Link-Adaptable Polling-based Link-Adaptable Polling-based MAC Protocol for Wireless MAC Protocol for Wireless

LANsLANsByung-Seo Kim, Sung Won Kim, Yuguang FaByung-Seo Kim, Sung Won Kim, Yuguang Fa

ng and Tan F. Wongng and Tan F. WongDepartment of Electrical and Computer Engineering UDepartment of Electrical and Computer Engineering U

niversity of Floridaniversity of Florida

GlobeCom 2004GlobeCom 2004

Ju-Mei LiJu-Mei Li

OutlineOutline

IntroductionIntroduction

Related WorkRelated Work

Two-Step Multi-Polling Scheme (TS-MP)Two-Step Multi-Polling Scheme (TS-MP)

Performance Evaluation Performance Evaluation

ConclusionsConclusions

Ju-Mei LiJu-Mei Li

IntroductionIntroduction

Point Coordination Function (PCF)Point Coordination Function (PCF)– supports time-bound servicessupports time-bound services

PC

A

B C

DBSS

Beacon: parameters to control the superframe

After Beacon:

PC use Round-Robin scheme to poll stations

polling

Data

pollingpolling

Data

pollingData

Ju-Mei LiJu-Mei Li

IntroductionIntroduction

Disadvantage of PCFDisadvantage of PCF– Overhead of polling frames, the throughput is Overhead of polling frames, the throughput is

lowlow– Round-Robin scheme is inefficientRound-Robin scheme is inefficient– Collision may be caused by stations in Collision may be caused by stations in

neighboring BSSneighboring BSS– Transmission time is unpredictable Transmission time is unpredictable

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Related WorkRelated Work

802.11e Hybrid Coordination Function 802.11e Hybrid Coordination Function (HCF) in the CFP use PCF with two (HCF) in the CFP use PCF with two enhancementsenhancements– RTS/CTS handshakingRTS/CTS handshaking

Collision avoidance Collision avoidance – The station in neighboring BSSThe station in neighboring BSS

– Use Transmission Opportunity (TXOP) Use Transmission Opportunity (TXOP) Transmission time of a station will be bounded Transmission time of a station will be bounded

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Related Work: Related Work: multi-pollmulti-poll

[8] CP-MP protocol[8] CP-MP protocol– After broadcasting beacon frame, PC sends a After broadcasting beacon frame, PC sends a

multipollingmultipolling frame containing frame containing the allocated TXOP the allocated TXOP

and initial backoff time for and initial backoff time for each stationeach station to be polled to be polled

– Each station send packet to PC in its TXOPEach station send packet to PC in its TXOPCSMA/CA with backoff timeCSMA/CA with backoff time

RTS/CTS handshaking in order to avoid collisionRTS/CTS handshaking in order to avoid collision

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MotivationMotivation

Polling schemes in HCF and CP-MP Polling schemes in HCF and CP-MP solvesolve– Collision problem caused by a station in neighboring Collision problem caused by a station in neighboring

BSSBSS– But more overheads due to the RTS/CTS exchangesBut more overheads due to the RTS/CTS exchanges

But CP-MP schemeBut CP-MP scheme– May introduce collision in the May introduce collision in the samesame BSS BSS

There are no rate-adaptive MAC protocol for the There are no rate-adaptive MAC protocol for the PCF proposed in the current literturePCF proposed in the current literture

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Two-Step Multi-Polling Scheme Two-Step Multi-Polling Scheme (TS-MP)(TS-MP)

Basic ideaBasic idea

Polling schedulerPolling scheduler– First scheduler for SRMPFirst scheduler for SRMP– Second scheduler for DTMPSecond scheduler for DTMP

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TS-MP: TS-MP: basic ideabasic idea

PC

A

B C

D

E

Status-Request Multi-Poll (SRMP)

Status-Response (SR)

Overhearing SR of C: set the Tentative-NAV

Data Transmission Multi-Poll (TDMP)

BSS

When E hears the data frame from C, E will reset its NAV

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TS-MP: TS-MP: basic ideabasic idea

PC

A

B

C

D

E

Beacon

SRMP DTMP

NAV

Status Response (SR) (Date Rate, Buffer

Status)

Data Transmission (TXOP)

Status Collection Period (SCP)

Data Transmission Period (DTP)

CFP CP

PCA

BC

D

E

BSS

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TS-MP: TS-MP: frame structureframe structure

Byte 2FrameControl

6

BSSID

1 2 2 4Polling

Count (N)AID 1 AID N FCS

(a) SRMP

Byte 2FrameControl

6

BSSID

2 1 2 4Tentative

NAVBufferStatus

AID FCS

1Down-Rate

(b) SR

Byte 2FrameControl

6

BSSID

1 5 * Polling Count (N) 4Polling

Count (N) AID FCSPolling Control

Up-Rate TXOP

(c) DTMP

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SPiSPi – inter-arrival time of frames at station inter-arrival time of frames at station ii

– wwii

related to related to SPiSPi in order to manage the polling time of in order to manage the polling time of station station ii

– is the is the normalized numbernormalized number of transmitted frame of transmitted frame

s s

TS-MP: TS-MP: polling schedulerpolling scheduler

kiE

Ju-Mei LiJu-Mei Li

12

)(,1

12

)(,1

12

)(,1

12

)(,1

/

8

1

iSFii

di

iSFii

di

iSFii

di

iSFii

di

i

i

k

Wkj

ji

ki

SFi

ii

wandTSP

Tifw

wandTSP

Tifw

wandTSP

TifSP

wandTSP

TifSP

w

MeE

TM

PSP

Pi: payload

Mi: the average data arrival rate in the MAC layer at node i

TSF: time duration of superframe

T id: from the time instant when frame is generated in MAC layer t

o the time instant when frame is transmitted by station i

The number of transmitted frames at station i in superframe j

Frame delay

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TS-MP: TS-MP: polling schedulerpolling scheduler

First scheduler for SRMPFirst scheduler for SRMP– If number of polled stations is very largeIf number of polled stations is very large

large amount of time is spent during large amount of time is spent during SCPSCP– OverheadOverhead

– Poor performancePoor performance

– In order to avoid this problemIn order to avoid this problemThe polled stationsThe polled stations NNii in the current CFP in the current CFP ii

– Is determined from previous CFPsIs determined from previous CFPs

If PC found If PC found shortage of DTPshortage of DTP in previous CFP in previous CFP– The number of polled stations in SRMP is The number of polled stations in SRMP is reduced by onereduced by one

If PC foundIf PC found that that DTPDTP is long enough is long enough in previous CFPin previous CFP– The number of polled stations in SRMP is The number of polled stations in SRMP is increased by oneincreased by one

CFP

SCP DTP

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TS-MP: TS-MP: polling schedulerpolling scheduler

First scheduler for SRMP (cont.)First scheduler for SRMP (cont.)– Lower Lower wwii value, higher priority value, higher priority

Low value of Low value of wwii means means– Station Station ii has high probability of having frames to transmit has high probability of having frames to transmit

– Lower value, higher priorityLower value, higher priorityStation with low has high probability to have penStation with low has high probability to have pending framesding frames

PC chooses PC chooses NNii stations with low stations with low wwii

Stations with the same Stations with the same wwii

– PC choose stations with lowerPC choose stations with lower

kiE

kiE

kiE

Ju-Mei LiJu-Mei Li

TS-MP: TS-MP: polling schedulerpolling scheduler

Second scheduler for DTMPSecond scheduler for DTMP– TXOPTXOPii = ( = (TTprepre++TTphy_hdrphy_hdr++TTMAC_hdrMAC_hdr+2+2TTSIFSSIFS++TTACKACK++LLpayloadpayload//

RRii)*)*QQii

– RRii: is data rate in the physical layer: is data rate in the physical layer

– QQii: number of frames in buffer station : number of frames in buffer station ii

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Performance EvaluationPerformance Evaluation

One BSS with diameter of 250mOne BSS with diameter of 250m

Two rear-time trafficTwo rear-time traffic– CBRCBR

On: 1 secOn: 1 sec

Off: 1.35 secOff: 1.35 sec

Generate a frame of 200bytes at every 0.1 sec Generate a frame of 200bytes at every 0.1 sec

– VBRVBRBit rate of 53Kbps ~ 900KbpsBit rate of 53Kbps ~ 900Kbps

Size of each video packet: 800 bytesSize of each video packet: 800 bytes

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Performance EvaluationPerformance Evaluation

CBR

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Performance EvaluationPerformance Evaluation

VBR

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Performance EvaluationPerformance Evaluation

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ConclusionsConclusions

New polling-based MAC protocol for the PCF in INew polling-based MAC protocol for the PCF in IEEE 802.11 WLANEEE 802.11 WLAN– First multipollingFirst multipolling

PC obtain the information about PC obtain the information about

The polling sequence for data transmissionThe polling sequence for data transmission

– Second multipolling Second multipolling Coordinates data transmission to avoid collisionCoordinates data transmission to avoid collision

– Reduced the overhead caused by polling framesReduced the overhead caused by polling frames– More efficient scheduling schemes as well as rate adaMore efficient scheduling schemes as well as rate ada

ptationptation

Thank You !!Thank You !!