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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