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Quality of Service Schemes for IEEE 802.11 Wireless LANs-An Evaluation

Quality of Service Schemes for IEEE 802.11 Wireless LANs-An Evaluation. 主講人 : 黃政偉. Outline. Introduction Overview of evaluated schemes Simulation Result Discussion Conclusion. Introduction.

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Quality of Service Schemes for IEEE 802.11 Wireless LANs-An Evaluation

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  1. Quality of Service Schemes for IEEE 802.11 Wireless LANs-An Evaluation 主講人: 黃政偉

  2. Outline • Introduction • Overview of evaluated schemes • Simulation • Result • Discussion • Conclusion

  3. Introduction • Wireless networks are superior to wired networks with regard to aspects such as ease of installation and flexibility. • Given the coverage and low price, it is likely that demands for the ability to run real-time applications such as voice over IP over these networks will increase. • Inherent problems: low medium utilization, risk of collisions and problem of providing differentiation between different types of traffic.

  4. Overview of evaluated schemesPoint Coordinator Function (PCF)

  5. Overview of evaluated schemes Enhanced DCF(EDCF) • The EDCF mechanism allows traffic to be classified into 8different traffic classes, • Method: the minimum contention window (CWmin) and the interframe space • Higher priority : smaller CWmin ; lower priority : larger CWmin • Different traffic classes: Arbitration interframe space (AIFS = DIFS + number of time slots)

  6. Overview of evaluated schemes Enhanced DCF(EDCF)

  7. Overview of evaluated schemes Distributed Fair Scheduling (DFS)(1/2) • Fair means that each flow gets bandwidth proportional to some weight that has been assigned to it. • Before transmitting a frame, the backoff process is always initiated • Where sizepacket is the size of the packet to send

  8. Overview of evaluated schemes Distributed Fair Scheduling (DFS)(2/2) • Fairness is achieved by using the size of the packet to be sent in the calculation of the backoff interval. • If a collision occurs, a new backoff interval is calculated using the 802.11 standard. (CWmin=3)

  9. Overview of evaluated schemes Blackburst (BB)(1/2) • Three interframe spacing: (τ: max propagation delay) • tshort + 2τ< tmed ; tmed + 2τ< tlong ; tshort < tmed < tlong • Black burst duration: • Duration = tbslot * [d / tunit]; tbslot is a length of a black slot . tunit : a system parameter defined shortly. d : to access the medium, STA has been waiting for d sec. • The scheduled access interval : tsch

  10. Overview of evaluated schemes Blackburst (BB)(2/2)

  11. Simulation Scenarios(1/2) • Network simulator : ns-2 • WLAN bandwidth : 2Mbit/s • In infrastructure mode, the mobile nodes always communicate directly with the AP. • There is no mobility in the system

  12. TU = 1024μs Simulation Scenarios(2/2) • table1

  13. SimulationsMetrics(1/2) • Throughput (Normalization) • Percentage of the offered data that is actually delivered to the destination. • Medium utilization • How large percentage of time that is used for successful transmission of data frames. • Collision rate • The average number of collisions that occur per second.

  14. SimulationsMetrics(2/2) • Access delay • as the time the Head-of-Line data packet spends at the MAC layer before being successfully transmitted out on the wireless medium. • Cumulative delay distribution • We present the cumulative distribution of the access delays for high priority traffic to find out the percentage of the packets that are below certain delay bounds.

  15. ResultDetermining PCF superframe size(1/3) • Size for high priority traffic • Short control frame polled delay throughput • long control frame polled delay throughput • The best performance for high priority traffic would be achieved by having a superframe size similar to the interval between the frames generated by the nodes.

  16. ResultDetermining PCF superframe size(2/3)

  17. ResultDetermining PCF superframe size(3/3) Superframe of 20 time units for PCF in the comparison of the different QoS shcemes

  18. Result1.Throughput(1/2) • The objective of DFS is to provide fair differentiation. • DFS always allocates a share of the bandwidth for low priority traffic and avoids starvation.

  19. Result1.Throughput(2/2) • The difference in performance between BB, PCF and EDCF is quite small. • Both EDCF and BB starve low priority traffic rather fast, and PCF only gives a very small share of the bandwidth to low priority traffic.

  20. Result2. Medium utilization

  21. Result3. Collision Rate

  22. ResultOverhead by BB and PCF

  23. Result4. Access delay(1/2)

  24. Result4. Access delay(2/2)

  25. Result5. Cumulative delay distribution(1/2)

  26. Result5. Cumulative delay distribution(2/2)

  27. Discussion • Admission control : PCF easily implements. • Starving low priority traffic: BB and EDCF. • If Blackburst could not be used, EDCF could be a suitable alternative. • Comparing EDCF and DFS at high loads. In Fig. 5 and 8, EDCF has both higher throughput and lower average delay than DFS for high priority traffic. • Different settings used to create the different scenarios of course affect the final result.

  28. Conclusions • When using PCF, it is important to select a proper size of the superframe. • The superframe should be approximately as long as the interval between packets generated by a high priority station. • Blackburst gives the best performance with regard to throughput and access delay. • Both Blackburst and EDCF starve low priority traffic at high loads of high priority traffic, which in many cases is not desirable. In the case, DFS can do a better job.

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