • Wireless packet scheduling (WPS) • Channel-condition independent fair queuing (CIFQ) • CBQ-CSDPS • Server based fairness approach (SBFA) • Wireless fair service (WFS)

17. CSDPS • CSDPS allows for the use of any error-free scheduling discipline – e.g. WRR with WFQ spread • When a flow is allocated a slot and is not able to use it, CSDPS skips that flow and serves the next flow • No measurement of lag or lead • No explicit compensation model

18. CSDPS (Contd.) • Lagging flows can thus make up lags only when leading flows cease to become backlogged or experience lossy channels sometime • No long-term or short-term fairness guarantees

19. IWFQ • WFQ is used for the error free service • Packets tagged as in WFQ. Of the flows observing a clean channel, the flow with the minimum service tag packet is served • Tags implicitly capture the service differences between flows (lagging flows will have a smaller service and hence will be scheduled earlier)

20. IWFQ (Contd.) • Channel capture by lagging flows possible resulting in short term unfairness and starvation • Even in-sync flows can become lagging during such capture periods • Coarse short-term fairness guarantees because of possible starvation • Provides long-term fairness

21. WPS • WRR with WFQ spread used for error free service • A frame of slot allocations generated by WPS based on WRR (with WFQ spread) • Intra frame swapping attempted when a flow is unable to use a slot • If intra-frame swapping is not possible lag incremented as long as another flow can use the slot

22. WPS (Contd.) • At the beginning of next frame, weights for calculating spread readjusted to accommodate lag and lead • If intra-frame swapping succeeds most of the time, in-sync flows not affected • Complete channel capture prevented as each flow has a non-zero weight when frame spread is calculated • No short-term fairness guarantees, but provides long-term fairness

23. CIFQ • STFQ (Start time fair queuing) used for the error free service • Lag or lead computed as the difference between the actual service and the error free service • A backlogged leading flow relinquishes slot with a probability p, a system parameter • A relinquished slot is allocated to the lagging flow with the maximum normalized lag

24. CIFQ (Contd.) • In-sync flows not affected since lagging flows use slots given up by leading flows • Lagging flows can still starve leading flows under pathological scenarios • Provides both short-term and long-term fairness

25. CBQ-CSDPS • Same as IWFQ except that no explicit error free service is maintained • Rather, lead/lag is measured based on the actual number of bytes s transmitting during each time window • A flow with normalized rate r is leading if it has received channel allocation in excess of s*r, and lagging if it has received channel allocation less than s*r • Lagging flows are allowed precedence

26. CBQ-CSDPS • Same problem as in IWFQ – lagging flows given precedence, and hence can capture channel • Short term fairness is thus not guaranteed • Additionally, leads and lags are computed not based on error-free service, but based on a time window of measurement … performance sensitive to the time window

27. SBFA • Any error free service model can be used • SBFA reserves a fraction of the channel bandwidth statically for compensation by specifying a virtual compensation flow • When a flow is unable to use a slot, it queues a slot-request to the compensation flow • Scheduler serves compensation flow just as other flows • When the compensation flow gets a slot, it turns the slot over to the flow represented by the head-of-line slot-request

28. Slot queued into compensation flow Compensation Flow of weight w SBFA (Contd.) Scheduled to Tx F1 Cannot transmit because of error Slot scheduled for Tx and handed over to F1

29. SBFA (Contd.) • No concept of a leading flow • All bounds supported by SBFA are only with respect to the remaining fraction of the channel bandwidth • Performance of SBFA is sensitive to the statically reserved fraction • No short-term fairness • Long-term fairness dependent upon the reserved fraction

30. Wireless Fair Service • Uses an enhanced version of WFQ in order to support delay-bandwidth decoupling • Lag of a flow incremented only if there is a flow that can use the slot • Both lead and lag are bounded by per-flow parameters • A leading flow with a lead of L and a lead bound of Lmax relinquishes a fraction L/Lmax of the slots allocated to it by the error-free service • This results in an exponential reduction in the number of slots relinquished

31. WFS (Contd.) • Service degradation is graceful for leading flows • In-sync flows are not affected • Tightest short-term fairness among all algorithms discussed • Compensation for lagging flows can take up more time than other algorithms

32. Recap • Wireless Fair Scheduling • Why wireline algorithms cannot be used • Key components of a a wireless fair scheduling algorithm • Different approaches for wireless fair scheduling

33. Puzzle • You are blindfolded • There is a square table in front of you • Four bottles places – one at each corner • Bottles can either be in UP or DOWN orientations • You can “feel” any two of the bottles at a time, switch their orientation however you want to – you win if all bottles are oriented alike • The table will be rotated arbitrary number of ¼ turns after each of your moves • Can you guarantee that you will win?