A Tool for Simulating IEEE 802.11e Contention-based Access

1. A Tool for Simulating IEEE 802.11e Contention-based Access Andreas Floros, Theodore Karoubalis

2. Introduction • WLAN QoS support through the IEEE802.11e specification • Contention-based access mechanism (EDCA) • Controlled-access mechanism (HCCA) • In this work, an EDCA simulation tool is presented • Basic simulation targets • Backoff mechanism • Traffic differentiation scheme efficiency • Admission control scheme • Advanced mechanisms for service adaptation • i.e. upon heavy link degradation • Optional mechanisms • Power-save, direct link etc

3. EDCA overview • Traffic differentiation scheme • Maps 8 user priorities to 4 Access Categories (ACs) • AC_BE, AC_BK, AC_VI, AC_VO • Each AC can be considered as an independent backoff entity • contends for medium access, using a set of EDCA parameter values

4. Simulation platform description • MS-Windows based application • Basic design-phase requirements • User friendly application interface • No special description language required (such as TCL/TK) • Hybrid event and time driven simulation core • Efficient handling of protocol signaling • Improved simulation execution times • Support of all mandatory EDCA mechanisms • TSPEC-based traffic source modeling • Performance evaluation using legacy and advanced networking criteria

5. Simulation platform description (cont’d) • Main application window

6. Defining simulation scenarios • Number and properties of QSTAs • Radio type (802.11b or .11g) • Local scheduler type • Number and TSPECs of active TS per QSTA • Maximum 4 • QAP properties • Radio type (802.11b or .11g) • Global EDCA parameters • Admission Control Scheme • Simulation duration • Channel error model • Other simulation parameters • i.e. TXOP continuation

7. Defining simulation scenarios (cont’d) • Traffic Specifications definition

8. Defining simulation scenarios (cont’d) • QAP properties

9. Defining simulation scenarios (cont’d) • Wireless channel model and QSTA properties

10. Scenario execution • Scenario validation through a pre-processor stage • Very fast execution times • Hybrid simulation core • Less than 10 seconds for 100 seconds scenarios

11. Simulation results and output • Simulation analysis • Represents a detailed analysis of all EDCA-related events occurred • Can be exported in .html format

12. Simulation results and output (cont’d) • Advanced statistics • Organized by AC, TS or QSTA • Plot window • Displays throughput, medium occupancy as a function of time

13. Simulator accuracy validation • Verification using WFA EDCA tests • Used for ensuring different vendor products interoperability • Included in the WME pre-standard • Traffic differentiation tests • AC fairness tests • All tests performed for • Ideal wireless channel • Non-ideal wireless channel (30% packet loss probability) • TSPECs defined by WFA • Optional TXOP continuation was also checked

14. Traffic differentiation test • Test description • two streams belonging to different ACs and QSTAs • the high priority traffic does not exceed the link capacity • the low priority stream provides enough traffic load, saturating the wireless link • PHY transmission rate equal to 1Mbps • Aim of the test is to confirm differentiation of streams belonging to different ACs

15. Traffic differentiation test (cont’d) • Due to channel saturation, the total number of TXOPs is almost constant in both wireless link cases • The high priority stream gets the bandwidth needed, while the low priority traffic gets only the bandwidth left • The high priority traffic packet delay is very low, even in the case of non-ideal wireless channel • Traffic differentiation is achieved!

16. AC fairness test • Test description • Two identical traffic flows mapped to equal AC priorities • The total intended load exceeds the channel capabilities • PHY transmission rate equal to 1Mbps • Aim of the test is to confirm that different streams belonging to the same AC obtain the same transmission rights

17. AC fairness test (cont’d) • Both streams get half of the link effective bandwidth • The same stands for non-ideal wireless channel • Although the throughput and packet delay performance is significantly degraded • AC fairness is achieved!

18. Case study: TXOP continuation test • Simulation scenario employed • PHY rate equal to 11Mbps • Aggregated bandwidth occupancy equal to 98%

19. Case study: TXOP continuation test (cont’d) • For ideal wireless links • The measured overall throughput is increased by nearly 27% • The additional throughput corresponds to lower priority ACs (AC_VI) • An increment of the packet delay is observed, due to the extended duration of the granted TXOPs • The total number of medium collisions is significantly decreased

20. Case study: TXOP continuation test (cont’d) • For non-ideal wireless links • Compared to the ideal wireless link case • the throughput increment with TXOP continuation is lower • the packet delay also slightly increases.

21. Conclusions • A contention-based EDCA simulation tool was presented • realizes a user friendly interface for defining the simulation scenarios • provides fast simulation execution times • The simulation output is realized using easy-to-use graphical interfaces and well-established formats • The accuracy of the tool was verified using WFA interoperability test • The effect of the TXOP continuation scheme was examined as a case study

22. Questions;

23. E-mail: floros@ionio.gr www.ionio.gr