11

A New ACK Policy for A New ACK Policy for Coexisting IEEE Coexisting IEEE

802.11/802.11e Devices802.11/802.11e Devices

Haithem A Al-Mefleh, J. Morris ChangHaithem A Al-Mefleh, J. Morris ChangElectrical and Computer Eng. Dept.Electrical and Computer Eng. Dept.

Iowa State UniversityIowa State University

U. S. A.U. S. A.

March 2008March 2008

22

OutlineOutline

IntroductionIntroduction IEEE 802.11IEEE 802.11

Problem StatementProblem Statement

Related WorkRelated Work

Proposed Solution - NZ-ACKProposed Solution - NZ-ACK

EvaluationEvaluation

ConclusionsConclusions

33

IntroductionIntroduction

IEEE 802.11 Std for Wireless LANSIEEE 802.11 Std for Wireless LANS 802.11b,g,a802.11b,g,a

DCFDCF

PCFPCF 802.11e 802.11e

backward compatiblebackward compatible

QoSQoS

EDCAEDCA

HCCAHCCA

44

IntroductionIntroductionDCF:DCF: DIFSDIFS CWCWminmin, ,

CWCWmaxmax

Busy Medium Backoff SlotsSIFS

PIFS

DIFS

AIFS[i]

Slot time

Next Frame

AIFS[j]

Defer Access

DIFS/AIFS

Select Slot and Decrement Backoff as long as medium is idle

Immediate access when Medium is free >= DIFS/AIFS[i]

Contention Window

AIFS[0]

CWmax[0]

AC0

CWmin[0]

AIFS[1]

CWmax[1]

AC1

CWmin[1]

AIFS[2]

CWmax[2]

AC2

CWmin[2]

AIFS[3]

CWmax[3]

AC3

CWmin[3]

(MSDU, UP)

TXOPLimit[0] TXOPLimit[1] TXOPLimit[2] TXOPLimit[3]

EDCA:EDCA: AIFS[AC]AIFS[AC] CWCWminmin[AC], [AC],

CWCWmaxmax[AC][AC] TXOP[AC]TXOP[AC]

Both:Both: Duration of Duration of

frames used for frames used for NAVNAV

55

Problem StatementProblem Statement

Coexisting EDCA and legacy DCF usersCoexisting EDCA and legacy DCF users EDCA contention parameters are MAC-LevelEDCA contention parameters are MAC-Level DCF contention parameters are PHY-LevelDCF contention parameters are PHY-Level

No Control over legacy usersNo Control over legacy users

66

Problem StatementProblem Statement

Smallest AIFS is equivalent to DIFSSmallest AIFS is equivalent to DIFS

DCF has no TXOPDCF has no TXOP

Smaller CW for EDCA Smaller CW for EDCA Higher Collisions Higher Collisions

Higher CW for EDCA Higher CW for EDCA Lower Priority Lower Priority

EDCA users may get lower priority andEDCA users may get lower priority and

performanceperformance

77

Problem StatementProblem Statement

Simple example:Simple example: A WLAN of EDCA and DCF usersA WLAN of EDCA and DCF users EDCA users: VoIP, CWEDCA users: VoIP, CWminmin 8, AIFS=50 8, AIFS=50uuss

DCF users: CWDCF users: CWminmin 32, DIFS=50 32, DIFS=50uuss An increase in number of EDCA users An increase in number of EDCA users The QAP The QAP

broadcasts new CWbroadcasts new CWmin min 3232

AIFS cannot be smaller, DCF users keep their CWAIFS cannot be smaller, DCF users keep their CWminmin

DCF and EDCA users are now having same priority, DCF and EDCA users are now having same priority, and therefore QoS of EDCA could be affectedand therefore QoS of EDCA could be affected

88

Problem StatementProblem Statement

There is a need for mechanisms to: There is a need for mechanisms to:

1.1. Mitigate the impact of legacy DCFMitigate the impact of legacy DCF

2.2. Maintain priority of service for EDCA Maintain priority of service for EDCA users - QoSusers - QoS

99

Related WorkRelated WorkSwaminathan and Martin 2006

Simulation Analysis on coexisting 802.11e/802.11bSimulation Analysis on coexisting 802.11e/802.11b Conclusions: Conclusions:

AIFS is best for delay, but would result in throughput starvation for DCFAIFS is best for delay, but would result in throughput starvation for DCF

to achieve fairness, both CWto achieve fairness, both CWminmin and AIFS should be adapted and AIFS should be adapted

G. Bianchi, I. Tinnirello, and L. Scalia 2005 Conclusions in addition to those aboveConclusions in addition to those above

the increase of collisions due to small CW reduces the difference the increase of collisions due to small CW reduces the difference between EDCA and DCFbetween EDCA and DCF

J. Majkowski and F. C. Palacio 2006 Suggests a scheme to improve DCF performance when they have Suggests a scheme to improve DCF performance when they have

multimedia trafficmultimedia traffic HTB (Hierarchal Token Bucket) discipline between IP layer and Layer HTB (Hierarchal Token Bucket) discipline between IP layer and Layer

2 at legacy DCF users to classify, police, and schedule and shape 2 at legacy DCF users to classify, police, and schedule and shape incoming traffic.incoming traffic.

Requires modifications to DCF usersRequires modifications to DCF users Does not show how to solve coexistence effectsDoes not show how to solve coexistence effects

1010

Related WorkRelated WorkACKS 2005ACKS 2005

A solution in which the QAP skips sending an ACK to a DCF user with A solution in which the QAP skips sending an ACK to a DCF user with probability Sprobability S

Waste of time needed for transmitting successful packet and its ACK, Waste of time needed for transmitting successful packet and its ACK, which is still reserved because of the use of NAV by all otherswhich is still reserved because of the use of NAV by all others

Not good for wireless medium which is noisyNot good for wireless medium which is noisy Proposed for a saturated network by fixing AIFS to DIFS for all ACs, Proposed for a saturated network by fixing AIFS to DIFS for all ACs,

CWCWmaxmax=CW=CWminmin, and adapting CW, and adapting CWminmin to achieve weighted throughput to achieve weighted throughput

ratiosratios

A. Banchs, A. Azcorra, C. Garcia, and R. Cuevas 2005 2005 even if weighted throughputs are met, EDCA users may be affectedeven if weighted throughputs are met, EDCA users may be affected DCF users do not deploy TXOP limitsDCF users do not deploy TXOP limits

1111

Related WorkRelated Work

J. Majkowski and F. C. Palacio 2006 2006 A mechanism to prevent DCF users from starting a data transmission if A mechanism to prevent DCF users from starting a data transmission if

such transmission would overlap with TBTT (Target Beacon such transmission would overlap with TBTT (Target Beacon Transmission Time).Transmission Time).

Requires modification to DCF usersRequires modification to DCF usersQAP broadcasts a parameter which is used by DCF users to determine QAP broadcasts a parameter which is used by DCF users to determine when not to transmitwhen not to transmit

Divides beacon interval intoDivides beacon interval intoFirst period: all are contending. During this period, no solution to coexistence First period: all are contending. During this period, no solution to coexistence effects. Contention of DCF is accumulatedeffects. Contention of DCF is accumulated

Second period: only EDCA are contending. What if no EDCA traffic is Second period: only EDCA are contending. What if no EDCA traffic is available?available?

1212

NZ-ACKNZ-ACKDCF/EDCA: duration of last ACK is 0DCF/EDCA: duration of last ACK is 0New ACK Policy – NonZero-ACKNew ACK Policy – NonZero-ACK

Only last ACK of ongoing transmissionOnly last ACK of ongoing transmission

Fragment 0A

SIFSDefer

Fragment 1

SIFS

ACK 1

SIFS

Others: EDCA users with or without NZ-ACK

Others: Legacy users with NZ-ACK

NAV (Fragment 1)

NAV (Fragment 0)NAV (ACK 0)

NAV (ACK 1)

NAV (Fragment 0)

NAV (Fragment 1)NAV (ACK 0)

ACK 0B

Defer

Defer

Backoff

Backoff

Backoff

Others: Legacy users without NZ-ACK

1313

NZ-ACKNZ-ACKImplementationImplementation

Backward CompatibilityBackward Compatibility

QAP issues NZ-ACK framesQAP issues NZ-ACK frames

EDCA users are only required to distinguish NZ-EDCA users are only required to distinguish NZ-ACK from ACK framesACK from ACK frames

Transparent to DCF usersTransparent to DCF users

Frame Control Field ofFrame Control Field of

control frames (RTS, CTS, ACK)control frames (RTS, CTS, ACK) All bits B8 – B15All bits B8 – B15

except B12 are always 0except B12 are always 0

B0

Bits: 2

Protocol Version

Type SubtypeFrom DS

To DS

More Frag

RetryPwr Mgt

More Data

WEP Order

2 4 1 1 1 1 1 1 1 1

B1B2 B3 B4 B7 B8 B9 B10 B11 B12 B13 B14 B15

1414

NZ-ACKNZ-ACK

Data 0A

SIFSDefer

Data 1

SIFS

ACK 1

SIFS

NAV (Data 2)

NAV (ACK 1)

NAV (Data 0)

NAV (Data 1)NAV (ACK 0)

ACK 0B

Backoff Data 2SIFS

ACK 2

SIFS

ImplementationImplementation

Last ACK:Last ACK: recognize time to recognize time to

send last ACKsend last ACK EDCA: duration is not EDCA: duration is not

enough for more framesenough for more frames DCF: More Fragment bit DCF: More Fragment bit

(0; last one) of Frame (0; last one) of Frame Control Field of data Control Field of data

frame fragmentframe fragment

1515

NZ-ACKNZ-ACK

Main challenges:Main challenges: WhenWhen to issue NZ-ACK frames to issue NZ-ACK frames What value What value should be used for the duration of should be used for the duration of

an NZ-ACKan NZ-ACK

1616

NZ-ACKNZ-ACKVirtual Queues (VQ) at QAPVirtual Queues (VQ) at QAPEDCA flow i:EDCA flow i:

Peak data ratePeak data rate Average data rateAverage data rate nominal data size (lnominal data size (lii))

rrii = average data rate for VBR = average data rate for VBRFlow Utilization: uFlow Utilization: uii = r = riiTTss / l / liiTTs s = AIFS+SIFS+T= AIFS+SIFS+Tdatadata+T+TACKACK

QAP estimates active EDCA usersQAP estimates active EDCA users

r 1

Packet Arrival with Piggybacked Information

r 2 r n

Dropping

1717

Virtual Queue Management Virtual Queue Management Add one Virtual Packet to the VQ whenAdd one Virtual Packet to the VQ when

The packet arrived QAP (from QSTA) with The packet arrived QAP (from QSTA) with piggybacked information to indicate more data in piggybacked information to indicate more data in QSTA QSTA andand the VQ is empty the VQ is empty

Every rEvery rii

Drop one Virtual Packet when:Drop one Virtual Packet when: Received one packet from QSTAReceived one packet from QSTA a maximum delay is met – 100ms for voicea maximum delay is met – 100ms for voice When used to issue an NZ-ACK frameWhen used to issue an NZ-ACK frame

Empty the VQ when Empty the VQ when piggybacked info indicates no more datapiggybacked info indicates no more data

Queues ordered with smallest uQueues ordered with smallest u i i firstfirst

QAP estimates active EDCA usersQAP estimates active EDCA users

r 1

Packet Arrival with Piggybacked Information

r 2 r n

Dropping

1818

NZ-ACKNZ-ACK

Issue NZ-ACK frames: Issue NZ-ACK frames: When there are non-empty virtual queues, and, When there are non-empty virtual queues, and, When UWhen UDCF_Measured DCF_Measured >= U>= UDCFDCF , and, , and, With probability With probability pp

Duration of NZ-ACK:Duration of NZ-ACK: ddc c = u= uccTT

uucc = utilization of first virtual frame found, which is also the smallest = utilization of first virtual frame found, which is also the smallest available. available.

The frame used to calculate dThe frame used to calculate dcc is dropped. is dropped.

TT is a design parameter – beacon interval(s) is a design parameter – beacon interval(s)

1919

NZ-ACKNZ-ACK

Features:Features: No change to legacy usersNo change to legacy users

Backward CompatibilityBackward Compatibility Adaptively provides control over legacy Adaptively provides control over legacy

usersusers Minimal overheadMinimal overhead

No change to IEEE 802.11 frames formatsNo change to IEEE 802.11 frames formats All processing is at the QAPAll processing is at the QAP

Works with contention-based operationsWorks with contention-based operations EDCA, DCFEDCA, DCF

2020

EvaluationEvaluationSimulation Simulation

Opnet Modeler 11.5.AOpnet Modeler 11.5.AImplement NZ-ACK, and ACKS by modifying 802.11e modelImplement NZ-ACK, and ACKS by modifying 802.11e modelCompare to 802.11e EDCA, and ACKSCompare to 802.11e EDCA, and ACKS

Performance measures:Performance measures: ThroughputThroughput

Total data bits transmitted successfullyTotal data bits transmitted successfully Fairness Index (FI) [12,13]Fairness Index (FI) [12,13]

SSi i : Throuput per user i: Throuput per user i 0 <= FI <=10 <= FI <=1The closer FI to 1, the higher the fairnessThe closer FI to 1, the higher the fairnessWe used FI to measure how fair among DCF usersWe used FI to measure how fair among DCF users

Delay Delay delay of a data frame is measured from frame arrival at MAC until it is delay of a data frame is measured from frame arrival at MAC until it is successfully received (until its ACK received correctly)successfully received (until its ACK received correctly)

Retransmission AttemptsRetransmission AttemptsNumber of retransmission attempts per data frameNumber of retransmission attempts per data frameUsed as an indication of collision ratesUsed as an indication of collision rates

2121

EvaluationEvaluationSaturationSaturation Stations always have frames to transmitStations always have frames to transmit 802.11g PHY, 54Mbps/24Mbps802.11g PHY, 54Mbps/24Mbps EDCA: 50 users, VoiceEDCA: 50 users, Voice DCF: 50 usersDCF: 50 users Compare to:Compare to:

EDCA with 2 settings of CWEDCA with 2 settings of CWmin/maxmin/max

ACKSACKS OneSlot: an NZ-ACK frame OneSlot: an NZ-ACK frame

is always issued with a is always issued with a

duration of 1 slotduration of 1 slot

2222

EvaluationEvaluation

2323

EvaluationEvaluation

OneSlot:OneSlot: The best performance for EDCAThe best performance for EDCA

lowest delays, highest throughputs, highest ratio lowest delays, highest throughputs, highest ratio of EDCA throughputs to that DCFof EDCA throughputs to that DCF

High degradation of DCF users’ performanceHigh degradation of DCF users’ performance Throughput is 20% lower than achieved with any Throughput is 20% lower than achieved with any other scenarioother scenario

Lowest FI valueLowest FI value

2424

EvaluationEvaluation

NZ-ACK vs. ACKSNZ-ACK vs. ACKS Average delay, and delay per EDCA are lowerAverage delay, and delay per EDCA are lower

e.g. with NZ-ACKe.g. with NZ-ACK22 by 6.7%, and 8.8% respectively by 6.7%, and 8.8% respectively

ACKS is about weighted throughputACKS is about weighted throughput Throughput ratio are 3/4 with both variants of NZ-Throughput ratio are 3/4 with both variants of NZ-

ACK, and 3.4 with ACKS (3 is the goal)ACK, and 3.4 with ACKS (3 is the goal) Higher fairness with NZ-ACK variantsHigher fairness with NZ-ACK variants Retransmissions lower than with ACKS by 11%, Retransmissions lower than with ACKS by 11%,

17.5%.17.5%. ACKS adds to collisions – skipping ACKACKS adds to collisions – skipping ACK

NZ-ACK reduces number of contending usersNZ-ACK reduces number of contending users

2525

EvaluationEvaluation

NZ-ACK vs. EDCANZ-ACK vs. EDCA Higher EDCA throughput (6.67%, 7.99%)Higher EDCA throughput (6.67%, 7.99%) Lower EDCA delay (10.9%, 13.2%)Lower EDCA delay (10.9%, 13.2%) Lower retransmissions (at least 14%) Lower retransmissions (at least 14%)

NZ-ACK reduces number of contending usersNZ-ACK reduces number of contending users Higher throughput ratiosHigher throughput ratios (46.7%, 42%) (46.7%, 42%)

2626

EvaluationEvaluation

Overall network performance with NZ-ACK Overall network performance with NZ-ACK is higher than that with EDCA and ACKSis higher than that with EDCA and ACKS Highest total throughputHighest total throughput Lowest average delayLowest average delay Lowest retransmission attemptsLowest retransmission attempts

NZ-ACK reduces number of contending usersNZ-ACK reduces number of contending users

2727

EvaluationEvaluationNon-SaturationNon-Saturation 802.11b PHY, 11Mbps/1Mbps802.11b PHY, 11Mbps/1Mbps DCF: DCF:

CWCWminmin/CW/CWmaxmax 32/1024, DIFS 50 32/1024, DIFS 50uuss starting with 1 users, another is added every 3 secondsstarting with 1 users, another is added every 3 seconds max 50 usersmax 50 users Traffic generator: Exponential(40ms), 1000 bytes per packetTraffic generator: Exponential(40ms), 1000 bytes per packet

EDCA: EDCA: CWCWminmin/CW/CWmaxmax 32/64, DIFS 50 32/64, DIFS 50uuss 18 users with one voice flow per user18 users with one voice flow per userON/OFF modelON/OFF model

Both ON/OFF periods are Exponential(0.352 seconds) Both ON/OFF periods are Exponential(0.352 seconds) G.711 (silence) encodes, 64kbps, 160 bytes per packet.G.711 (silence) encodes, 64kbps, 160 bytes per packet.

Simulation period: 170 secondsSimulation period: 170 seconds T = Beacon IntervalT = Beacon Interval Virtual packets dropped after a delay of 0.1 secondVirtual packets dropped after a delay of 0.1 second

2828

EvaluationEvaluation

ThroughputThroughput EDCA: slight enhancement EDCA: slight enhancement

starts at 40s, i.e. when starts at 40s, i.e. when there are about 14 DCF there are about 14 DCF user – Almost the sameuser – Almost the same

DCF users not affectedDCF users not affected

Delay, Delay VariationDelay, Delay Variation 40s – 14 DCF users40s – 14 DCF users With NZ-ACK, kept smallWith NZ-ACK, kept small

0

0.05

0.1

0.15

0.2

0 20 40 60 80 100 120 140 160 180Time (seconds)

Pac

ket

Del

ay (

seco

nd

s)

802.11

NZ-ACK

0

500000

1000000

1500000

2000000

2500000

3000000

0 20 40 60 80 100 120 140 160 180

Time (Seconds)

Th

rou

gh

pu

t (b

ps)

per Voice - NZ-ACK per Voice - 802.11 per DCF - 802.11

per DCF - NZ-ACK Total - NZ-ACK Total - 802.11

2929

EvaluationEvaluation

RetransmissionsRetransmissions 40s -14 DCF users40s -14 DCF users Reduction of number of Reduction of number of

contending userscontending users

Delay (DCF)Delay (DCF) EDCA:EDCA:

up to 0.2sup to 0.2s

Prob[delay>0.1s] > 0.2Prob[delay>0.1s] > 0.2 NZ-ACK: NZ-ACK:

less than 0.026sless than 0.026s

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 20 40 60 80 100 120 140 160 180

Time (Seconds)

Ret

ran

smis

sio

n A

ttem

pts

802.11

NZ-ACK

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.05 0.1 0.15 0.2 0.25

Packet Delay (seconds)

CD

F 802.11

NZ-ACK

3030

ConclusionsConclusions

There is a need for mechanisms that There is a need for mechanisms that address the coexistence of EDCA and address the coexistence of EDCA and DCF users in future IEEE 802.11 WLANsDCF users in future IEEE 802.11 WLANsNZ-ACK adaptively controls DCF users, NZ-ACK adaptively controls DCF users, and maintains priority of service of EDCA and maintains priority of service of EDCA users while providing acceptable users while providing acceptable throughput performance for DCF usersthroughput performance for DCF usersNZ-ACK does not require any modification NZ-ACK does not require any modification to legacy DCF usersto legacy DCF users

3131

ReferencesReferences[1] IEEE Std 802.11b-1999, “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY)

Specifications: Higher-Speed Physical Layer Extension in the 2.4 GHz Band.”[2] IEEE Std 802.11g-2003, “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY)

specifications Amendment 4: Further Higher Data Rate Extension in the 2.4 GHz Band.”[3] IIEEE Std 802.11a-1999, “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY)

specifications Amendment 1: High-speed Physical Layer in the 5 GHz band.”[4] IEEE Std 802.11e-2005, “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY)

specifications. Amendment 8: Medium Access Control (MAC) Quality of Service Enhancements.”[5] A. Swaminathan and J. Martin, “Fairness issues in hybrid 802.11b/e networks,” in Consumer

Communications and Networking Conference, 3rd IEEE CCNC, vol. 1, 8-10 Jan 2006, pp. 50–54.[6] G. Bianchi, I. Tinnirello, and L. Scalia, “Understanding 802.11e contention-based prioritization mechanisms

and their coexistence with legacy 802.11 stations.” IEEE Network, vol. 19, no. 4, pp. 28–34, 2005.[7] J. Majkowski and F. C. Palacio, “Coexistence of ieee 802.11b and ieee 802.11e stations in qos enabled

wireless local area network.” in Wireless and Optical Communications, A. O. Fapojuwo and B. Kaminska, Eds. IASTED/ACTA Press, 2006, pp. 102–106.

[8] L. Vollero, A. Banchs, and G. Iannello, “Acks: a technique to reduce the impact of legacy stations in 802.11e edca wlans,” in Communications Letters, IEEE, vol. 9, no. 4, April 2005, pp. 346–348.

[9] A. Banchs, A. Azcorra, C. Garcia, and R. Cuevas, “Applications and challenges of the 802.11e edca mechanism: an experimental study.” IEEE Network, vol. 19, no. 4, pp. 52–58, 2005.

[10] J. Majkowski and F. C. Palacio, “Qos protection for ieee 802.11e in wlan with shared edca and dcf access.” in Communication Systems and Networks, C. E. P. Salvador, Ed. IASTED/ACTA Press, 2006, pp. 43–48.

[11] Opnet, “Opnet Modeler.” www.opnet.org. [12] D. R. Jain, Chiu, and W. Hawe, “A Quantitative Measure of Fairness and Discrimination for Resource

Allocation in Shared Computer Systems,” DEC Research Report TR-301, September 1984.[13] C. Koksal, H. Kassab, and H. Balakrishnan, “An Analysis of Short- Term Fairness in Wireless Media

Access Protocols,” In Proc. of ACM SIGMETRICS, 2000.

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Thank YouThank You

Questions?Questions?