Applying a Deficit Round Robin for Downlink Resource Allocation in Mobile WiMAX Networks

1. Applying a Deficit Round Robin for Downlink Resource Allocation in Mobile WiMAX Networks Chakchai So-In, Raj Jain and Abdel Karim Al-Tamimi 31 March 2009 Washington University in Saint LouisSaint Louis, MO 63130{cs5, jain, aa7}@cse.wustl.edu

2. Outline • Overview of Mobile WiMAX • Resource Allocation in Mobile WiMAX • Fair Scheduling Algorithms • Throughput Fair Allocation • Extensions of the scheduling • Performance Evaluation • Conclusions

3. Broadband Wireless Networks High speed over long distance e.g., up to 70Mbps or 50kms Focus on Point to Multipoint networks Focus on downlink direction Overview of Mobile WiMAX Fixed Station Uplink (UL) Base Station (BS) Downlink (DL) Mobile Stations (MSs)

4. Resource Allocation in Mobile WiMAX Queues • Which queue(s) to serve in the next WiMAX frame, how much? • Goals: • Maximize throughput (minimize overhead and unused space) • Achieve fair allocation (throughput fairness) • Support Mobile WiMAX QoS parameters Scheduler Frame 1 Frame 2

5. Two Common Scheduling Algorithms • General Processor Sharing (GPS) [1] • Allocate the fair share regardless of packet sizeE.g., WiMAX frame capacity = 300B, 4 MSs with 125B packet size → 300/4 = 75B fragmented packet is transmitted. • Deficit Round Robin (DRR) [2] • Allow only full packet to be transmitted • IF the quantum (or the fair share)+ deficit ≥ the packet size, the packet is transmitted and the deficit is updated. • ELSE the deficit is remembered.

6. Example of DRR • 4 Mobile Stations (MSs) • Packet size = 125B • Frame capacity = 300B • Quantum (fair share)= 300/4 =75 • RED = transmitted packets • Waste 50B each frame Frame #Transmitted Packet Size/ Deficit Counter

7. DRR with Fragmentation Awareness • Mobile WiMAX allows fragmentation. • DRR with Fragmentation (DRRF) • Similar to DRR: Transmit full packet → Reduce overhead: MAC and fragmentation headers • IF there are some left-over spaces, DRRF allocates those to some MSs. → Achieve full frame utilization

8. Example of DRRF #Transmitted Packet Size/ Deficit Counter • 4 MSs, 125B packet size and 300B frame capacity • RED = transmitted packets • RED= transmitted fragmented packets • Unlike DRR, do not waste 50B in each frame

9. Throughput Fair Allocation • Broadband Wireless Networks • Link Capacity is not constant over time/distance. • WiMAX allows different Modulation and Coding (MCS) for different wireless condition. • Slot = smallest allocation unit • Slot capacity → variable • We uses #slots not #bytes as an allocation unit. MCS_size = #bytes/slot given MCS level Slot WiMAX Frame Frequency . . . Time

10. Max-Min Fairness Algorithm • Some MSs may not have enough traffic and may not be able to use the fair share. • Max-Min Fairness for GPS, DRR and DRRF • For GPS; we use #slots derived from Max-Min algorithm as an actual allocation. • For DRR and DRRF; we use #slots derived from Max-Min algorithm as a actual quantum. • We do scaling to make the quantum at least one packet size . (We use 1500 bytes.)

11. DRRF Extensions • Minimum Guaranteed Rate (MGR) and Maximum Sustained Rate (MSR) Extensions • Derive #slots required to meet MGR and MSR • Update #freeslots • Apply Max-Min algorithm to derive the proper quantum • Traffic Priority Extension • Flows of higher priority are serviced before those of the lower priority. • The proper quantum number is derived from hierarchical Max-Min allocation from the highest to the lowest priority.

12. Single Base Station (BS) with multiple mobile stations Simulation configuration follows WiMAX Methodology [3] NS2 Simulator [4] Simulation Topology MS1 MS2 . . . 100Mbps, 1ms Server MSn BS

13. Scenarios • Throughput vs. overhead: 15 flows • Throughput Fairness: 5 flows (different wireless conditions) • Minimum Rate: 5 flows with 1 flow at 2Mbps rate guaranteed • Maximum Rate: 5 flows with 1 flow at 0.5Mbps maximum rate • Traffic Priority: 5 flows, 2 flows with priority 2, and 3 flows with priority 1. (Each flow is 1.5Mbps.)

14. Scenario 1: Throughput vs. overhead • All three algorithms achieve perfect fairness. • GPS: High link utilization BUT more overhead • DRR: less overhead BUT also less throughput • DRRF = GPS (High link utilization) + DRR (less overhead)

15. Scenario 2: 5 Flows with Various MCSs 1.47 Mbps 1.78 Mbps 1.78 Mbps • All three algorithms achieve perfect throughput fairness. (Jain Fairness Index = 1) • An average system throughput for GPS and DRRF is 1.78 Mbps versus 1.47 Mbps for DRR GPS DRRF DRR

16. Scenario 3: 5 Flows with 2Mbps (rmin) • MS1’ throughput is 2Mbps + 0.6Mbps (fair share). • The left-over bandwidth is distributed fairly (MS2 to MS5), 0.6Mbps.

17. Scenario 4: 5 Flows with 0.5Mbps (rmax) • MS1’ throughput is limited to 0.5Mbps. • The left-over bandwidth is distributed fairly (MS2 to MS5), 1.2Mbps.

18. Scenario 5: Traffic Priority • Setup: 2 flows with priority 2 and others with priority 1. • Results: 2 flows with priority 2 receive full throughput, 1.5Mbps. The left-over bandwidth is distributed fairly.

19. Conclusions • Resource allocation in Mobile WiMAX networks • Maximize throughput, achieve fairness and support QoS • Achieving the goals • Deficit Round Robin (DRR) with Fragmentation = Full frame utilization (similar to GPS) + less overhead (similar to DRR) • With slot-based allocation, GPS, DRR and DRRF achieve throughput fairness. • With the proper quantum and deficit counter, DRRF can support three main QoS parameters for Mobile WiMAX.

20. Thank you and Questions?