A Novel APSD Scheduler for WLAN

IEEE 802.11e

COMMUNICATION SYSTEMS, NETWORKS AND DIGITAL SIGNAL PROCESSING

University of LecceUniversity of Lecce

Speaker: Sebastiano Elia

CSNDSP 2006 Fifth International Symposium

CSNDSP 2006 2/23

Introduction

Wireless LAN

Broadband

Application Quality of

Service

Mobility

CSNDSP 2006 3/23

Power Management

Device with limited battery

capacityLimited time in

action

Need of Power Saving mechanisms in order to increase

the life time of batteries

CSNDSP 2006 4/23

Goal

Defining of Power Saving schema aiming to reduce the power consumption of a mobile IEEE 802.11e device while guaranteeing QoS requirements for delay-sensitive applications like multimedia streaming.

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IEEE 802.11e APSD

CSNDSP 2006 6/23

IEEE 802.11e APSD

IEEE 802.11e defines a new method to deliver the frames buffered at the AP while the station is in PS Mode, the Automatic Power Save Delivery (APSD)

With APSD the station in PS mode remains in awake state for the duration of a Service Period.

CSNDSP 2006 7/23

IEEE 802.11e APSD

Unscheduled APSD

Scheduled APSD

The stations sends a trigger

to the AP in order to begin

a Service Period

The Service Periods are

repeated every Service Interval

CSNDSP 2006 8/23

IEEE 802.11e APSDAPSD scheduler

The channel time is slotted into APSD channels repeating at fixed time intervals equal to the Basic Service Interval. An APSD channel may be assigned to a single APSD station with service interval equal to the Basic Service Interval, or can be shared by two or more PS stations with TSs having longer SI.

CSNDSP 2006 9/23

IEEE 802.11e APSDAPSD scheduler

Fixed Service Interval

Short life time of battery in presence of Variable Bit

Rate traffic

The transition between sleep to

awake wastes energy

CSNDSP 2006 10/23

Our Proposal

Defining of an Enhanced APSD Scheduler

CSNDSP 2006 11/23

•A novel APSD Scheduler has been designed for WLAN IEEE 802.11e, referred as Enhanced APSD (E-APSD) scheduler.

•The E-APSD scheduler is able to exploit information related to PS-Buffer statistics in order to optimize (growing up or down) the duration of sleep periods for the stations in APSD mode.

Proposal Enhanced APSD Scheduler

Thmax

CSNDSP 2006 12/23

• According to the value of PS-Buffer occupancy (n), at the end of every Service Period, the E-APSD behaves as follow:

• If n < Thmax, then the AP communicates to the APSD Station the number of SI that the Station must jump, by using the QAP PS Buffer State subfield of the QoS Control field included into the header of the QoS Null frame.

• If n ≥ Thmax, then the E-APSD scheduler tries to assign additional free slots to the admitted APSD reducing the time that the APSD Station spends in sleep state.

Proposal Enhanced APSD Scheduler

CSNDSP 2006 13/23

Enhanced APSD Scheduler

N min N ',MaximumServiceIntervalSI j

,DelayBound2SI j

n j N ' j SI j

NominalMSDUSize

R j

N 'R j n j NominalMSDUSize j SI j

0

nj Rj

Rj nj Thmax

Buffered MSDUMean Data Rate

Maximum number of MSDU sent during a SP

CSNDSP 2006 14/23

Enhanced APSD Scheduler

• After the end of the first SP the number of buffered packets is less than Thmax.

• The AP calculates N (equal to two SI in this example) and communicates how many SIs the PS-Station must jump.

Time

Service Interval

1 1 1 1 1

CSNDSP 2006 15/23

• A slot is considered free when another Station will jump it or when it has not yet been assigned to any APSD stream. The presence of free APSD channels is always known to AP.

• At the end of a Service Period, if the number of MSDU buffered at the AP is higher than Thmax, the AP will assign to the STA the first free slot among next m, where m is the number of APSD channel before the next slot regurarly assigned to the STA after a SI.

Enhanced APSD Scheduler

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Time1 3 5 7 22 4 6 1 3

Station A: SI 10 ms Station B: SI 10 ms Station C: SI 10 ms

Occupied Slots

• STA-A jumps one Service Period.

• STA-B has more than Thmax packets stored into the PS-Buffer, so the AP tries to assign to STA-B one extra-slot.

• STA-B gains the slot released by STA-A, in addition to that one assigned after a SI.

• STA-C wakes up.

Enhanced APSD Scheduler

CSNDSP 2006 17/23

Enhanced APSD Scheduler

• Thmax represents the minimum amount of data in a PS-Buffer that allows a Station to use an additional slot, reducing the sleep time.

nj 2Rj j SI j

Thmax 2Rj j SI j

CSNDSP 2006 18/23

Simulation Model

Streaming Server

Access Point

Client wireless

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Simulation Hypothesis

Hypotesis

• Audio/Video Streaming Traffic• 13 Stations in the WLAN• IEEE 802.11a phisyc layer• SNR: from 22 dB to 26 dB• No mobility and no congestion

All simulation results, obtained using the Network Simulator v2 (ns2) tool, are characterized by a 95% confidence interval whose maximum relative error is equal to 5%.

A comparison between the proposed E-APSD scheduler and that presented in Appendix H of Draft 10 of IEEE 802.11e specifications is been performed.

CSNDSP 2006 20/23

CBR VBR 1 VBR 2

Simulation HypothesisTraffic Generators

Packet size 512 byte

Data rate 128 Kbps

Service Interval 30 ms

Delay Bound 200 ms

Thmax 14

R 7

N° of streams 9

Packet size 512 byte

Minimum rate 320 Kbps

Maximum rate 1.5 Mbps

Rate time 4 sec

Burst time 1 sec

Time deviation 0.5

Rate deviation 0.25

Number of changes 10

Service Interval 30 ms

Delay Bound 200 ms

Thmax 14

R 7

N° of streams 2

Packet size 512 byte

Minimum rate 1 Mbps

Maximum rate 6 Mbps

Rate time 1.5 sec

Burst time 1 sec

Time deviation 0.5

Rate deviation 0.25

Number of changes 10

Service Interval 10 ms

Delay Bound 200 ms

Thmax 12

R 7

N° of streams 2

CSNDSP 2006 21/23

Simulations ResultsEnergy Bit Ratio

22 22.5 23 23.5 24 24.5 25 25.5 260

1

2

3

4

5

6

7

8 x 10-7

SNR (dB)

Enhanced Scheduler - CBRStandard Scheduler - CBREnhanced Scheduler - VBR 1Standard Scheduler -VBR 1Enhanced Scheduler -VBR 2Standard Scheduler -VBR 2

EB

R (J

/bit)

22 22.5 23 23.5 24 24.5 25 25.5 260

1

2

3

4

5

6

7

8 x 10-7

SNR (dB)

Enhanced Scheduler - CBRStandard Scheduler - CBREnhanced Scheduler - VBR 1Standard Scheduler -VBR 1Enhanced Scheduler -VBR 2Standard Scheduler -VBR 2

EB

R (J

/bit) up to

50% of energy less …

CSNDSP 2006 22/23

• The simulation results have shown that adapting the duration of sleep period according to the current traffic load together with the absence of triggers are very effective in terms of power saving.

• Moreover, the proposed scheduler is very simple and should be very easily implemented in a IEEE 802.11e compatible Access Point or network adapter.

ConclusionsConclusions

CSNDSP 2006 23/23

Thank for your attention1Giovanni Ciccarese, 2Gabriella Convertino, 1Mario De Blasi,

1Sebastiano Elia, 1Cosimo Palazzo, 1Luigi Patrono1 Dep. of Innovation Engineering, University of Lecce, Via

Monteroni, 73100 Lecce, Italye-mail: {cosimo.palazzo, mario.deblasi, gianni.ciccarese,

luigi.patrono, sebastiano.elia} @unile.it2 STMicroelecronics, Lecce, Italy

gabriella.convertino@st.com

CSNDSP 2006 24/23

Simulations Results

22 22.5 23 23.5 24 24.5 25 25.5 260

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

SNR (dB)

Enhanced Scheduler - CBRStandard Scheduler - CBREnhanced Scheduler - VBR 1Standard Scheduler - VBR 1Enhanced Scheduler - VBR 2Standard Scheduler - VBR 2

Mea

nP

acke

tDel

ay(s

ec)

22 22.5 23 23.5 24 24.5 25 25.5 260

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

SNR (dB)

Enhanced Scheduler - CBRStandard Scheduler - CBREnhanced Scheduler - VBR 1Standard Scheduler - VBR 1Enhanced Scheduler - VBR 2Standard Scheduler - VBR 2

Mea

nP

acke

tDel

ay(s

ec)

Mean Packet Delay