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Service differentiation mechanisms for IEEE 802.11-based wireless networks. Srikant Kuppa , Ravi Prakash University of Texas at Dallas WCNC2004. Outline. Instruction Possible MAC policies for Service Differentiation SAB (schedule after backoff) SBB (schedule before backoff)
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Service differentiation mechanisms forIEEE 802.11-based wireless networks Srikant Kuppa , Ravi Prakash University of Texas at Dallas WCNC2004
Outline • Instruction • Possible MAC policies for Service Differentiation • SAB (schedule after backoff) • SBB (schedule before backoff) • Comparative Performance Evaluation • Conclusions
Instruction • 802.11 DCF • All nodes contest for the medium for each transmitted frame • Simplicity • Offer only best-effort service • No QoS guarantee
Instruction • Two MAC policies SAB/SBB are proposed and intend to • offer some “guarantees” for high-priority data • Prevent starvation of low-priority traffic
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 CWfor high-priority frames
Backoff instances for each ACs Possible MAC policies for Service Differentiation (SAB)
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 )
Performance Evaluation • Simulator NS-2.26 • Single hops / Stationary • Simulation time = 110s
Performance Evaluation • Scenario A • Saturation traffic • Traffic flows = 4 * n • Constant Bit Rate • Frame Size = 512 byte • Date Rate = 1Mbps • Scenario B • Non-Saturation traffic • Traffic flows = 2 * n
Performance Evaluation • improve the chances of successful transmission in High contention environment • larger values of CW • more number of backoff stages
Performance Evaluation :the effective bandwidth used for successful transmissions and does not include bandwidth spent on collided or retransmitted frames
Conclusions • 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