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