IEEE 802.11e QoS on WLANs

1. IEEE 802.11eQoS on WLANs Speaker : Min-Hua Yang Advisor : Ho-Ting Wu Date: 10/25/05

2. Overview • 802.11 background information • DCF • PCF • Qos limitations of 802.11 • 802.11e (QoS support) • EDCA • HCCA • Simulation result

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

5. IEEE 802.11 Topology • Basic Service set networks (BSS) • Independent BSS networks • Infrastructure BSS • Extended Service Set (ESS) networks

6. The Basic Service Set (BSS) • The BSS consists of a group of any number of stations. • The basic building block of IEEE 802.11 LAN

7. Independent BSS (IBSS) network • stations communicate directly • no connection to a wired network • Ad hoc network

8. Infrastructure BSS (BSS) • Include an access point (AP) • The AP may have connection to an wired network • All stations communicate with the AP • Communication between stations must go through AP • consume twice the bandwidth

9. The Extended Service Set(ESS) • Distribution system (DS) provides logical services necessary to handle address-to-destination mapping and seamless integration of multiple BSSs • An AP is a STA that provides access to the DS

10. Terminology- Interframe Space • Time interval between frames. • SIFS – Short Interframe Space • PIFS – PCF Interframe Space • DIFS – DCF Interframe Space • DIFS > PIFS > SIFS • Fixed for each PHY • Provide priority levels

11. IEEE 802.11 • 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.

12. Distributed Coordination Function( DCF) • Possible in both Infrastructure and Ad hoc mode • 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.

13. Basic Access Mechanism-CSMA/CA • Sense the medium before transmitting. • If the medium is not busy, the transmission may proceed. • If the medium is busy, invoke backoff procedure. • Transmit after the backoff procedure • If the transmission is not successful, invoke backoff procedure.

14. Virtual Carrier Sense Mechanism • Network Allocation Vector (NAV) • Set NAV based on duration information (available in MAC header, RTS, CTS) • NAV counts down to zero at a uniform rate. • IF NAV=0, medium is idle; otherwise, it is busy.

15. Basic Access Mechanism-CSMA/CA

16. Example of backoff intervals

17. IEEE 802.11 DCF (cont.) • BackoffTime = Random() × aSlotTime where • Random( ) = [0, CW] • CWmin≤ CW ≤CWmax. • aSlotTime fixed for each PHY • CWnew= (CWold+ 1) ×PF –1 (where PF=2) • PF : Persistence Factor

18. A example of CW

19. Other Collision Avoidance Mechanism – RTS/CTS • Hidden Terminal Problem: • Hidden Terminal are STAs that the receiver can hear but that cannot be detected by other senders. • In order to solve the problem,a optional RTS/CTS is introduced. • The source sends a short RTS frame before each data transmission,and the receiver replies with a CTS frame if it is ready to receive.

20. Basic access scheme v.s RTS/CTS access scheme

21. Point Coordination Function (PCF) [Optional in Standard] • 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.

22. PCF(cont.)

23. IEEE Legacy 802.11 MAC • SuperFrame consists of Contention Period (CP) and Contention Free Period (CFP). • PCF used during CFP and DCF used during CP.

24. 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

25. IEEE 802.11e – Overview • 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)

26. IEEE 802.11e – MAC frame format

27. IEEE 802.11e Overview - 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 • Each AC may contains more than one UP • Traffic of higher UP will be transmitted first in one AC

28. IEEE 802.11e Overview-User Priority (UP) • 8 User Priority • Identical to IEEE 802.1D priority tags.

29. Basic concepts – 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

30. 802.11e EDCF – Access Category • 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

31. EDCA use different IFSs SIFS – ACKs, between multiple frames with the continuation EDCA TXOP DIFS / AIFS – Used by DCF and EDCA for different access categories EDCA – Accessing the medium

32. EDCF - Arbitration Interframe Space (AIFS) • QSTA use AIFS to defer the contention window or transmission for each AC • AIFS[AC] = AIFSN[AC]x aSlotTime+ aSIFTime • AIFSN for each AC is broadcast via beacon frame containing ‘EDCA Parameter Set’element • DIFS = 2 x aSlotTime+ aSIFTime

33. EDCF – Some elements of the Beacon frame

34. Default EDCF Parameter Set

35. Each AC has own Interframe space – AIFS Backoff Counter (BO) CWmin, CWmax, CW TXOP limit QSTA listens beacon frames to receive this information Each QSTA implements own queues for each AC traffic From the queues the frame with the highest priority is sent if internal collision happens EDCA details

36. Legacy 802.11 station and 802.11e station with four ACs within one station.

37. 802.11e HCCA Overview - Hybrid Coordination Function(HCF) • Designed to increase efficiency by reducing the contention on the medium • Uses ”polling” • Like PCF • Thus, HCCA can send ”polling” both under CFP and CP • PCF – Only polling in CP • Specifically assigned transmit times for every frame • Enables QoS guarantees

38. 802.11e 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 controll of the medium by sending a QoS CF-Poll

39. 802.11e HCCA (cont.) • Differences between hybrid coordinator (HC) and point coordinator (PC): • HC can poll QSTAs in both CP and CFP • HC grants a polled TXOP to one QSTA, which restricts the duration of the QSTA’s access to the medium.

40. An example of an 802.11e superframe

41. Improve Efficiency – 802.11e • Block Acknowledgment • Send multiple MSDUs without a bunch of ACKs • Group ACK • Direct Link Protocol (LDP) • No support for DLP in legacy MAC – Needs to talk through the AP • Less use of the channel • The STAs is enable to talk directly to each other • Sounds easy, lots of things to think about: • Power saving • Security

42. Simulation-QoS , does it work? • EDCF Parameters for three queues

43. EDCF maintains the throughput of high-priority audio and video flows by punishing the background traffic. Throughput and delay performance for EDCF Throughput(KB/s)

44. The total throughput of EDCF is lower than that of DCF when the traffic load is larger than 48%:EDCF reduces the throughput of low-priority flows considerable and therefore results in decreasing the total throughput. Comparison of total throughput between EDCF and DCF

45. The result show that the HCF controlled channel access mechanism can guarantee the minimum delay requierment(50ms) for the admitted flows in different load rate EDCF works very well under low load conditions but suffers from delay degradation in high-load condition Mean delay of audio,CBR video versus channel load for EDCF and HCF

46. Reference • Qiang Ni, Lamia Romdhani, and Thierry Turletti. "A Survey of QoS Enhancements for IEEE 802.11 Wireless LAN". Wiley Journal of Wireless Communication and Mobile Computing (JWCMC), John Wiley and Sons Ltd., 2004; Volume 4, Issue 5: 547-566. • “Analysis of IEEE 802.11e for QoS support in wireless LANs”Mangold, S.; Sunghyun Choi; Hiertz, G.R.; Klein, O.; Walke, B.;Wireless Communications, IEEE Volume 10,  Issue 6,  Dec. 2003 Page(s):40 - 50 • Yu-Sun Liu Ph. D. WLAN上課講義 • Google.com