Towards Backoff Range-Based Service Differentiation over IEEE 802.11 Wireless LAN Networks

1. Towards Backoff Range-Based Service Differentiation over IEEE 802.11 Wireless LAN Networks SCW

2. 多媒體網路盛行，及時性的應用廣泛使用，而不同的應用對 bandwidth, delay and jitter 有不同的需求（QoS） Abstract Best effort 傳統傳輸方式對及時性traffic將有所影響

3. Abstract DCF 802.11 MAC 基本存取方法，不同型態的traffic使用相同參數（相同priority）來競爭媒介，無法提供任何QoS的保證 改 良 支援QoS EDCF（enhanced DCF） AEDCF（adaptive EDCF） AF-EDCF（adaptive fair EDCF） SCW（sliding contention window）

4. Introduction－EDCF • 改良DCF而來用不同 priority 來提供差異性的存取控制 （有8個 priority queues） • DCF的一個queue增加成為EDCF 8 種不同型態的queue，稱為TC（traffic class） • 每個TC均是一virtual DCF station（獨立的傳送queue），亦即在一個station內會有8個virtual DCF station同時對不同優先權的frame分別傳輸

5. Introduction－EDCF（Cont.） • AIFS [i] ：（arbitration interframe space） • 取代DCF的DIFS（AIFS＞＝DIFS） • 所有TC均有不同的AIFS，優先權愈高的TC其TC的AIFS愈短 3 mechanisms Allow each TC’s window to have different behavior • CWmax [i] ：maxmun contention window value • CWmin [i]：minimum contention window value

6. Introduction－EDCF（Cont.） • In DCF： • In EDCF： CWnew＝ 2 ×CWold CWnew[i] ＝ （CWold[i] ＋1）× PF[i] 每個TC有自己的PF（ Persistence factor ）  優先權愈大，PF則愈小，如此可讓優先權較高的TC擁有較多機會可傳送

7. Introduction－AEDCF EDCF每個TC的PF值均為固定的，如此會造成”scalability problem” 改 良 AEDCF（adaptive EDCF）將PF視為動態的參數，可依照網路的情況（collision rate）動態調整CW值

8. Introduction－AF-EDCF EDCF、AEDCF當通道負載增加時，無可避免的將會快速產生碰撞，而導致通到利用效能不彰，且高優先權之TC仍會佔用較多的bandwidth，對於通道的使用較無公平性 改 良 AF-EDCF（adaptive fair EDCF）將 channel load 考慮進來，以增加對高優先權TC的保護

9. Introduction－SCW EDCF、AEDCF、AF-EDCF SCW（sliding contention window） • 僅站在 node 的角度去做應變措施，如此考量並不完全，因為一個節點的 traffic 可能有多個來自不同 TC的 flows，而這些 flows 均會造成很大的變化 • backoff time 是隨機選取的，很難滿足 QoS guarantees, fairness and bandwidth efficiency • 站在 flows 的角度去做討論 • 每個 TC 有不同的 CW range，所以 backoff counter is selected dependent on the type of traffic being transmitted 改 良

10. Introduction－SCW （Cont.） SCW [i] is associated with each TC [i]

11. Introduction－SCW （Cont.） • CW [i] LB：SCW [i]’s lower bound • CW [i] UP：SCW [i]’s upper bound • The lower and upper bounds delimit the interval from which TC[i]’s flow select a random backoff value

12. Introduction－SCW （Cont.） The bounds of the window change as the window slides, but stay within the interval [CW [i]min, CW [i]max]

13. Introduction－SCW （Cont.） • Sliding factor，取代 PF [i] • TC [i] priority 愈大，SF [i] 愈小 • “stride”，決定 CW up or down

14. Introduction－SCW （Cont.） • 初始狀態 When loss are low & packets are transmitted successfully When loss are high

15. Introduction－SCW （Cont.） SLIDING ALGORITHM

16. Introduction－SCW （Cont.） SLIDING ALGORITHM Loss rate（drop rate）

17. Introduction－SCW （Cont.） SLIDING ALGORITHM A threshold value for the maximum tolerated loss rate for TC [i]

18. Introduction－SCW （Cont.） SLIDING ALGORITHM If the loss rate is too high，gives the TC higher priority and reducing the loss rate

19. Introduction－SCW （Cont.） SLIDING ALGORITHM If the loss rate is low，give more opportunity to lower-priority flows

20. Introduction－SCW （Cont.） SLIDING ALGORITHM Does not require any QoS metric thresholds

21. Introduction－SCW （Cont.） SLIDING ALGORITHM Network load

22. Simulation model • 10 wireless terminals（WT [i] , i = 1…10） • A single access point（ AP ） • Each WT generates up to 3 different flows at a time • High priority（HP） • Medium priority（MP） • Best effort（BE）

23. Simulation model－HP flows’ throughput

24. Simulation model－HP flows’ throughput Mean throughput SCW=72.27 kb/s EDCA=70.37 kb/s AEDF=69.88 kb/s

25. Simulation model－HP flows’ throughput SCW 7 kb/s The throughput is not good due to high intraclass contention provoked by a too narrow backoff range for HP flows

26. Simulation model－HP flows’ throughput AEDCF EDCA The throughput is relatively good due to the use of a scenario that the bit rate peaks do not occur at the same time

27. Simulation model－HP flows’ delay

28. Simulation model－HP flows’ delay AEDCF and EDCA suffer from higher queuing delays at the MAC level

29. Simulation model－MP flows’ throughput

30. Simulation model－MP flows’ throughput Too narrow backoff range provokes EDCA’s MP flows high intra-TC contention and a serious drop in throughput

31. Simulation model－MP flows’ delay

32. Simulation model－MP flows’ delay EDCA’s MP flows experience occasional degradations that have consequences for the delay

33. Simulation model－BE flows’ throughput

34. Simulation model－BE flows’ throughput SCW achieve better overall network utilization during this period

35. Simulation model－BE flows’ throughput SCW outperforms AEDCF with 100 kb/s in network utilization gain

36. Simulation model－BE flows’ throughput Compared to SCW, AEDCF has excessive throughput oscillations because it uses a per-flow collision rate to adjust the backoff range

37. Simulation model－overall network throughput

38. Simulation model－overall network throughput EDCA suffers a devastating drop in network utilization during the period

39. Conclusions and future works • Simulations show that the SCW protocol performs better than EDCA & ADECF • Reduces oscillation • Increases fairness • Currently, SCW class-specific MAC parameters must be carefully adjusted, based on the WLAN deployment scenario and predicted traffic. • Hence, future work should focus on finding mechanisms for dynamically adapting these parameters to particular network scenarios.