The Study of the QoS Provisioning Mechanisms in IEEE 802.11e Wireless LANs

1. The Study of the QoS Provisioning Mechanisms in IEEE 802.11e Wireless LANs Student : Min-Hua Yang Advisor : Ho-Ting Wu Date : 2006.7.30

2. Outline • Background of IEEE 802.11 • IEEE 802.11 MAC Layer • QoS limitations of IEEE 802.11 • Introduction to IEEE 802.11e(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

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 802.11 (IEEE) • HiperLAN (European Telecommunications Standards Institute, ETSI)

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 802.11 devices operate in the DCF mode only

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

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

7. CSMA/CA

8. BackoffTime = Random() × aSlotTime Random( ) = [0, CW] CWmin≤ CW ≤CWmax. aSlotTime fixed for each PHY CWnew= (CWold+ 1) ×2 -1 DCF(cont.)

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-sensitiveapplications.

10. PCF(cont.)

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

12. Introduction to IEEE 802.11e • New terminology • QAP – QoS Access Point • QSTA – QoS Station • HC – Hybrid Coordinator • In order to support QoS in 802.11 WLAN , 802.11e 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)

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

14. EDCA-User Priority(UP)

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

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

17. 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 EDCA details

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

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)

20. An example of an 802.11e superframe

21. The concept of proposed Call Admission Control algorithm in 802.11e EDCA • In 802.11e, 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.

22. The concept of proposed Call Admission Control algorithm in 802.11e 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.

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 .

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.

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

26. Algorithm detail

27. Simulation – parameter setting • System parameters are defined as below:

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

29. Simulation results-connection delay

30. connection delay (cont.)

31. Simulation results-Blocking probability

32. Simulation results-Packet delay

33. Packet delay (cont.)

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)

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.

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 802.11 MAC protocol,”ACM/BaltzerWireless Networks , Volume 7, 2001 pp.159-171

37. 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. Reference Material • The value of AF[i] depend on AIFS[i]

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

39. Algorithm detail

40. Simulation results – packet delay • The related simulation parameter setting is the same as the simulation of CAC algorithm

41. Packet delay (cont.)

42. Conclusion • In this work, we proposed two QoS provisioning mechanisms to be employed in the IEEE 802.11 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 802.11 WLANs.

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