1. Student : Min-Hua Yang Advisor : Ho-Ting Wu Date : 2006.7.30
The Study of the QoS Provisioning Mechanisms in IEEE e Wireless LANs
Student : Min-Hua Yang
Advisor : Ho-Ting Wu
Date :

2. Outline Background of IEEE 802.11
IEEE MAC Layer
QoS limitations of IEEE
Introduction to IEEE e(QoS support)
EDCA
HCCA
Proposed Call Admission Control Algorithm and simulation result
Proposed Adaptive Contention Window Adjustment Algorithm and simulation result
Conclusion and Future Work
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3. IEEE 802.11 background information
WLAN( Wireless Local Area Network )
A LAN to which mobile users (clients) can connect and communicate by means of high-frequency radio waves rather than wires.
WLAN Standard
IEEE (IEEE)
HiperLAN (European Telecommunications Standards Institute, ETSI)
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4. IEEE 802.11 MAC Layer Two coordination functions are defined
the mandatory Distributed Coordination Function (DCF) based on CSMA/CA
optional Point Coordination Function (PCF) based on poll-and-response mechanism.
Most of today’s devices operate in the DCF mode only
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5. InterFrame Space (IFS)
Time interval between frames.
SIFS-Short IFS
PCF-PCF IFS
DIFS-DCF IFS
EIFS-Extented IFS
Fixed for each PHY
Provide priority levels
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6. Distributed Coordination Function ( DCF)
Known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)
Used During Contention period (CP)
DCF can support best-effort services , not any QoS guarantees
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7. CSMA/CA
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8. DCF(cont.) CWmin≤ CW ≤CWmax. BackoffTime = Random() × aSlotTime
Random( ) = [0, CW]
CWmin≤ CW ≤CWmax.
aSlotTime fixed for each PHY
CWnew= (CWold+ 1) ×2 -1
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9. Optinal Point Coordination Function (PCF)
Used during Contention-Free Period (CFP)
A single AP controls access to the medium, and a Point Coordinator (PC) Agent resides in the AP.
AP polls each station for data, and after a given time interval moves to the next station.
No stations are allowed to transmit unless it is polled.
AP could have a priority scheme for stations.
PCF is useful for time-sensitive applications.
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10. PCF(cont.)
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11. QoS Limitations of 802.11 DCF (Distributed Coordination Function)
Only support best-effort services
No guarantee in bandwidth, packet delay and jitter
Throughput degradation in the heavy load
PCF (Point Coordination Function)
Inefficient central polling scheme
Unpredictable beacon frame delay due to incompatible cooperation between CP and CFP modes
Transmission time of the polled stations is difficult to control
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12. Introduction to IEEE 802.11e New terminology
QAP – QoS Access Point
QSTA – QoS Station
HC – Hybrid Coordinator
In order to support QoS in WLAN , e has defined a new mechanism , namely, Hybrid Coordination Function (HCF).
HCF is implemented by all QAPs and QSTAs
HCF has two access mechanisms
Contention based
Enhanced distributed channel access (EDCA)
Controlled channel access
HCF Controlled Channel Access (HCCA)
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13. Enhanced distributed channel access (EDCA)
EDCA defines four Access Categories (AC)
Background
Best Effort
Voice
Video
EDCA supports 8 User Priority (UP) values
Priority values (0 to 7) identical to the IEEE 802.1D priorities
Rules
One UP belongs to one AC(Access Category)
Each AC may contains more than one UP
Traffic of higher UP will be transmitted first in one AC
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14. EDCA-User Priority(UP)
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15. Transmission opportunity (TXOP)
Time interval permitted for a particular STA to transmit packets.
During the TXOP, there can be a series of frames transmitted by an STA separated by SIFS.
TXOP types
EDCA TXOP initiation
Obtained by winning a successful EDCF contention
Polled TXOP (HCCA TXOP)
Obtained by receiving a QoS CF-poll frame from the QAP
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16. EDCA-Access Category (AC)
EDCF defines access category (AC) mechanism to support the priority mechanism at the non-AP QSTAs.
Each QSTA has four ACs.
An AC is an enhanced variant of the DCF which contends for transmission opportunity (TXOP) using the set of parameters such as CWmin[AC], CWmax[AC], AIFS[AC], etc.
Each AC queue works as an independent DCF STA and uses its backoff parameter.
In EDCA, the size of Contention-Window (CW) and Inter-frame space (IFS) is dependent on AC
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17. EDCA details Each AC has own
Interframe space – AIFS
Backoff Counter
CWmin, CWmax, CW
TXOP limit
QSTAs listen beacon frames to receive this information
Each QSTA implements own queues for each AC
If internal collision happens , the frame with higher priority will be sent
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18. HCF Controlled Channel Access (HCCA)
Designed to increase efficiency by reducing the contention on the medium
Using “Polling”
Like PCF
But HCCA can send “Polling” both under CFP and CP
PCF-only polling in CF
Specially assigned transmit times for every frame
Enable QoS guarantee
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19. HCCA (cont.)
Has higher priority than EDCA .Under HCCA, HC(Hybrid Coordinator) has full controll over the wireless medium
If HC needs it, it could take over the control of the medium by sending a QoS CF-Poll and the control duration is called Controlled Access Phase(CAP)
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20. An example of an 802.11e superframe
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21. The concept of proposed Call Admission Control algorithm in 802
The concept of proposed Call Admission Control algorithm in e EDCA
In e, an ADDTS request shall be transmitted by a QSTA to the HC in order to request admission of traffic by employing an AC that requires admission control.
In order to make such a request, the QSTA shall transmit a TSPEC element contained in an ADDTS request management frame with the following fields specified: Mean data rate, Minimum data rate..,etc.
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22. The concept of proposed Call Admission Control algorithm in 802
The concept of proposed Call Admission Control algorithm in e EDCA (cont.)
On receipt of an ADDTS request, the QAP shall make a determination as to whether to accept the request.
The QSTA may choose to tear down the explicit request at any time by transmitting a DELTS frame containing the TSID which specify the TSPEC to the QAP.
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23. The criteria of QAP’s determination
Base on this scenario, QAP needs a criteria to determine whether to accept the new requested flow.
We make the determination criteria depend on whether system has enough bandwidth to let the new requested flow join in.
We let the physical rate and data rate are denoted as total system bandwidth and bandwidth requirement of new requested flow respectively.
The QAP can get the information of the bandwidth requirement from TSPEC .
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24. The criteria of QAP’s determination (cont.)
QAP can not allow total system bandwidth to be used by QSTAs because the medium access mechanism is based on CSMA/CA.
Besides, two key ideas are introduced to enable QoS guarantee:
the sum of the bandwidth requirement of the active traffic streams with higher priority than new requested flow can not be influenced.
QAP should reserved minimum bandwidth requirement for traffic classes with lower priority than new requested flow in case starvation.
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25. Proposed Call Admission Control (CAC) Algorithm
We first define the terms used in the Algorithm
R – total system bandwidth, 36Mbps
α – system parameter, 0 ≦ α ≦ 1
AC[ i ]new – the mean requested bandwidth of new traffic stream i. The value of AC[i]new is obtained from the field of Mean data rate of TSPEC
AC[ j ]total_average –the sum of the mean bandwidth requirement of active traffic strams with traffic class j
AC[ j ]reserved_min –the minimum bandwidth of traffic class j reserved by QAP
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26. Algorithm detail
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27. Simulation – parameter setting
System parameters are defined as below:
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28. Simulation - traffic generation
During ON state period, the traffic of each AC is constant bit rate
We assume the request arrival rate of each AC is λ and the offered load of each AC is the same
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29. Simulation results-connection delay
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30. connection delay (cont.)
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31. Simulation results-Blocking probability
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32. Simulation results-Packet delay
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33. Packet delay (cont.)
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34. Proposed Adaptive Contention Window Adjustment (ACA) Algorithm(1/3)
Problem description
The initial values of CWmin and CW max of each kind of AC are useful for service differentiation
But the more the number of active traffic streams, the more collisions will occur, especially for higher priority ACs(voice,video) AC
CWmin
CWmax
AC_BK 15
1023
AC_BE
AC_VI 7
AC_VO 3
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35. Proposed Adaptive Contention Window Adjustment (ACA) Algorithm (2/3)
In order to solve this problem, we make QAP dynamically adjust the contention window size of each AC and broadcast the result to all QSTAs every beacon interval .
The contention window size of each kind of AC should be adaptive to system loading.
By CAC, the connection number of different ACs can be denoted as indicator of system loading.
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36. Proposed Adaptive Contention Window Adjustment (ACA) Algorithm (3/3)
How should we use the information of connection number of different ACs to dynamically adjust contention window size?
We have surveyed a paper that can help us .
Y.C. Tay , K.C.Chua , “A capacity analysis for the IEEE MAC protocol,” ACM/Baltzer Wireless Networks , Volume 7, 2001 pp
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37. Reference Material
From the paper, the authors have proved how to determine the value of CWmin according to the number of stations in a wireless cell. The formula can be written as bellow:
We employ the formula to define Adjustment Factor (AF) for each kind of AC.
The value of AF[i] depend on AIFS[i]
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38. Term Definition We first define the terms used in the Algorithm
connection_weight : the affection degree of the number of connection for each kind of AC
connection[ i ] : the number of active connection of AC[i] , 0 ≦ i ≦ 3
ratio[i] : the default proportion of CWmax to CWmin of AC[i], 0 ≦ i ≦ 3
AF[i]: the Adjustment Factor of AC[i], 0 ≦ i ≦ 3
CW[i]min_default,CW[i]max_default : the default value of CWmin and CWmax of AC[i], 0 ≦ i ≦ 3
CW[i]min, CW[i]max : the value of CWmin and CWmax of AC[i], 0 ≦ i ≦ 3
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39. Algorithm detail
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40. Simulation results – packet delay
The related simulation parameter setting is the same as the simulation of CAC algorithm
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41. Packet delay (cont.)
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42. Conclusion
In this work, we proposed two QoS provisioning mechanisms to be employed in the IEEE EDCA.
With the call admission control algorithm, we not only guarantee the QoS of established connections of higher priority ACs traffics but also protect the minimum reserved bandwidth for lower priority ACs traffics.
With the adaptive contention window adjustment algorithm, we dynamically adjust the contention window size of each AC according to the established connection number of different ACs.
Performance results via simulations have demonstrated the advantages of employing these two mechanisms in the IEEE WLANs.
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43. Future work
Comparisons with other related research work about call admission control and adaptive contention window adjustment are
necessary
A comprehensive call admission control mechanism in mixed EDCA and HCCA is not available
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