1 qos schemes for ieee 802.11 wireless lan – an evaluation by anders lindgren, andreas almquist...
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QoS Schemes for IEEE 802.11 Wireless LAN – An Evaluation
by Anders Lindgren, Andreas Almquist and Olov SchelenPresented by Tony Sung, 10th Feburary 2004
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Outline
Introduction – Why QoS?
Existing IEEE 802.11 MAC Algorithms DCF and PCF
Proposed QoS Mechanisms Enhanced DCF Blackburst Distributed Fair Queuing
Performance Comparison
Conclusion
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Introduction – Why QoS?
Shared Medium Medium Utilization can be Low Collision is Possible
To Support Real-time / Multimedia Traffic Require Service Differentiation -> QoS Prioritization Resource Sharing
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Existing IEEE 802.11 MAC Algorithms
Currently Two Methods are used to provide Medium Access:
Distributed Coordination Function (DCF)
Mobile Stations try to Compete for Accessing the
Medium
Point Coordination Function (PCF)
Access Point polls the Stations and Grant Access
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Existing IEEE 802.11 MAC Algorithms
Distributed Coordination Function (DCF) Based on CSMA/CA Algorithm
Sense the Medium before Sending with Contention Window and Backoff
Has Data
to Send
Sense Medium( for DIFS )
Has Data
to Send
Backoff( for a random
time in [0, CW) )
Backoff Timer Suspended
DIFS
Has Data
to Send
Has Data
to Send
Backoff
Backoff
DIFS
Start TX
Start TX
Timeout
Timeout
Increase CWND & Backoff
Increase CWND & Backoff
Start TX
ACK
Start TX
Resume
ACK
Unknown Delay, Unknown Bandwidth, Low Medium Utilization
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Existing IEEE 802.11 MAC Algorithms
Point Coordinate Function (PCF) Extends and Coexists with DCF
Controlled by Point Coordinator (i.e. Access Point) Keeps a List of Stations to be Polled
Contention-Free Period
Start
Backoff
DIFSACK
PIFS
Poll 1
Station 1:ACK & Data
Poll 2
Station 2:ACK & Data
Send Beacon Frame
Declare End of CFP
Contention Free Period (CFP)
Higher Utilization, But Delay and Bandwidth may still be an Unknown in High Load situation.
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Proposed QoS Mechanisms
IEEE 802.11e Enhanced DCF
Stations wait for the Channel to become Idle for a pre-defined Time called Inter-frame Spacing (IFS) before sending
Shorter IFS will gain Higher Priority
When congested, Backoff time is determined by size of Congestion Window (CW)
Smaller CW will gain Higher Priority
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Proposed QoS Mechanisms
IEEE 802.11e Enhanced DCF Defines a new IFS called Arbitration IFS Provide Packet Prioritization
Classifies Packets into 8 Different Traffic Classes, Each with different IFS and CW
Packet Bursting
Has Data
to Send
Has Data
to Send
Backoff
Backoff
AIFS 1
AIFS 2
Start TX
Backoff
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Proposed QoS Mechanisms
Blackburst Reduce Delay Jitter of High Priority Traffics Send out Black Burst by Jamming the Channel Station that has Waited Longer sends Longer
Normal Traffic
High Priority Stations Start Black Bursting
PIFS
Detects!
Detects!
Detects!
Winner
Winner TX
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Proposed QoS Mechanisms
Distributed Fair Scheduling Prioritization Completely Sacrifice Performance
of Low Priority Traffic DFS Provides Proportional Sharing Between
Flows according to Assigned Weight Utilizes the Backoff Mechanism of DCF
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Proposed QoS Mechanisms
Distributed Fair Scheduling Calculate Backoff Interval as follow:
_ packetsizeB Scaling Factor
Smaller Packets have Higher Chance to be Sent
Weight is Added Here
Larger Weight means Smaller Backoff Interval, hence Higher
Chance for Sending
Scale the Backoff Interval to a Reasonable Length
Random Variable to Provide Randomness of the Backoff
Interval
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Performance Comparison
Objective Compare
Throughput, Medium Utilization, Collisions, Delay Of
PCF ○ , EDCF △ , DFS ▓ , BB ●
Types of Traffic High Priority (H-P)
300 bytes (Normal Dist.) 25ms Inter-packet Interval (96kb/s)
Low Priority (L-P) 800 bytes (Normal Dist.) 50ms Inter-packet Interval (128kb/s)
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Performance Comparison
Average Throughput
All Schemes Achieved Similar Throughput for H-P Traffic
BB is best for # H-P Nodes < 13
H-P Traffic Loss Performance 1st in DFS, while maintaining Finite Throughput for L-P Traffic
L-P Traffic Starves Rapidly
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Performance Comparison
Medium Utilization
BB has Highest Peak, but Drops at Higher # H-P Nodes ( > 13)
EDCF and DFS has Substantially Low Utilization
Reasons in the next slides…
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Performance Comparison
Overhead
High when # of H-P Nodes > 13
=>Low Utilization
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Performance Comparison
Collisions
EDCF Collides Easily
=>Low Utilization
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Performance Comparison Delay
Many H-P Nodes,One CFP cannot Accommodate
Medium # of H-P Nodes
Many H-P Nodes,Packet Bursting Causes Low Delay for Bursting Packets, and Very High Delay for Waiting Packets
Most Cases has Low Delay
Worst Case has Delay < 50ms
Delay is Proportional to Packet Size, Span Out a Large Range
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Conclusion
Blackburst and EDCF Starve L-P Traffic Blackburst
Delay is minimal, Best for Real-time Good at Avoiding Collision
EDCF Starving can be Reduced if using same AIFS for all Traffic in
EDCF -> Close to DFS Impl. Already in IEEE 802.11e
DFS Can be an Alternative if Starving L-P Traffic is Unfavorable
PCF Polling Overhead is High
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Thank You
Questions are Welcomed
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Appendix
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Appendix