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802.11e Wireless LANs

Speaker ： Fu-Yuan Chuang Advisor ： Ho-Ting Wu Date ： 2007.01.02. 802.11e Wireless LANs. Outline. Introduction to IEEE 802.11e Tuning of 802.11e Network Parameters Adaptive Contention-Window MAC Algorithms for QoS-Enabled Wireless LANs. Introduction to IEEE 802.11e. New terminology

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802.11e Wireless LANs

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

  2. Outline • Introduction to IEEE 802.11e • Tuning of 802.11e 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 802.11e -- 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-Pollframe 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 802.11D 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. ACM : admission control mandatory ACI : Access category identify EDCA Parameter Set element

  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

  13. Juliana Freitag, Nelson L. S. da Fonseca, and Jos´e F. de Rezende, “Tuning of 802.11e Network Parameters,” IEEE Communications Letters , Volume 10,  Issue 8,  Aug. 2006 Page(s):611 - 613 Tuning of 802.11e Network Parameters

  14. Introduction • A novel mechanism for tuning the access parameters of 802.11e 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 802.11e 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

  16. 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

  17. 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

  18. Mechanism • Load最小的Class將其TXOP設為0 • 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

  19. 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

  20. 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

  21. 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. 368- 374, June 2005 Adaptive Contention-Window MAC Algorithms forQoS-Enabled Wireless LANs

  22. Saturation Throughput

  23. 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)

  24. 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

  25. The Throughput Derivative Algorithm

  26. References • IEEE-802.11WG, “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 802.11e Network Parameters,” IEEE Communications Letters , Volume 10,  Issue 8,  Aug. 2006 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. 368- 374, June 2005

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