Service differentiation mechanisms forIEEE 802.11-based wireless networks

Srikant Kuppa , Ravi PrakashUniversity of Texas at Dallas

WCNC2004WCNC2004

OutlineOutline

InstructionInstruction Possible MAC policies for Service

Differentiation– SAB (schedule after backoff)– SBB (schedule before backoff)

Comparative Performance Comparative Performance EvaluationEvaluation

ConclusionsConclusions

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

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

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

Possible MAC policies for Service Differentiation (SAB)

Backoff instances for each ACs

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]

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 )

Possible MAC policies for Service Differentiation (SBB)

Performance Performance EvaluationEvaluation Simulator NS-2.26Simulator NS-2.26 Single hops / StationarySingle hops / Stationary Simulation time = 110sSimulation time = 110s

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

Performance Performance EvaluationEvaluation

Performance Performance EvaluationEvaluation

Performance Performance EvaluationEvaluation

improve the chances of successful transmission in High contention environment– larger values of CW – more number of backoff stages

Performance Performance EvaluationEvaluation

Performance Performance EvaluationEvaluation

Performance Performance EvaluationEvaluation

： the effective bandwidth used for successful transmissions and does not include bandwidth spent on collided or retransmitted frames

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