Balancing Uplink and Downlink Delay of VoIP Traffic in 802.11 WLANs

Balancing Uplink and Downlink Delay Balancing Uplink and Downlink Delay of VoIP Traffic in 802.11 WLANsof VoIP Traffic in 802.11 WLANs

Sangho ShinHenning Schulzrinne

Columbia UniversityDepartment of Computer Science

May 2 2006

MotivationMotivation

VoIP in IEEE 802.11 WLANsDistribution Network (DS)

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Access Point (AP)

STAs

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VoIP in IEEE 802.11 WLANs

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Access Point (AP)

STAs

Distribution Network (DS)

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VoIP in IEEE 802.11 WLANs

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Access Point (AP)

STAs


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0

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26 27 28 29 30 31 32 33 34

Number of VoIP sources

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Uplink (90th%tile)Downlink (90th%tile)Uplink (AVG)Downlink (AVG)

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Downlink

Uplink

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When the channel is congested ….

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Queue Queue Queue Queue

Queue


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Queue


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Queue


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Queue


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Queue


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Solutions?Give a higher priority to the AP than STAs

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Queue


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OutlineOutline

Background Adaptive Priority Control (APC) Simulation results Implementation Issues Conclusions Future work

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BackgroundBackground

DIFS

Busy Medium

DIFS

Frame

Defer Access

Contention Window

Backoff

Slot time

Number of Backoff = (0, CWmin)

Medium Access in DCF (Distributed Coordination Function)

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BackgroundBackground

How to control the priority of the AP? Inter-Frame Spacing (IFS) Contention Window (CW) Contention Free Transmission

Busy Medium

DIFS DIFS

Frame

Contention Window

Backoff

Defer Access Slot time

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Transmission ControlTransmission Control

Control Contention Window (CW)

Frame3DIFS Backoff Frame1 DIFS Backoff Frame2 DIFS Backoff

Number of Backoff = (0, CWmin)

Frame3DIFS BO Frame1 DIFS Frame2 DIFSBO BO

Number of Backoff = (0, CWmin/2)

DIFS

DIFS BO

STA

AP Frame

BO1 DIFSDefer Access BO2

DIFS BO Frame

Defer AccessDIFS BO

DIFS BO

BO BO

Frame

Defer Access

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Remaining BO time

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Control Contention Window (CW) It is hard to control the priority accurately.

Backoff time is randomly decided between (0, CWmin).

Shorter CW Higher collision probability Decrease the capacity.

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Control IFS

Frame3DIFS Backoff Frame1 DIFS Backoff Frame2 DIFS Backoff

IFS = DIFS

STA2

AP

Frame

DIFSDefer Access BO

BO Frame

Defer Access Frame

IFS

STA1

Defer Access

DIFS

IFS = DIFS/2

Frame3IFS Backoff Frame1 Backoff Frame2 BackoffIFSIFS

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Contention Free Transmission (CFB)

Control the number of frames (P)

Frame3IFS Frame1 IFS Frame2 IFS

Frame3IFS Frame1 IFS Frame2 IFS

P = 3DIFS BO Defer Access

IFS Frame1 IFS Frame2 DIFS BO Defer Access

P = 2

IFS Frame3

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Contention Free Transmission (CFB) Precise priority control

P Priority Transmitting three frames contention free thr

ee times higher priority than other STAs. No overhead

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Adaptive Priority ControlAdaptive Priority Control

Optimal priority of the AP to balance the uplink and downlink delay ? Number of active wireless STAs

Semi-Adaptive Method

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Semi-AdaptiveSemi-Adaptive10 packets

P = 4

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P = 4

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Optimal priority of the AP to balance the uplink and downlink delay ? Number of active wireless STAs

Semi-Adaptive Method Simple Adaptive to the change of the number of active

wireless STAs. Not adaptive to the change of the traffic volume

of uplink and downlink.

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P = 4

2 packets

P = 4

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Adaptive Priority ControlAdaptive Priority Control Optimal priority of the AP to balance the

uplink and downlink delay ? Uplink/Downlink delay ≈Queuing delay

The same packet processing time in Queue of the AP and STAs The same queuing delay

Queuing dely

Access delayPropagation delay

Network

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Adaptive Priority Control Adaptive Priority Control

Optimal Priority = QAP/QSTA (QSTA>0)

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APCAPC10 packets

P = 10 / 2 = 5

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APCAPC10 packets

P = 10 / 2 = 5

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APCAPC

P = 5/1 = 5

5 packets

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Optimal Priority = QAP/QSTA

Simple Adaptive to change of number of active STA

s Adaptive to change of uplink/downlink traffic

volume Results in the same packet processing time

between the AP and STAs same queuing delay in the AP and STAs same uplink and downlink delay

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20 ms packetization interval (64kb/s)

Downlink

Uplink

Simulation ResultsSimulation Results- - DCFDCF

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Simulation ResultsSimulation Results

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Semi-Adaptive MethodSemi-Adaptive Method

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Simulation Results - Simulation Results - APCAPC

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Simulation Results Simulation Results APC vs Semi-AdaptiveAPC vs Semi-Adaptive


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Semi-Adaptive APC

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Simulation Results Simulation Results DCF vs APCDCF vs APC

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En

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Uplink (90th%tile)

Downlink (90th%tile)

Uplink (AVG)

Downlink (AVG)

Capacity = 28 Capacity = 35

DCF APC

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30%

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Simulation Results Simulation Results Semi-Adaptive MethodSemi-Adaptive Method

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Simulation Results Simulation Results APC vs Semi-AdaptiveAPC vs Semi-Adaptive MethodMethod

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Uplink (90th%tile)


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Downlink (AVG)

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Uplink (90th%tile)


Uplink (AVG)

Downlink (AVG)

Semi-Adaptive APC

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Simulation Results -Simulation Results - APC APC

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Implementation IssuesImplementation Issues How can the AP know the queue size of

nodes? Add the queue size of each node to VoIP packets.

It requires the changes in clients Estimate the average queue size of nodes

Queue size = Number of packets generated at APP layer – Number of packets transmitted

Number of packets transmitted ≈ Number of packets received

Number of packets generated at APP layer ≈ Number of active nodes x packetization interval

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Implementation IssuesImplementation Issues APC with estimated Queue sizeAPC with estimated Queue size

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Implementation IssuesImplementation Issues

How to implement Contention Free Transmission? IEEE 802.11e : Contention Free Bursty (CF

B) Change the CFB duration.

Wireless Media Extension (WME) A subset of IEEE 802.11e Implemented in many chipsets Allows change of IFS, CFB duration

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ConclusionsConclusions

Uplink and downlink delay VoIP traffic in DCF are significantly unbalanced.

APC, in which AP transmits QAP/QNodes packets, balances the uplink and downlink delay.

APC improves the capacity for VoIP traffic from 28 calls to 35 calls, by 25%.

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Future WorkFuture Work

Integrate APC to IEEE 802.11e. Measure the performance of APC with

background traffic. Measure the performance of APC with

actual wireless nodes in ORBIT test-bed.

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Thank you

Questions?

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VoIP in IEEE 802.11 WLANsDistributed Network (DS)

May 2 2006

SimulationsSimulations VoIP traffic model

ITU-T P59

Parameter Duration (s)

Rate (%)

Talk-spurt 1.004 38.53

Pause 1.587 61.47

Double-Talk 0.228 6.59

Mutual Silence

0.508 22.48

0.51.0 0.23 0.30.9

1.5

Duration (s)

Rate (%)

0.9 37.5

1.5 62.5

0 0

0.3 25.0

Our Model

Documents

Balancing Uplink and Downlink Delay of VoIP Traffic in 802.11 WLANs