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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 主講人: 黃政偉
Outline • Introduction • Overview of evaluated schemes • Simulation • Result • Discussion • Conclusion
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.
Overview of evaluated schemesPoint Coordinator Function (PCF)
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)
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
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)
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
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
TU = 1024μs Simulation Scenarios(2/2) • table1
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.
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.
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.
ResultDetermining PCF superframe size(3/3) Superframe of 20 time units for PCF in the comparison of the different QoS shcemes
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.
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.
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.
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.
