IEEE 802.11 EDCF: a QoS Solution for WLAN

Javier del Prado1, Sunghyun Choi2 and Sai Shankar1

1Philips Research USA - Briarcliff Manor, NY2Seoul National University – Seoul, Korea

Email: {javier.delprado,sai.shankar}@philips.com, sunghyun.choi@ieee.org

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Outline

• IEEE 802.11 WLAN• IEEE 802.11e EDCF • EDCF Bursting • Performance Evaluation• Conclusions

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IEEE 802.11 WLAN

• IEEE 802.11 MAC Can be considered a wireless version of

Ethernet Best-Effort Traffic Referred as legacy MAC

• Currently the IEEE 802.11 Working Group is defining a supplement to support Quality of Service (QoS): IEEE 802.11e MAC Multimedia services

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IEEE 802.11 WLAN MAC

• MAC is based on logical functions: Distributed Coordination Function (DCF)

– Based on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)

Point Coordination Function (PCF)– Poll and Response mechanism

• MAC works with a single FIFO queue

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The DCF of the 802.11 MAC

• Distributed MAC based on local assessment of the channel: is the medium busy?

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Backoff Process

• Each station maintains its Contention Window (CW) value to select the Backoff Count (BO)

• The BO is a pseudorandom integer drawn from [0,CW]

• The CW is determined as follows: Originally is assigned CWmin After unsuccessful transmission:

CW := 2 · (CW + 1) – 1 Upper Bound of CWmax CW is reset to CWmin after successful

transmission

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The Enhanced DCF for QoS: EDCF

EDCF Bursting

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The Enhanced DCF (EDCF)

• Extension of the legacy DCF MAC for QoS• Defined in 802.11e Draft • Ability to differentiate frames with

different priorities Each frame from higher layers carries its

user priority (UP) value: supported up to 8 UPs

• Provides differentiated channel accesses to frames with different UPs

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EDCF

• AIFS[UP] and CWmin[UP] instead of DIFS, and CWmin

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EDCF Contention Parameters

• AIFS[UP] PIFS to protect Acknowledgement (ACK) transmission

• AIFS[UP] and CWmin[UP] announced by AP in beacon frames

• The smaller AIFS[UP], CWmin[UP] the shorter the channel access delay for UP

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EDCF Multiple Queues

• Multiple FIFO queues in the MAC: up to 8 queues• Every queue is an independent contention entity

with its own contention parameters

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EDCF Bursting

• EDCF TXOP limit announced in beacon frames by AP

• Legacy 802.11 allows single frame transmission

• EDCF Bursting: allows transmission of multiple frames within a time limit called Transmission Opportunity (TXOP)

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Performance Evaluation

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Simulation Parameters

• IEEE 802.11b PHY layer: 11 Mbps physical rate

• 3 types of trafficType Inter-arrival

Time (Avg. in sec)

Frame Size

(bytes)

On Time (sec)

Off Time (sec)

Data Rate

(Mbps) Voice Constant

(0.02) 92 Always 0 0.0368

Video Constant (0.001)

1464 0.012 0.088 1.4

Data Exponential (0.012)

1500 Always 0 1.0

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Simulation Parameters

• EDCF parameters per traffic type

Type AIFS CWmin Voice PIFS 15 Video DIFS 15 Data DIFS 31

• May not be optimal parameters. These should be adapted dynamically by the AP

• Data traffic type parameters are equivalent to legacy DCF

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DCF vs. EDCF

• 4 voice stations• 2 video stations• 4 data stations• Every Station sends a single traffic type

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Simulation Results

• A) Throughput

DCF EDCF

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EDCF

Simulation Results

• B) Data Dropped

DCF

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Simulation Results

• C) Delay

DCF EDCF

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EDCF Bursting

• 4 voice stations• 4 video stations• EDCF TXOP limit = 3.5 ms

2 video frames at 11 Mbps

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Simulation Results

Throughput Data Dropped

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Simulation Results

Video Delay Voice Delay

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Conclusions• Comparison between DCF and EDCF

EDCF can provide differentiated access among different user priorities

• Evaluated EDCF Bursting Increases throughput performance at the cost

of larger delays for voice Reduces contention overhead

• Admission control unit and traffic policer are needed More than two video stations can not be

accommodated