Wireless Scheduling

Puzzle Monty Hall Problem You are a contestant on a game show. In front of you are three closed doors. The game show host informs you that behind one of these doors is the motor car of your dreams, but behind the other two doors lies a peanut (which you're allergic to anyway!). The quiz-master asks you to select a door. After you have selected, he then opens one of the other two doors that does not contain the car. He does this every week to build up the suspense for the watching millions. He asks if you would like to open the door you originally selected and take home that prize, or switch to the remaining door and go home with that prize.  Is it in your best interests to switch, or to remain with your original selection ?

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

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)

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

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

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)

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

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

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

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

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

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

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

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

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

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

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

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

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

Puzzle Man in a boat floating in a swimming pool  He has a large solid iron ball If he drops the ball into the water, what happens to the level of water in the swimming pool? (increases, decreases, stays the same?)