Claudio Cicconetti, Luciano Lenzini, Enzo Mingozzi, Giovanni Stea Dipartimento di Ingegneria dell'Informazione University of Pisa, Italy IPS-MoMe 2005

Claudio Cicconetti, Luciano Lenzini,Enzo Mingozzi, Giovanni Stea

Dipartimento di Ingegneria dell'InformazioneUniversity of Pisa, Italy

IPS-MoMe 2005Warsaw, Poland, 14-15th March 2005

A Software Architecture for Simulating IEEE 802.11e HCCA

Dipartimento di Ingegneria dell'Informazione – University of Pisa, ITALY

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Summary

Introduction

Software Architecture

Implementation and preliminary results

Conclusions


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Introduction

New (EuQoS) applications require QoS videoconference VoIP online gaming ...

The legacy IEEE 802.11 lacks QoS support Distributed Coordination Function Point Coordinated Function

Solution: 802.11e Enhanced Distributed Channel Access (prioritized access) HCF Controlled Channel Access (parametrized access)


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Legacy 802.11 - DCF

Distributed protocol, based on CSMA/CA: listen before transmit collisions detected with positive acknowledgment binary exponential backoff procedure if collision

All the stations access the medium with the same procedure Only suitable for best-effort traffic


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Legacy 802.11 - PCFCFP

AP

STA

PIFS

CP CFP CP

P

CF-Data

P

STA CF-Data

P

CF-Data

P

CF-Data

CP

Contention-Free Periods (CFPs) alternates to Contention-Periods (CPs) at fixed intervals

During CFPs the AP cyclically polls the STAs Still unsuitable for providing QoS


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802.11e Enhancements

QoS Access Point (QAP), with enhanced scheduling

capabilities Admission control

QoS Stations (QSTAs), capable of Contention-based access

DCF, EDCA Responding to polls

PCF, HCCA

IEEE 802.11eQoS Access Point

(QAP)

IEEE 802.11eQoS Station

(QSTA)

IEEE 802.11eQoS Station

(QSTA)


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802.11e - EDCA

Distributed protocol

Up to four Access Categories

Based on the differentiation of the CSMA/CA parameters

Achieves relative differentiation of traffic from different ACs

If in infrastructure mode, admission control at the QAP

AC1

AC2

AC3

AC4

Mapping (MSDU, UP) to Access Categories

internal collision resolution

tran

smit

que

ues

per-

AC

wireless medium access

Link Layer


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802.11e – HCCA (i)

Centralized Up to 8 Traffic Streams (TSs) for each QSTA Traffic specs and requirements (TSPEC) negotiation

Mean data rate, delay bound, etc.

Admission control of TSs at the QAP Absolute QoS guarantees enforced by

scheduling Controlled Access Phases (CAPs)

busy

busy

busy

QAP

QSTA

QoSCF-Poll

QoS Data

QoSCF-Ack

QoSData

QoSCF-Ack

QSTA

TXOP

CAPDCF/EDCA

PIFS


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HCCA Scheduling

The 802.11e does not define a standard scheduling algorithm at the QAP

A reference scheduler is provided TDM-like scheduling: TXOP of fixed duration at fixed time

intervals for all admitted TSs

Different schedulers are currently being proposed in the literature We need a common framework for evaluation and

testing

TXOPi

TXOPj

TXOPk

TXOPi

TXOPj

TXOPk

Service Interval (SI)

HCCADCF

EDCA


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Software Architecture

Modular architecture MAC fully compliant to the standard HCCA

function MAC functions separated from HCCA scheduling Allows for flexible integration of various

schedulers

ClassifierMedium Access Control

HCCA Scheduler

Measurement

outgoingpackets

outgoingpackets

incomingpackets

Upper Layers


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MAC (i)

CAP_Hand Has_Control/Lost_Control Transmit

Receive/Data_Receive Success

QoSCF-Poll

QoS CF-Ack

QoS Data

RECVDATA_RECV

DATA_RECV

TRANSMIT

RECVDATA_RECV

RECVSUCCESS

LOST_CONTROL

RECVHAS_CONTROL

HAS_CONTROL

RECVSUCCESS

LOST_CONTROL

LOST_CONTROL

QAP

QSTA

HAS_CONTROLTRANSMIT

SIFS

SIFS SIFS SIFS

SIFS SIFS

SIFS

SIFS

PIFS

QoS Data

QoS CF-Ack

QoS Data

QoS CF-Ack

busy

busy

CAP_HAND TRANSMIT

RECVSUCCESS

TRANSMIT TRANSMIT

RECV

TRANSMIT

the HCCA scheduler notifies that there are not any packets to transmit

the HCCA scheduler notifies that there are not any packets to transmit

TRANSMIT

contention resumed

MAC is implemented as a FSM driven by a set of events


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MAC (ii)

LOST CONTROL

has_control_ = 0

CANCEL NEXT CAP

do/ stop the mhCap_ timer

HCCA_START

HCCA_STOPLOST CONTROL

has_control_ = 0

HCCA_HAS_CONTROL

(QoS_CF-poll reception)

HAS CONTROL

has_control_ = 1

HCCA_LOST_CONTROL

(end of TXOP)

QSTA

START NEXT CAPdo/ query the HCCA Scheduler to know the start time of the next CAP(start accordingly the mhCap_ timer)

HAS CONTROLhas_control_ = 1

HCCA_START

HCCA_STOP

HCCA_LOST_CONTROL(uplink frame)

WAIT FOR CAP

SENSE IDLEdo/ sense the medium idlefor a PIFS/SIFS duration

mhCap_ timer is busy

CAP_HAND

HCCA_HAS_CONTROL

QAP

QoS_CF-polltransmission

HCCA_LOST_CONTROL(end of CAP)

TXOP end


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HCCA Scheduler (i)

Listens to a subset of the MAC events Has_Control/Lost_Control Possibly others, depending on its actual needs

General interface: enque(): adds a new packet coming from the LL deque(): pops the HOL packet

QAP-specific functions get_next_cap(): returns the expiration time of

the next CAP addTSPEC(): requests the admission of a new TS


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HCCA Scheduler (ii) QSTA

BUSY

DEQUEdo/ dequeue()do/ get_queue_size()

IDLE

HCCA_HAS_CONTROL

HCCA_LOST_CONTROL

HCCA_START

HCCA_STOP

enque()

ACTIVE

QAP

BUSY

DEQUE

do/ dequeue()

NEXT CAPaddTSPEC()

enque()

IDLE

HCCA_HAS_CONTROL

HCCA_START

HCCA_STOP

HCCA_LOST_CONTROL

do/ get_next_cap()

ACTIVE


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Implementation

The software architecture was implemented in the Network Simulator 2 simulation environment

QAP scheduler: reference 802.11e

QSTA scheduler:FIFO with only one TS

...

TS 1 TS 2 TS n

QAP

TS n

QSTA n

TS 1 TS 2 TS n

TS n

QSTA 2

TS n

QSTA 1


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Preliminary Results (i)

1 QAP 3 QSTAs (bidirectional video streaming session) 3 legacy STAs (asymptotic condition) 802.11b @ 11 Mbps Error-free channel No RTS/CTS, no MAC fragmentation

802.11eQAP

802.11eQSTAs legacy 802.11

STAs


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Preliminary Results (ii)

200 400 600 800 1000 1200

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

HCCAdata+video

EDCAdata+video

Ma

xim

um

de

lay

(s)

Time (s)

uplink downlink

EDCAvideo only

Maximum delay of video flows


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Preliminary Results (iii)

400 600 800 1000 12000

200000

400000

600000

800000

1000000

1200000

1400000

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

Th

rou

gh

pu

t (b

ps)

Time (s)

data (throughput)

Co

llisi

on

pro

ba

bili

ty (

%)

data (collisions) video downlink video uplink


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Conclusions and future work

A software architecture for simulating the IEEE 802.11e HCCA was defined

The scheduling and MAC functions were decoupled by a generic communication interface

The contributed framework has been implemented using ns2

Future (ongoing) work Defining, implementing and comparing different

HCCA scheduling algorithms for 802.11e


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End of presentation

Thanks for your attention

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Documents

Claudio Cicconetti, Luciano Lenzini, Enzo Mingozzi, Giovanni Stea Dipartimento di Ingegneria dell'Informazione University of Pisa, Italy IPS-MoMe 2005