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IEEE 802.11 EDCF: a QoS Solution for WLAN Javier del Prado 1 , Sunghyun Choi 2 and Sai Shankar 1 1 Philips Research USA - Briarcliff Manor, NY 2 Seoul National University – Seoul, Korea Email: {javier.delprado,sai.shankar}@philips.com, [email protected]. Outline. IEEE 802.11 WLAN - PowerPoint PPT Presentation
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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, [email protected]
2
Outline
• IEEE 802.11 WLAN• IEEE 802.11e EDCF • EDCF Bursting • Performance Evaluation• Conclusions
3
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