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

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

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

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  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.

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