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

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

3. Introduction Wireless networks are superior to wired networks with regard to aspects such as ease of installation and flexibility. 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. 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. Inherent problems: low medium utilization, risk of collisions and problem of providing differentiation between different types of traffic.

4. Overview of evaluated schemes Point Coordinator Function (PCF)

5. Overview of evaluated schemes Enhanced DCF(EDCF) The EDCF mechanism allows traffic to be classified into 8 different traffic classes, The EDCF mechanism allows traffic to be classified into 8 different traffic classes, Method: the minimum contention window (CW min ) and the interframe space Method: the minimum contention window (CW min ) and the interframe space Higher priority : smaller CW min ; lower priority : larger CW min Higher priority : smaller CW min ; lower priority : larger CW min Different traffic classes: Arbitration interframe space (AIFS = DIFS + number of time slots) 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. 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 Before transmitting a frame, the backoff process is always initiated Where size packet is the size of the packet to send Where size packet 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. 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. (CW min =3) If a collision occurs, a new backoff interval is calculated using the 802.11 standard. (CW min =3)

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

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

11. Simulation Scenarios(1/2) Network simulator : ns-2 Network simulator : ns-2 WLAN bandwidth : 2Mbit/s WLAN bandwidth : 2Mbit/s In infrastructure mode, the mobile nodes always communicate directly with the AP. In infrastructure mode, the mobile nodes always communicate directly with the AP. There is no mobility in the system There is no mobility in the system Packet size (m, σ 2 ) Inter-packet interval Bit rate High priority 300bytes 40 bytes 25-40 ms 60-96 kbit/s Low priority 800 bytes 150 bytes 50ms 128 kbit/s

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

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

14. Simulations Metrics(2/2) Access delay 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. 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 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. 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. Result Determining PCF superframe size(1/3) Size for high priority traffic Size for high priority traffic Short control frame polled delay Short control frame polled delay throughput throughput long control frame polled delay long control frame polled delay throughput 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. 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. Result Determining PCF superframe size(2/3)

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

18. Result 1.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. Result 1.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. Result 2. Medium utilization

21. Result 3. Collision Rate

22. Result Overhead by BB and PCF

23. Result 4. Access delay(1/2)

24. Result 4. Access delay(2/2)

25. Result 5. Cumulative delay distribution(1/2)

26. Result 5. Cumulative delay distribution(2/2)

27. Discussion Admission control : PCF easily implements. Admission control : PCF easily implements. Starving low priority traffic: BB and EDCF. Starving low priority traffic: BB and EDCF. If Blackburst could not be used, EDCF could be a suitable alternative. 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. 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. 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. 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. 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. 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. 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.