1. Speaker：Fu-Yuan Chuang Advisor：Ho-Ting Wu Date：2007.01.02
802.11e Wireless LANs
Speaker：Fu-Yuan Chuang
Advisor：Ho-Ting Wu
Date：

2. Outline Tuning of 802.11e Network Parameters
Introduction to IEEE e
Tuning of e Network Parameters
Adaptive Contention-Window MAC Algorithms for QoS-Enabled Wireless LANs

3. Introduction to IEEE 802.11e New terminology
QAP – QoS Access Point
QSTA – QoS Station
HC – Hybrid Coordinator
A new mechanism defined in IEEE e -- 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)

4. Architecture

5. Comparison of 802.11 and 802.11e CAP controlled access phase :
A time period when the HC maintains control of the medium after gaining medium access by sensing the channel to be idle for a PIFS duration

6. Transmission Opportunity (TXOP)
Under HCF, TXOP is basic unit of transmission
TXOP types
EDCA TXOP
Obtained by a QSTA winning an instance of EDCA contention during the CP
HCCA TXOP
Obtained using the controlled channel access
Polled TXOP
by a non-AP QSTA receiving a QoS (+)CF-Poll frame during the CP or CFP

7. Enhanced Distributed Channel Access (EDCA)
EDCA defines four Access Categories (AC)
Voice
Video
Best Effort
Background
EDCA supports 8 User Priority (UP) values
Priority values (0 to 7) identical to the IEEE D 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 within one AC

8. EDCA-Access Category (AC)
Four access categories (ACs) that support the delivery of traffic with differentiated UPs
An AC is an enhanced variant of the DCF which contends for TXOP using the following parameters: CWmin[AC], CWmax[AC], AIFS[AC].
Each AC queue functions as an independent DCF STA and uses its own backoff parameter.
In EDCA, the Contention-Window (CW) size and the Inter-frame space (IFS) is AC dependent

9. EDCA Details Each AC has its own Interframe space – AIFS
Back off Counter
CWmin, CWmax, CW
TXOP limit
QSTAs obtains these info from beacon frames
Each QSTA implements own queues for each AC
If internal collision happens, the frame with higher priority will be sent

10. EDCA Parameter Set element
ACM : admission control mandatory
ACI : Access category identify

11. Contentions among Different ACs in EDCA
Contention among EDCAFs (AC, AIFS, CWmin , CWmax ) to win a TXOP

12. Default Values for Each AC
AIFS[AC] = AIFSN[AC] × aSlotTime + aSIFSTime.
DIFS=2*aSlotTime +aSIFSTime AC
AIFSN
CWmin/CWmax
AC_VO 2
3-7
AC_VI
7-15
AC_BE 3
AC_BK 7

13. Tuning of 802.11e Network Parameters
Juliana Freitag, Nelson L. S. da Fonseca, and Jos´e F. de Rezende, “Tuning of e Network Parameters,” IEEE Communications Letters , Volume 10,  Issue 8,  Aug Page(s):

14. Introduction
A novel mechanism for tuning the access parameters of e QAP and QSTAs
To solve the asymmetry problem
To produce balanced uplink and downlink delays
The network can operate under much higher loads

15. Introduction - asymmetry problem
the existing asymmetry between the uplink and downlink delays which occurs when using the e contention method
The QAP is responsible for forwarding all traffic to/from QSTAs
Since both QAP and QSTAs have the same probability of accessing the medium, the queues in the QAP can rapidly build up, increasing the downlink delay

17. The Proposed Approach
The adjustment of TXOP value is used to improve the throughput of the classes at the QAP leading to more balanced delay

18. Idea - TXOP 假設每個QSTA在每個Class中有一個flow，第i個Class的TXOP允許q個frames被傳送
則在QAP中第i個Class的TXOP的值調整為可以傳送k*q個frames (k為第i個class中downlink flow的數目)
可讓downlink throughput接近uplink throughput

20. Mechanism Load最小的Class將其TXOP設為0 其他class根據一個ratio調整TXOP
只可傳送一個frame
其他class根據一個ratio調整TXOP
The ratio between the load that has arrived at their queues and the load that has arrived at the queue of the class with the lowest load
可讓load大的queue獲得較大的傳送時間

22. CW 當STA數目不多時，low CWmin 可以減少idle的時間並增進channel utilization
當STA數目過多時，high CWmin 可以避免collision
As the number of stations with active flows of a certain class increases, the CWmin value of this class should increase as well as that of all classes with lower access priority

23. Mechanism
若STA(在class i中有active flows)數目大於CWmin(i)，則CWmin(i) is increased to the next power of 2 minus 1
反之，則CWmin(i) value is reduced to the immediate lower power of 2，縮短idel的時間

26. Adaptive Contention-Window MAC Algorithms for QoS-Enabled Wireless LANs
Samer El Housseini, Hussein Alnuweiri, “Adaptive Contention-Window MAC Algorithms for QoS-Enabled Wireless LANs,” wireless Networks, Communications and Mobile Computing, 2005 International Conference on , Vol.1,  pp , June 2005

27. Saturation Throughput

28. The Throughput Derivative Algorithm
after each CW change, the TD Algorithm employs measurements of the throughput taken in the AP
CWmax is also changed while keeping the same ratio (CWmax / CWmin)

29. The Throughput Derivative Algorithm
The throughput derivative is taken over the present and a few past measurements of the throughput
If the derivative is positive
The AP continues to increase the CW
If the derivative is negative
The AP decrease the CW

30. The Throughput Derivative Algorithm

31. References
IEEE WG, “IEEE Standard for Information technology - Telecommunications and information exchange between systems - Local and metropolitan area networks - Specific requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications Amendment 8: Medium Access Control (MAC) Quality of Service Enhancements,” November 2005
Juliana Freitag, Nelson L. S. da Fonseca, and Jos´e F. de Rezende, “Tuning of e Network Parameters,” IEEE Communications Letters , Volume 10,  Issue 8,  Aug Page(s):611 – 613
Samer El Housseini, Hussein Alnuweiri, “Adaptive Contention-Window MAC Algorithms for QoS-Enabled Wireless LANs,” wireless Networks, Communications and Mobile Computing, 2005 International Conference on , Vol.1,  pp , June 2005