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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.
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Quality of Service Schemes Quality of Service Schemes for IEEE 802.11 Wireless for IEEE 802.11 Wireless
LANs-An EvaluationLANs-An Evaluation主講人主講人 : : 黃政偉黃政偉
OutlineOutline IntroductionIntroduction Overview of evaluated schemesOverview of evaluated schemes SimulationSimulation ResultResult DiscussionDiscussion ConclusionConclusion
IntroductionIntroduction Wireless networks are superior to wired networks with regard Wireless networks are superior to wired networks with regard
to aspects such as to aspects such as ease of installation and flexibilityease of installation and flexibility.. Given Given the coverage and low pricethe coverage and low price, it is likely that demands for , it is likely that demands for
the ability to run real-time applications such as voice over IP the ability to run real-time applications such as voice over IP over these networks will increase.over these networks will increase.
Inherent problems: low medium utilization, risk of collisions Inherent problems: low medium utilization, risk of collisions and problem of providing differentiation between different and problem of providing differentiation between different types of traffic.types of traffic.
Overview of evaluated schemesOverview of evaluated schemes Point Coordinator Function (PCF)Point Coordinator Function (PCF)
Overview of evaluated schemes Overview of evaluated schemes Enhanced DCF(EDCF)Enhanced DCF(EDCF)
The EDCF mechanism allows traffic to be classified iThe EDCF mechanism allows traffic to be classified into nto 88 different traffic classes, different traffic classes,
MethodMethod: the : the minimum contention window (CWminimum contention window (CWminmin)) an and d the interframe spacethe interframe space
Higher priorityHigher priority : : smaller CWsmaller CWmin min ; ; lowlower priorityer priority : : larger CWlarger CWminmin
Different traffic classesDifferent traffic classes: : Arbitration interframe space Arbitration interframe space (AIFS = DIFS + number of time slots)(AIFS = DIFS + number of time slots)
Overview of evaluated schemes Overview of evaluated schemes Enhanced DCF(EDCF)Enhanced DCF(EDCF)
Overview of evaluated schemes Overview of evaluated schemes Distributed Fair Scheduling (DFS)(1/2)Distributed Fair Scheduling (DFS)(1/2)
Fair means that each flow gets bandwidth proportional to sFair means that each flow gets bandwidth proportional to some weight that has been assigned to it.ome weight that has been assigned to it.
Before transmitting a frame, the backoff process is always Before transmitting a frame, the backoff process is always initiatedinitiated
Where sizeWhere sizepacketpacket is the size of the packet to send is the size of the packet to send
packetsize
B ctorScaling_fa
magnitude. suitable of valuestointervals backoff thescale toused isctor Scaling_Fa
1mean with variablerandom a is ,station theof weight theis
Overview of evaluated schemes Overview of evaluated schemes DistriDistributed Fair Scheduling (DFS)(2/2)buted Fair Scheduling (DFS)(2/2)
Fairness is achieved by using the size of the packet to Fairness is achieved by using the size of the packet to be sent in the calculation of the backoff interval.be sent in the calculation of the backoff interval.
If a collision occurs, a new backoff interval is calculatIf a collision occurs, a new backoff interval is calculated using the 802.11 standard. (CWed using the 802.11 standard. (CWminmin=3)=3)
Overview of evaluated schemes Overview of evaluated schemes BlackBlackburst (BB)(1/2)burst (BB)(1/2)
Three interframe spacing: Three interframe spacing: ((ττ: max propagation delay): max propagation delay) ttshort short + 2+ 2ττ< t< tmed med ; t; tmed med + 2+ 2ττ< t< tlonglong ; ; ttshort short < < ttmedmed < t < tlonglong
Black burst duration:Black burst duration: Duration = tDuration = tbslotbslot * [d / t * [d / tunitunit]; t]; tbslotbslot is a length of a black slot . is a length of a black slot .
ttunitunit : a system parameter defined shortly. : 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 : tThe scheduled access interval : tsch sch
Overview of evaluated schemes Overview of evaluated schemes BlackBlackburst (BB)(2/2)burst (BB)(2/2)
Simulation Simulation Scenarios(1/2)Scenarios(1/2)
Network simulator : ns-2Network simulator : ns-2 WLAN bandwidth : 2Mbit/sWLAN bandwidth : 2Mbit/s In infrastructure mode, the mobile nodes always communicate In infrastructure mode, the mobile nodes always communicate
directly with the AP.directly with the AP. There is no mobility in the systemThere is no mobility in the system
Packet size Packet size (m, (m, σσ22))
Inter-packet Inter-packet intervalinterval
Bit rateBit rate
High priorityHigh priority 300bytes300bytes40 bytes40 bytes
25-40 ms25-40 ms 60-96 kbit/s60-96 kbit/s
Low priorityLow priority 800 bytes800 bytes150 bytes150 bytes
50ms50ms 128 kbit/s128 kbit/s
Simulation Simulation Scenarios(2/2)Scenarios(2/2)
table1table1
TU = 10TU = 102424μμss
SimulationsSimulationsMetrics(1/2)Metrics(1/2)
Throughput (Normalization)Throughput (Normalization) Percentage of the offered data that is actually delivered to Percentage of the offered data that is actually delivered to
the destination.the destination. Medium utilizationMedium utilization
How large percentage of time that is used for successful How large percentage of time that is used for successful transmission of data framestransmission of data frames..
Collision rateCollision rate The average number of collisions that occur per second.The average number of collisions that occur per second.
SimulationsSimulationsMetrics(2/2)Metrics(2/2)
Access delayAccess delay as the time the Head-of-Line data packet spends at the as the time the Head-of-Line data packet spends at the
MAC layer before being successfully transmitted out on MAC layer before being successfully transmitted out on the wireless medium.the wireless medium.
Cumulative delay distributionCumulative delay distribution We present the cumulative distribution of the access delays We present the cumulative distribution of the access delays
for high priority traffic to find out the percentage of the for high priority traffic to find out the percentage of the packets that are below certain delay bounds.packets that are below certain delay bounds.
ResultResultDetermining PCF superframe size(1/3)Determining PCF superframe size(1/3)
Size for Size for high priority traffichigh priority traffic Short control frame polled delay Short control frame polled delay
throughputthroughput long control frame polled delay long control frame polled delay
throughputthroughput The best performance for high priority traffic would be achThe best performance for high priority traffic would be ach
ieved by having a superframe size similar to ieved by having a superframe size similar to the interval bethe interval between the frames generated by the nodes.tween the frames generated by the nodes.
ResultResultDetermining PCF superframe size(2/3)Determining PCF superframe size(2/3)
ResultResultDetermining PCF superframe size(3/3)Determining PCF superframe size(3/3)
Superframe of 20 time units for PCF in the comparison of the different QoS shcemes
ResultResult1.Throughput(1/2)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.
ResultResult1.Throughput(2/2)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.
ResultResult2. Medium utilization2. Medium utilization
ResultResult3. Collision Rate3. Collision Rate
ResultResultOverhead by BB and PCFOverhead by BB and PCF
ResultResult4. Access delay(1/2)4. Access delay(1/2)
ResultResult4. Access delay(2/2)4. Access delay(2/2)
ResultResult5. Cumulative delay distribution(1/2)5. Cumulative delay distribution(1/2)
ResultResult5. Cumulative delay distribution(2/2)5. Cumulative delay distribution(2/2)
DiscussionDiscussion 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 suitIf Blackburst could not be used, EDCF could be a suit
able alternative. able alternative. Comparing EDCF and DFS at high loads. In Fig. 5 anComparing EDCF and DFS at high loads. In Fig. 5 an
d 8, EDCF has both higher throughput and lower averd 8, EDCF has both higher throughput and lower average delay than DFS for high priority traffic.age delay than DFS for high priority traffic.
Different settings used to create the different scenarioDifferent settings used to create the different scenarios of course affect the final result.s of course affect the final result.
ConclusionsConclusions When using PCF, it is important to select a proper size of the sWhen using PCF, it is important to select a proper size of the s
uperframe. uperframe. The superframe should be approximately as long as the intervaThe superframe should be approximately as long as the interva
l between packets generated by a high priority station.l between packets generated by a high priority station. Blackburst gives the best performance with regard to throughpBlackburst gives the best performance with regard to throughp
ut and access delay.ut and access delay. Both Blackburst and EDCF starve low priority traffic at high lBoth Blackburst and EDCF starve low priority traffic at high l
oads of high priority traffic, which in many cases is not desiraoads of high priority traffic, which in many cases is not desirable. In the case, DFS can do a better job.ble. In the case, DFS can do a better job.