Service differentiation mechanisms for IEEE 802.11-based wireless networks
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Transcript of Service differentiation mechanisms for IEEE 802.11-based wireless networks
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Service differentiation mechanisms forIEEE 802.11-based wireless networks
Srikant Kuppa , Ravi PrakashUniversity of Texas at Dallas
WCNC2004WCNC2004
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OutlineOutline
InstructionInstruction Possible MAC policies for Service
Differentiation– SAB (schedule after backoff)– SBB (schedule before backoff)
Comparative Performance Comparative Performance EvaluationEvaluation
ConclusionsConclusions
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InstructionInstruction
802.11 DCF802.11 DCF– All nodes contest for the medium for All nodes contest for the medium for
each transmitted frameeach transmitted frame– SimplicitySimplicity– Offer only best-effort serviceOffer only best-effort service– No QoS guaranteeNo QoS guarantee
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InstructionInstruction
Two MAC policies Two MAC policies SAB/SBBSAB/SBB are are proposed and intend to proposed and intend to – offer some “guarantees” for high-offer some “guarantees” for high-
priority datapriority data– Prevent starvation of low-priority Prevent starvation of low-priority
traffictraffic
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Possible MAC policies for Service Differentiation (SAB) Schedule after backoff (SAB) policy
– 802.11e Enhanced DCF(EDCF) is based on SAB policy
– a node accesses the medium depending on AC of the frame
– Access Category (AC) mechanism is used to realize service differentiation
Four ACs (AC 3 > AC 2 > AC 1 > AC 0) Different IFS for different ACs Shorter CW for high-priority frames
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Possible MAC policies for Service Differentiation (SAB)
Backoff instances for each ACs
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Possible MAC policies for Service Differentiation (SAB) Schedule after backoff (SAB) policy
– Each AC with the nodes behaves like a virtual node
– Arbitration IFS(AIFS) is used AIFS = DIFS + some time slot
– Internal collision: solve by frame-scheduler– External collision– Problem: Under high lodes , EDCF suffer
high collision and starve low-priority traffic [7]
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Possible MAC policies for Service Differentiation (SBB) Schedule before backoff (SBB) policy
– provides service differentiation without maintaining concurrent backoff instances
– 2 Steps selection of the next frame to transmit
– RR / WRR (weight Round Robin) channel access functionality for the selected
frame– VCW (Various CW )
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Possible MAC policies for Service Differentiation (SBB)
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Performance Performance EvaluationEvaluation Simulator NS-2.26Simulator NS-2.26 Single hops / StationarySingle hops / Stationary Simulation time = 110sSimulation time = 110s
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Performance Performance EvaluationEvaluation
Scenario AScenario A– Saturation trafficSaturation traffic– Traffic flows = 4 * nTraffic flows = 4 * n– Constant Bit Rate Constant Bit Rate – Frame Size = 512 Frame Size = 512
bytebyte– Date Rate = 1MbpsDate Rate = 1Mbps
Scenario BScenario B– Non-Saturation Non-Saturation
traffictraffic– Traffic flows = 2 * nTraffic flows = 2 * n
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Performance Performance EvaluationEvaluation
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Performance Performance EvaluationEvaluation
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Performance Performance EvaluationEvaluation
improve the chances of successful transmission in High contention environment– larger values of CW – more number of backoff stages
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Performance Performance EvaluationEvaluation
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Performance Performance EvaluationEvaluation
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Performance Performance EvaluationEvaluation
: the effective bandwidth used for successful transmissions and does not include bandwidth spent on collided or retransmitted frames
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ConclusionsConclusions
Service differentiation in IEEE 802.11-based wireless ad hoc networks– SAB =>EDCF : AC + AIFS– SBB => RR/WRR + VCW
Through simulation experiments :– EDCF starves low-priority frames under high
loads of high-priority traffic.– Reasonable performance guarantees shown
by schemes WRR and WRR with VCW