1. Methods for providing Quality of Service in WLANs W.Burakowski, A. Beben, J.Sliwinski Institute of Telecommunications, Warsaw University of Technology, Poland

2. Slide 2 COST 279 Final Seminar, Lisbon, 27-29 June 2005 Outline  Class of Service (CoS) concept for providing end_to_end QoS in heterogeneous and multi- domain networks  How to provide CoSs in WLANs  Method for improving packet delay characteristics in WLANs  Summary

3. Slide 3 COST 279 Final Seminar, Lisbon, 27-29 June 2005 Problem statement  Objective: Assuring strict end_to_end QoS guarantees at the packet layer  Network environment: heterogeneous and multi-domain networks  Different access network technologies: WiFi, xDSL, UMTS, LAN/Ethernet  IP core  To follow concept of Classes of Service (acc. to the IETF proposal) „Service class” represents a set of traffic that requires specific delay, loss and jitter characteristics from the network for which a consistent and defined per hop-behaviour applies

4. Slide 4 COST 279 Final Seminar, Lisbon, 27-29 June 2005 CoSs: IETF proposal (1) 11 Basic CoSs and 4 aggregated CoSs

5. Slide 5 COST 279 Final Seminar, Lisbon, 27-29 June 2005 CoSs: IETF proposal (2), exemplary applications

6. Slide 6 COST 279 Final Seminar, Lisbon, 27-29 June 2005 QoS requirements from chosen applications Different QoS requirements with respect to: throughput delay delay variation loss ratio

7. Slide 7 COST 279 Final Seminar, Lisbon, 27-29 June 2005 Exemplary set of CoSs Basic CoSs – visible by the users and can be deployed in some access networks (e.g. in LAN/Ethrenet) Aggregated CoSs – can be deployed in some parts of the networks (e.g. WLANs, inter-domain links)

8. Slide 8 COST 279 Final Seminar, Lisbon, 27-29 June 2005 CoSs: An example of deployment in network

9. Slide 9 COST 279 Final Seminar, Lisbon, 27-29 June 2005 CoSs: An example of deployment in network

10. Slide 10 COST 279 Final Seminar, Lisbon, 27-29 June 2005 CoSs in WLANs (1) Ethernet xDSL Cable Access Point Edge Router Possible bottlenecks

11. Slide 11 COST 279 Final Seminar, Lisbon, 27-29 June 2005 CoSs in WLANs: status Currently available Near future: 802.11e (September 2005 ?) DCF - no QoS - contention based operation - widely deployed PCF -irregular polling -no equipment -not deployed (!) EDCAHCCA - relative QoS - just hitting the market - polling based - available in long term WLAN Standard IEEE 802.11 DCF – Distributed Coordination Function PCF – Point Coordination Function EDCA - Enhanced Distributed Channel Access HCCA - Hybrid Coordinated Channel Access

12. Slide 12 COST 279 Final Seminar, Lisbon, 27-29 June 2005 CoSs in WLANs: status Investigated approaches for handling QoS traffic in WLANs:  Tuning of MAC parameters to get „a priority” of QoS traffic over the best effort  Enhancement of MAC layer for improving polling mechanism Any of the proposed approaches is not sufficient for supporting CoSs

13. Slide 13 COST 279 Final Seminar, Lisbon, 27-29 June 2005 CoSs in WLANs: Mechanisms in 802.11  DCF access  Distributed, random, contention based access  Assures „equal” access to medium No traffic differentiation, no QoS guaranties – a single best effort service

14. Slide 14 COST 279 Final Seminar, Lisbon, 27-29 June 2005 Proposed solution (1)  Use 802.11 DCF mode

15. Slide 15 COST 279 Final Seminar, Lisbon, 27-29 June 2005  To separate in a physical way the QoS traffic from the best effort traffic by keeping two APs – one for QoS traffic and one for best effort traffic Proposed solution (1)  Use 802.11 DCF mode

16. Slide 16 COST 279 Final Seminar, Lisbon, 27-29 June 2005 General strategy:  Terminal has access to 2 associated APs  One for BE traffic  One for QoS traffic (RT and N-RT)  Problems to be solved:  Switching between APs  Differentiation between „real” and „non-real” time traffics  How to perform admission control Proposed solution (2)

17. Connection scenario AP-BE AP-QoS TTTTT Switch HUB Router RM WiFi-QoS topology SIP proxy RM – Resource Manager

18. Connection scenario AP-BE AP-QoS TTTTT Switch HUB Router RM QoS request Terminal sends by AP- BE its QoS request to SIP Proxy SIP proxy

19. Connection scenario AP-BE AP-QoS TTTTT Switch HUB Router RM QoS request SIP Proxy sends the QoS request to RM where CAC is performed SIP proxy

20. Connection scenario AP-BE AP-QoS TTTTT Switch HUB Router RM RM reads the MAC address

21. Connection scenario AP-BE AP-QoS TTTTT Switch HUB Router RM RM adds the MAC address to „allowed” list

22. Connection scenario AP-BE AP-QoS TTTTT Switch HUB Router RM RM adds the MAC address to „denied”

23. Connection scenario AP-BE AP-QoS T TTTT Switch HUB Router RM QoS confirmation Terminal is attached to the AP-QoS and QoS confirmation is send

24. Slide 24 COST 279 Final Seminar, Lisbon, 27-29 June 2005 Traffic handling: Differentiation of RT and NRT  The bottleneck is DOWNLINK only  Need to implement QoS mechanisms on the top of MAC:  Separate queues for RT and NRT  Policers (RT) and shapers (NRT)  Admission Control

25. Slide 25 COST 279 Final Seminar, Lisbon, 27-29 June 2005 Admission Control  Engineering approach  The main idea: WLAN is stable until the offered load does not exceed serving capabilities PPS – Packets per second

26. Slide 26 COST 279 Final Seminar, Lisbon, 27-29 June 2005 Simulation results (1)  Single AP QOS with a number of terminals  Two types of streams:  RT service: symmetrical voice connection G.729 codec; 20 ms packet inter-arrival time (50 pkt/s) for each direction; 60 bytes packets (IP+UDP+RTP+payload).  NRT service: Greedy TCP flows in downstream direction only, 512 kbit/s shaped throughput;

27. Slide 27 COST 279 Final Seminar, Lisbon, 27-29 June 2005 Simulation results (2) Conclusions:  Relatively large delay variation is caused by „random access” mechanism in MAC  To fulfill e2e QoS requirements additional mechanism may by required! (a) average delay(b) 99.9% quantile of IPDV Acceptance region

28. Slide 28 COST 279 Final Seminar, Lisbon, 27-29 June 2005 Method for improving delay characteristics in WLANs (1)  Delay variation in WLANs is caused by transmission backoffs and collisions coming from MAC protocol behaviour  To overcome we need synchronisation between streams, like in TDMA

29. Slide 29 COST 279 Final Seminar, Lisbon, 27-29 June 2005 Method for improving delay characteristics in WLANs (2)  The „self synchronisation” mechanism enforces synchronization between CBR sources by introducing for each CBR stream different initial delay  D i  The value of  D i is fixed independently in each station based on the last successful transmission as the interval between the moment when packet transmission starts and the moment of its arrival

30. Slide 30 COST 279 Final Seminar, Lisbon, 27-29 June 2005 Simulation results (1)  Single AP QOS with a number of terminals  Only VoIP streams:  RT service: symmetrical voice connection G.729 codec; 20 ms packet inter-arrival time (50 pkt/s) for each direction; 60 bytes packets (IP+UDP+RTP+payload).

31. Slide 31 COST 279 Final Seminar, Lisbon, 27-29 June 2005 Simulation results (2) standard WLAN enhanced WLAN Conclusion: The method assures constant delay! Histogram of the transfer delay differences between two consecutive packets after synchronisation phase (11 VoIP streams)

32. Slide 32 COST 279 Final Seminar, Lisbon, 27-29 June 2005 Numerical results (3) VoIP streams start transmission at the random moments Cumulative distribution function of the synchronization time VoIP streams start transmission at the same time (worst case) Conclusion: For the random start synchronization is reached quickly

33. Slide 33 COST 279 Final Seminar, Lisbon, 27-29 June 2005 Summary  Providing e2e QoS requires to assure QoS in each part of the network  The concept of CoSs is exploited  Providing CoSs in WLANs requires:  Separation between QoS and best effort traffics  Additional traffic handling mechanisms  Application of admission control  The solution is verified by simulation and is planned for implementation  We can get excellent delay charactaristics for VoIP by adding „self synchronisation” mechanism