Session 1544
Quality of Service(QoS) in IEEE 802.11 Wireless LANs:
Evaluation of Distributed Coordination Function (DCF)
& Point Coordination Function (PCF)
Praveen Durbha, Matthew Sherman
Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties.
1544 Wireless LANs
Overview
 Introduction
 IEEE 802.11 Standard
 Types of 802.11 WLANs
 802.11 WLAN Mobility Types
 WM Access Mechanisms
 WLAN Problems
 Quality of Service
 Simulation Models
 Simulation Results & Analysis
 Conclusions
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1544 Wireless LANs
Introduction
A wireless LAN is one in which a mobile user can connect to a local area
network (LAN) through a wireless (radio) connection.
A standard, IEEE 802.11, specifies the technologies for WLANs.
IEEE 802.11 WLAN architecture
HL
Datalink Layer
LLC
MAC
PHY
Wireless Function
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1544 Wireless LANs
The IEEE 802.11 Standard
 Original 802.11
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2.4 GHz operating frequency
Data rates of 1 Mbps/2Mbps
Frequency Hopping (802.11 FHSS)
Direct Sequence (802.11 DSSS)
 Supplements of 802.11
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802.11a – Operation up to 54 Mbps using OFDM in
the 5 GHz frequency range
802.11b – Extension of the initial DSSS 2.4 GHz band
up to 11 Mbps
802.11e – MAC layer DCF and PCF enhancements for
QoS assurance
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1544 Wireless LANs
Types of 802.11 WLANs
 Ad hoc Wireless Network
This type of wireless network does not have any backbone
infrastructure and has at least two wireless stations. It is also referred
to as Independent Base Service Set (IBSS).
 Infrastructure Wireless Network
This type of wireless network consists of multiple cells interconnected
by Access Points (APs) and a Distribution System (DS) such as
Ethernet. It is also called as Extended Service Set (ESS).
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1544 Wireless LANs
802.11 Mobility Types
 No transition
Refers to stations that do not move and are moving within a BSS.
(Supported)
 BSS transition
Refers to stations that move from one BSS to another BSS within the
same ESS. (Supported)
 ESS transition
Refers to stations that move from a BSS in one ESS to another BSS in a
different ESS. (Not supported)
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1544 Wireless LANs
The 802.11 MAC
 Distributed Coordination Function (DCF)
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Mandatory implementation
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Uses CSMA/CA protocol
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No service differentiation
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Works for both IBSS & ESS
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Operates during the Contention Period (CP)
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Waits a period of DIFS interval before transmission
 Point Coordination Function (PCF)
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Optional Implementation
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Uses a Point Coordinator (PC) which resides in the AP
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Works only for ESS
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Operates during the Contention Free Period (CFP)
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Waits a period of PIFS interval before transmission
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1544 Wireless LANs
DCF Algorithm
• Wireless station senses the medium
• If medium free for DIFS, transmit frame else wait till the end of current
transmission
• Receiver sends ACK to sender after SIFS (Success)
• If collision, wait for EIFS and then back-off (Failure)
Back-off = Random_number () * Slot_time
Random_number range [0,CW]
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Contention window (CW) doubled for every repeated collision
Back-off timer decremented
Station with least back-off has access to the medium
Stations wanting to transmit additional packets also back-off
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1544 Wireless LANs
PCF Algorithm
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Point Coordinator senses medium
Idle for PIFS, send out polling packet
Polled stations send packet to AP after SIFS
AP sends ACK to polled stations after SIFS
Medium busy, defer access till the end of current transmission
No back-off involved
Process repeats till all packets are forwarded through AP.
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1544 Wireless LANs
Hidden Terminal Problem
Statement
Every station in a wireless network has limited radio transmitting range.
This may lead to two stations communicating with the same receiving station
which results in a collision. The station causing the collision is termed as
“hidden” with reference to the receiving station.
Solution
The RequestToSend(RTS) & ClearToSend(CTS) mechanism is used to resolve
the hidden terminal problem.
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1544 Wireless LANs
RTS/CTS Mechanism
Algorithm
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Sender transmits RTS frame
Receiver acknowledges RTS with CTS frame
Sender receives the CTS frame and the channel is reserved
Sender sends the DAT frame
Receiver sends the ACK frame to the sender to end the 4-way handshake
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1544 Wireless LANs
Quality of Service
Definition
QoS is a broad term used to describe the overall experience the end-user or
application will receive over a wireless network.
Standard parameters used for measuring QoS are
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Media Access Delay
Throughput
Packets Dropped (Packet Loss Rate - PLR)
Network Availability
DCF and PCF are evaluated using Media Access Delay and Throughput
parameters.
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Simulation Model – DCF
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DCF Station Parameters
Values
Type of Network
Infrastructure
Length of Simulation
60 seconds
Packet Size Range
64 -1500 Bytes
Packet Size Distribution
uniform_int
Interarrival Time (Low Load)
0.025-0.05 seconds
Interarrival Time (High Load)
0.0125-0.025 seconds
Interarrival Time (Overload)
0.00625-0.0125 seconds
Interarrival Time Distribution
Uniform
RTS Threshold
256/1024 Bytes
Number of Stations
10
Station Traffic Destination
Node_0 (AP)
AP Traffic Destination
Node_1
Physical Characteristics
Direct Sequence
Data Rate
11 Mbps
PCF Functionality
Disabled
Offered Load (Low)
1.7 Mbps
Offered Load(High)
3.4 Mbps
Offered Load(Overload)
6.8 Mbps
Center Node (Node_0)
Access Point
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Simulation Model – PCF
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PCF Station Parameters
Values
Type of Network
Infrastructure
Length of Simulation
60 seconds
Packet Size Range
64 -1500 Bytes
Packet Size Distribution
uniform_int
Interarrival Time (Low Load)
0.05-0.1 seconds
Interarrival Time (High Load)
0.025-0.05 seconds
Interarrival Time (Overload)
0.0125-0.025
Interarrival Distribution
uniform
Number of Stations
10
Station Traffic Destination
Node_0 (AP)
AP Traffic Destination
Node_1
Physical Characteristics
Direct Sequence
Data Rate
11 Mbps
PCF Functionality
Enabled
CFP Beacon Multiple
1
CFP Offset
0
CFP Interval
0.01
Max Failed Polls
2
Beacon Interval
0.02 seconds
Offered Load (Low)
1.7 Mbps
Offered Load (High)
3.4 Mbps
Offered Load (Overload)
6.8 Mbps
Center Node (Node_0)
Access Point
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Simulation Results – Low Load
• Delay least for DCF no
RTS/CTS scenario
• PCF records higher delay
• Good QoS as delay < 1 sec
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Simulation Results – Low Load
• Throughput same for all
scenarios as no data packets
dropped (see next slide)
• Full Load carried
• Load=Throughput + Data Dropped
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1544 Wireless LANs
Simulation Results – Low Load
• No data dropped (PLR) in any of
the scenarios
• Good QoS
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Simulation Results – High Load
• PCF remains stable.
• No effect on DCF with no
RTS/CTS.
• DCF with RTS/CTS approaching
criticality.
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1544 Wireless LANs
Simulation Results – High Load
• Offered Load doubled
• Load=Throughput for PCF,DCF
no RTS & DCF with 1024 RTS
• Load≠Throughput for DCF with
256 RTS
• Some packet drops (see next slide)
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1544 Wireless LANs
Simulation Results – High Load
• PLR = 0 for PCF, DCF no RTS
& DCF with 1024 RTS
• PLR = 325 Kbps for DCF with
256 RTS
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Simulation Results – Overload
• All Scenarios in overload
• Packets being dropped
• PCF best delay performance
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1544 Wireless LANs
Simulation Results – Overload
• Maximum throughput for PCF
• PCF offers highest capacity
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1544 Wireless LANs
Simulation Results – Overload
• Maximum PLR for DCF with
256 RTS
• Minimum PLR for PCF
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Conclusions
 Summary
This presentation evaluated the two basic wireless medium access
mechanisms DCF and PCF under low and high load conditions. DCF was
observed to outperform PCF under low load and PCF outperform DCF
under high load conditions. RTS/CTS offered no advantage, but there were
no hidden terminals. The observations were done using Media Access
Delay, Throughput & Data Dropped (PLR) metrics.
 Future Work
DCF and PCF do not provide prioritized access to the wireless medium.
IEEE 802.11e is currently working on enhancing the 802.11 MAC with
mechanisms like Enhanced DCF (EDCF) and HCF. It will be interesting to
evaluate these mechanisms using OPNET once the 802.11e draft has
stabilized.
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References
[1] IEEE 802.11 Working Group, IEEE 802.11 standard for wireless LANs,
1999, Reaffirmed 2003.
[2] Jim Geier,”Wireless LANs: Implementing High Performance IEEE 802.11
Networks”, Second Edition, SAMS publishing 2002
[3] Qiang Ni, Lamia Romdhani, Thierry Turletti, Imad Aad, “QoS Issues and
Enhancements for IEEE 802.11 Wireless LAN, INRIA, Sophia Antipolis,
Cedex, France, November 2002.
[4] Dongyan Chen, Sachin Garg, Martin Kappes, Kishore Trivedi, “Supporting
VBR VoIP Traffic in IEEE 802.11 Wireless LAN in PCF Mode
OPNETWORK'02, Washington DC, August 2002.
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