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PLANETE group, INRIA Sophia-Antipolis
July 1, 2003
Adaptive Channel allocation for QoS
Enhancement in IEEE 802.11 Wireless LANs
Presented by:
Mohammad Malli
Advisors:
Qiang Ni, Thierry Turletti, and Chadi Barakat
Université de Nice Sophia-Antipolis
Ecole Doctorale STIC
DEA Réseaux et Systèmes Distribués
July 1, 2003 2
Outline
IEEE 802.11 and 802.11e
Problems and Solutions
Simulation Topologies and Parameters
Our scheme: Adaptive EDCF
Our scheme: Adaptive DCF
Conclusions and Future Work
July 1, 2003 3
IEEE 802.11
New technologyNew technology Provide end-users the benefits of increased mobility &
productivity Enable network connectivity at locations where cabling is either
difficult or costly to install
IEEE 802.2
Logical Link Control (LLC)
IEEE 802.11 IEEE 802.11 Media Access Control Media Access Control (MAC)(MAC)
IEEE 802.11 Physical Layer
(FHSS, DSSS, IR)
July 1, 2003 4
IEEE 802.11 MAC
MAC layer uses twotwo kinds of protocols to
access to the medium
DCF : Distributed Coordination Function is used to
support asynchronous data transmissions
PCF : Point Coordination Function is designed for
time-bounded multimedia applications
July 1, 2003 5
CSMA/CA
Source
Destination
Data
Others
Ack
Data
SIFS
DIFSBackoff Time
Defer Access
Time
Backoff_Time = Rand(0, CW) * aSlotTime
Backoff Counter must be decreased each slot time by one slot time whenever the channel is idle
CW is initially set to CWmin
CW is doubled after a failed transmission
CW is set to Cwmin, when the packet is successfully transmitted
July 1, 2003 6
DCF Limitations
DCF is unsuitable for real time applications because it doesn’t support service service differentiationdifferentiationDCF suffers from significant throughput degradation and high delay at high load due to high collision ratecollision rate and wasted number of idle idle slots slots in each backoff contention cycle
CollisionCollisionss
Idle backoff slotsIdle backoff slots (at each contention period)
Virtual transmission time
SIFS ACK DIFS DIFS DIFSDIFSACKSIFS
July 1, 2003 7
FCR (Fast Collision Resolution)
Proposed by Florida U. (Infocom ’03)
Extends the basic DCF (no service differentiationno service differentiation), to improve the
throughput
Main features:
- Static Backoff ThresholdStatic Backoff Threshold value = 2 * (CWmin + 1) - 1
- Increasing CWIncreasing CW when the channel is busy during deferring periods Weakness:
Backoff Threshold must be adapted to the medium state because during high load, the period of exponential state must be shorter to reduce aggressivity
When the channel is busy, it is better that the node waits with its remaining backoff time because doubling the CW during deferring periods increases the number of idle slots in low and medium load cases
July 1, 2003 8
IEEE 802.11e
Upcoming IEEE 802.11e MAC:
HCF: Hybrid Coordination Function has Controlled Channel Access Mechanism, it is used in infrastructure network
EDCFEDCF: Enhanced Distributed Coordination Function doesn’t need a central coordinator point, it is used in Ad-hoc network
Extends the basic DCF to support service service differentiationdifferentiation
July 1, 2003 9
EDCF - Enhanced DCF
DCFDCFEDCFEDCF
CWCW[TCi]
AIFS[TCi]
Transmission attempt
Transmission attempt
Many classes provide per flow differentiation
Class0 Class2Class1
Class7
TC7TC2TC1TC0
DIFS
Internal Scheduler (Resolve Virtual Collisions)
July 1, 2003 10
EDCF Limitations
EDCF fails to provide QoS at high load
Bad Video Quality
Low Total Throughput
Audio Video Background
Throughput (in B/s)
Time (in sec)
July 1, 2003 11
Our approach: Adaptive EDCF (1)
improve the QoSQoS for multimedia applications in all medium states
increase the total throughputtotal throughput in all medium states
Goals :
Idea : To protect Audio and Videoprotect Audio and Video transmissions, best effort queue increases its CW larger and reset a new backoff time, when it senses the channel is busy, during deferring periods
To decrease the wasted number of idle slotsdecrease the wasted number of idle slots due to backoff in each contention cycle, a queue must decrease faster its backoff time after it senses the channel idle during a certain time Each extension will help to realize the above goals
July 1, 2003 12
AEDCF(2): 1st extension
Our Solution :
First, extend the Fast Backoff mechanism, proposed for DCF in Florida U., to an adapted approach that differentiates between the different priority levels 2 Backoff states :
LinearLinear decrease (old)
ExponentialExponential decrease (new)
- Adapted to medium state
- Differentiate between traffic classes
aSlotTimepriAIFSN
priCWpriCWpriCWpriThresholdBackoff *
][1
1][]max[]min[][_
Linear state exponential state
0Backoff_ThresholdBackoff_Time
Backoff_Counter decreaseBackoff_Counter decrease
Slot T.
July 1, 2003 13
AEDCF (3): 2nd extension
Second, increase the contention window size and reset a new backoff time, when the channel is sensed busy, during deferring periods :
CW[pri] = min(CWmax[pri], 2 * CW[pri])
Priority
CWmax
0 1 2 3
1023 1023 31 15
Low priority flows will be punished
High priority flows will be protectedHigh priority flows will be protected
July 1, 2003 14
Simulations Topology and parameters (1)
Node0Node0Node Node 11
Node2Node2
Node Node nn
Audio
Video
Background
Audio
Audio
Video
Video
Background
Background
Low load : n = 5, 1 Mbytes/s
Medium load : n = 11, 2.5 Mbytes/s
High load : n = 15, 3.5 Mbytes/s
Medium Bandwidth = 4.5 Mbytes/s
July 1, 2003 15
Simulation Topologies and parameters (2)
Audio Video Background
Priority 3 2 0
CWmin 7 15 31CWmax 15 31 1023AIFSN 1 1 2
Transport UDP UDP UDPPacket Size (bytes) 160 1280 1500Packet Interval (ms) 20 10 12.5
Sending rate (Kbytes/s) 8 128 120
MAC parameters for the three flowsMAC parameters for the three flows
July 1, 2003 16
AEDCF: Flows Throughput
Good multimedia flow performance in medium and high load cases
Also, Background flows have better throughput than in EDCF case
Throughput with AEDCF in Throughput with AEDCF in 11 nodes topology11 nodes topology
Throughput with AEDCF in Throughput with AEDCF in 15 nodes topology15 nodes topology
Throughput (in B/s)
Throughput (in B/s)
Time (in sec) Time (in sec)
July 1, 2003 17
AEDCF: Total Throughput
Our scheme also provides highest Total Throughputhighest Total Throughput in high load case
Total Throughput in 15 nodes topologyTotal Throughput in 15 nodes topology
With our AEDCF scheme, the T.T is higher about 55 % more than with EDCF and 10 % more than DCF in this high load
topology
3.5 Mbytes/s Total sending rate
Total Throughput (in B/s)
Time (in sec)
July 1, 2003 18
Our approach: Adaptive DCF (1)Extends DCF by our adapted fast backoffadapted fast backoff approach
Throughput with DCFThroughput with DCF Throughput with FCRThroughput with FCR
Throughput with ADCFThroughput with ADCF
ADCF provides best best medium utilisationmedium utilisation in this medium load case
Throughput (in B/s)
Throughput (in B/s)
Throughput (in B/s)
Time (in sec) Time (in sec)
Time (in sec)
July 1, 2003 19
ADCF (2): High load
We still need service service differentiationdifferentiation to maintain a stable multimedia flows quality in high load
Throughput with DCFThroughput with DCF Throughput with FCRThroughput with FCR
Throughput with ADCFThroughput with ADCF
Throughput (in B/s)
Throughput (in B/s)
Throughput (in B/s)
Time (in sec)
Time (in sec)
Time (in sec)
July 1, 2003 20
ADCF (3): Total Throughput
In medium and high load cases, ADCF provides the highest highest Total Throughput Total Throughput
Total Throughput in 11 nodes topologyTotal Throughput in 11 nodes topology Total Throughput in 15 nodes topologyTotal Throughput in 15 nodes topology
In this medium load case, DCF is better than FCR
FCR is better than DCF only in high load case
3.5 Mbytes/s Total sending rate
2.75 Mbytes/s Total sending rate
Total Throughput (in B/s)
Total Throughput (in B/s)
Time (in sec)Time (in sec)
July 1, 2003 21
Conclusions and Future Work
QoS support in IEEE 802.11 and 802.11e WLANs is not good enough
We propose an extension to the proposed 802.11e EDCF: Adaptive EDCFAdaptive EDCF Uses adaptive fast backoff mechanism Provides more transmission opportunity to multimedia applications
and higher total throughput during high load situations
we propose an extension to the standard 802.11 DCF: Adaptive DCFAdaptive DCF Uses adaptive fast backoff mechanism Provides better medium utilisation and higher total throughput in
medium and high load cases It is not good enough for multimedia applications in high load state In this case, it is better to use AEDCF
Future work: Analytic modeling & Real Experimentation
