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Design of Fair Scheduling for WLANs

Design of Fair Scheduling for WLANs. Huei-Wen Ferng ( 馮輝文 ), Ph.D. Associate Professor Department of Computer Science and Information Engineering (CSIE) Nation Taiwan University of Science and Technology (NTUST) Wireless Communications and Networking Engineering (WCANE) Lab

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Design of Fair Scheduling for WLANs

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  1. Design of Fair Scheduling for WLANs Huei-Wen Ferng (馮輝文), Ph.D. Associate Professor Department of Computer Science and Information Engineering (CSIE) Nation Taiwan University of Science and Technology (NTUST) Wireless Communications and Networking Engineering (WCANE) Lab E-mail: hwferng@mail.ntust.edu.tw URL: http://web.ntust.edu.tw/~hwferng

  2. Outline • Introduction • Design I - DERR • Design II – EDDRR-BI and E-DERR-BI • Conclusions

  3. Introduction • IEEE 802.11/IEEE 802.11e • Popular wireless LAN standards • But some drawbacks exist. • QoS guarantee • Fairness

  4. Introduction - DCF

  5. Introduction – Enhance DCF (EDCF) or EDCA • Multiple queues of different priorities is used to take care of different QoS requirements.

  6. Design I Design of Distributed Elastic Round Robin (DERR)

  7. Related Work • Distributed Fair Scheduling (DFS) • adjusts backoff intervals to implement fairness. • Distributed Weighted Fair Queuing (DWFQ) • adjusts contention windows to fulfill fairness. • Both DFS and DWFQ have poor performance in throughput and delay due to latent collisions although better fairness can be achieved. • Distributed Deficit Round Robin (DDRR) • is designed based on Deficit Round Robin (DRR) • is a distributed fair scheduling scheme. • employs the concept of deficit count to achieve fairness. • is a collision-free scheme. • employs a mapping between deficit count and the inter frame space (IFS)

  8. Main Ideas of DDRR

  9. Main Ideas • Allowance • previously employed by elastic round robin (ERR) • The minimum amount of data allowed to be transmitted by a host. • Allowance is adjustable and smaller than the amount of data sent. • Classification • Hosts with different QoS requirements are classified into different classes. • Weight • defined according to QoS requirements

  10. How to Define Allowance? • The excess amount: where and are the excess amount and the total amount of transmitted frames • The allowance is then calculated using i: class j: host

  11. Why? • is larger for a host with higher throughput and vice versa. • Thus, is proportional to the desired throughput specified by host j within class i • The excess amount during the previous transmission is deducted for the sake of fairness.

  12. AMapping Function • Constant is used to make fall within PIFS and DIFS specified by the IEEE 802.11 standard • rand is a random number which results in different values of IFS to avoid possible collisions • A host with shortest IFS gets the right to transmit frames until the total length of frames exceeds the value of allowance.

  13. Definitions of Weights • Weight of class i is defined by Where KE is a pre-specified constant and TE,i (in seconds)is a time period.

  14. Numerical Results and Discussions • Ad hoc mode under transmission rate of 2 Mbps • No RTS/CTS • 8 hosts exist in the system • Frame generation rate: 110 kbps • data rate: 100 kbps • Frame length: 1000 bytes • Other parameters:

  15. Throughput and Delay Throughput Delay

  16. Fairness Index • Fairness index The difference between DERR and ideal: 0.032, DDRR and ideal: 0.071

  17. Throughput/weight (DDRR) Std. deviation: 0.018

  18. Throughput/weight (DERR) Std. deviation: 0.00063

  19. Concluding Remarks • A fair scheduling mechanism, i.e., DERR is proposed and discussed. • DERR avoids collisions through a random mapping between allowance and IFS. • DERR offers a better fair sharing algorithm than DDRR.

  20. Design II Design of EDDRR-BI and EDERR-BI

  21. Goal • Designing two scheduling mechanisms • Enhanced distributed deficit round robin with backoff interval (EDDRR-BI) and • Enhanced distributed elastic round robin with backoff interval (EDERR-BI)

  22. Main Ideas • These two scheduling mechanisms • should consider both priority and fairness, • are designed based on DDRR and DERR, • are targeted for IEEE 802.11e.

  23. Design of EDDRR-BI • DDRR vs. EDDRR-BI • Both employ the concept of the deficit count. • DDRR defines one type of deficit count, while EDERR-BI defines three. • No backoff procedure is involved in DDRR, but EDDRR-BI has the backoff procedure. • Cross-layer design is further considered in EDDRR-BI.

  24. Design of EDDRR-BI • For the ith flow of access category l in station j at time t, DC (t) is defined to denote the deficit count. • When no data is transmitted, j l, i l = a denotes audio, l = v denotes video, l = d denotes data

  25. Design of EDDRR-BI • When a frame for the ith flow of access category l in station j is transmitted at time t. the frame size is then deducted from the deficit count, i.e., • Temporary backoff interval • Backoff interval

  26. Design of EDERR-BI • Unlike EDDRR-BI, EDERR-BI adapts allowance, which is an elastic and adjustable amount of traffic data allowed for transmission, to govern the kernel of scheduling.

  27. j F (t’) is the total amount of traffic data transmitted at time t’ by the ith flow of access category l in station j. l,i Design of EDERR-BI j Where E (t’) stands for the excess amount for the ith flow of access category l in station j at time t’ l,i • Allowance is defined in proportional to the desired throughput. • Allowance increases linearly as time goes. • The excess amount at the latest time instant should be deducted to enforce fair scheduling.

  28. Design of EDERR-BI • Temporary backoff interval • Backoff interval

  29. Definitions of Weights • For both EDDRR-BI and EDERR-BI, weights should be properly defined so that one can gauge fairness based on weights. • We define weights for flows within the same access category l (l = a, v, d) w ([s] is used to denote the same access categories), [s],j l,i

  30. Definitions of Weights • Weights for flows within different access category w ([d] is used to stand for different access categories) • Weights for different stations w [d],j l,i j

  31. Simulation Results and Discussions We use the well-know network simulator ns-2 as the simulation platform. The desired flow rates for audio, video, data are fixed at 8 KB/s, 128 KB/s, 120 KB/s.

  32. Simulation Results and Discussions • To gauge the performance of EDDRR-BI, EDERR-BI, EDCA and EEDCF. Throughput comparison (Video) Fixing the number of stations at 18, we have 8% and 1.5% improvement for both EDDRR-BI and EDERR-BI compared to EDCA and EEDCF. (Data) We have 14% and 41% (28% and 58%) of improvement for both EDDRR-BI and EDERR-BI compared to EDCA and EEDCF.

  33. Simulation Results and Discussions Delay comparison Collision rate comparison When the number of stations is 18 (Video) mean delay for EDCA (EEDCF) is 84% and 126% (17% and 44%) higher than EDDRR-BI and EDERR-BI. (Data) mean delay for EDCA (EEDCF) is 5% and 8% (9% and 12%) higher than EDDRR-BI and EDERR-BI. When the number of stations is 18. We have 22% of improvement for EDDRR-BI and 31% for EDERR-BI compared to EDCA

  34. Simulation Results and Discussions Fairness comparison

  35. Simulation Results and Discussions We have 120% and 7% (97% and 19%) of improvement for EDDRR-BI and 144% and 33% (230% and 98%) of improvement for EDERR-BI compared to EDCA and EEDCF.

  36. Simulation Results and Discussions 50% and 6% (77% and 18%) of improvement for EDDRR-BI and 100% and 41% (192% and 94%) of improvement for EDERR-BI compared to EDCA and EEDCF are obtained. 1/ (1-fairness index)

  37. Concluding Remarks • Two fair scheduling mechanisms, i.e., EDDRR-BI and EDERR-BI, which can interplay with QoS requirements in the IEEE 802.11e LAN, are proposed and studied. • Through simulations, we successfully illustrate that EDDRR-BI and EDERR-BI outperform EDCA and EEDCF in terms of QoS related performance and fairness. • EDERR-BI performs better than EDDRR-BI. • EDERR-BI is most suggested for use in the IEEE 802.11e LAN to achieve better fairness and guaranteed QoS.

  38. Conclusions • Two designs of fair scheduling are discussed. • Future work? • Actually, we have done the design for EDDRR and EDERR which are not covered in this talk.

  39. Q & A

  40. References • Huei-Wen Ferng, Chung-Fan Lee, Jeng-Ji Huang, and Ge-Ming Chiu, “Designing a fair scheduling mechanism for IEEE 802.11 wireless LANs,” IEEE Commun. Letters, vol. 9, no. 4, pp. 301-303, April 2005. -> DERR • Huei-Wen Ferng, Chung-Fan Lee, Han-Yu Liau, and Cheng-Ching Peng, “A fair scheduling mechanism for the IEEE 802.11e wireless LAN,” in Proc. International Computer Symposium (ICS) 2006, Taipei, Taiwan, December 2006. -> EEDCF • Huei-Wen Ferng, Han-Yu Liau, and Jeng-Ji Huang, “Fair Scheduling Mechanisms with QoS Consideration for the IEEE 802.11e Wireless LAN,” in Proc. IEEE VTC 2007-Spring, Dublin, Ireland, April 2007. -> EDDRR-BI and EDERR-BI

  41. Thank You! E-mail: hwferng@mail.ntust.edu.tw URL: http://web.ntust.edu.tw/~hwferng

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