1. 1 Real-Time Traffic over the IEEE 802.11 Medium Access Control Layer Tian He

2. 2 Outline Motivation & Research issues Existing Solutions Access Procedures of Real-Time Station Dynamics of Real-Time station Simulation Result Conclusions

3. 3 Motivation Real time traffic is ever more important – VOIP Market $5b by 2005 – Videoconferencing ever more popular – Myriad of streaming services: VOD, Video Phone, D-Sharing. Major service under wireless network. – Currently data is dominate service in wired network, while Data is “special service” in wireless network. real-time streaming is dominate market in wireless environment IP networking towards wireless, mobile environment. Inherently it is a Interesting research problem

4. 4 Required QoS Real time traffic is not too sensitive to delay – ~400ms for VOIP, ~250ms for video conferencing Very sensitive to jitter – As little as 150ms can be unpleasant Effect of lost packets is strongly codec dependant.

5. 5 Even harder in wireless Narrow bandwidth available – 802.11a:54Mbps 802.11b:11Mbps 802.11g/e:22Mbps – IEEE 802.3ae : 10Gbps 909 times faster High control overhead – large synchronization fields – larger MAC headers 34B vs 14B in 802.3 – more management packets (AP registration) Inherent contention media (open space)

6. 6 Why not 802.11 DCF A wants to transmit but channel is busy B A C Packet to Node C RTS CTS Contention slots ACK positive acknowledgment

7. 7 Existing solution 802.11 centralized approach: PCF to guaranteed QoS.

8. 8 Why not 802.11 PCF Centralized Scheme – Single point of failure – Single media, no space multiplexing – High overhead. (Registration, Polling ) – In-compatible with multi-cell setting.

9. 9 Other Solutions for Real-time Time Division Multiple Access (TDMA) – Fixed Slotting: Inefficient – dynamic Slotting: complex scheduling algorithm Code Division Multiple Access (CDMA) – Fixed Coding: inefficient – Dynamic Coding: needs dynamic code assignment Token Ring Passing – Only suitable for single contention media

10. 10 Key Idea in this paper DCF mode for data stations. Special mode for real-time stations. Real-time stations have priority over data station by using shorter IFS. Real-time stations proactively send “black bursts”, of length proportional to waiting time. Guarantee one and only one wins

11. 11 Definitions

12. 12 How it Works: Data Station CSMA/CA as access procedure. Contention Window Backoff-Window T long Busy Medium T long T short

13. 13 How it Works: Real-time station Prioritization. Contention Window Backoff-Window T long Busy Medium T long T short T med

14. 14 How it Works (cont.) Round Robin among real-time station T med T long Busy Medium T obs Backoff T med T long Busy Medium T obs Data Real-time Station A Real-time Station B Schedule Time

15. 15 How it Works (cont.) How to set T obs. – T obs must be shorter than a black burst slot, otherwise we station A will not back off. – Tobs must be shorter than T med, so that no real-time station will access channel during observation. Real-time Station B T med T long Busy Medium T obs Backoff T med T long Busy Medium T obs Data Real-time Station A Schedule Time

16. 16 How it Works (cont.)

17. 17 A implicit assumption in the paper No two real-time station schedule packet at same time. T med T obs Data Real-time Station A Schedule Time T med T obs Data Real-time Station B

18. 18 Negative Acknowledgement Positive acknowledgement occurs an efficiency penalty. The receiver expects a packet after t sch +t obs. When we do not receive the expected packet, we may define t neg to be the time we send a negative acknowledgement. Will contend to send packet in real time.

19. 19 Negative Acknowledgement Robustness against hidden stations Stations near receiver may not know of the sender, and will block the receiver. Sending negative acknowledgement messages will silence these hidden stations.

20. 20 InteractionReal-Time station If no data station If there are data stations – Packet from data STA will “perturb” the channel – Real-Time STA will contend the channel by sending black-burst Finally, real-time STA will recover from perturbation to a state with no longer access delay (Stabilization)

21. 21 Stability Definition 1: – The system is stable if and only if whatever the initial conditions, there is an L >=0, such that the access delay is zero after L rounds Definition 2: – The system is unconditionally stable if and only if it is stable no matter the magnitude of the perturbation, T

22. 22 Stability (contd)

23. 23 Nominal values

24. 24 Simulation Result (1)

25. 25 Simulation Result (2)

26. 26 Simulation Result (3)

27. 27 Simulation Result (4)

28. 28 Conclusions Distributed access Higher priority to access the channel Can be overlaid on 802.11 without changing data stations Virtual TDM access structure for real- time stations  constant access rate. Under stable condition  bounded access delay

29. 29 Unsolved issue Possible contention between real-time stations Fix and signal access interval for real- time station Only consider initial disturbance T. No stable analysis for periodic or sporadic disturbance. It is a open question whether system is stable under such condition