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Real-Time Traffic over the IEEE 802.11 Medium Access Control Layer

Real-Time Traffic over the IEEE 802.11 Medium Access Control Layer. J. Sobrinho and A. krishnakumar. Tian He. Outline. Motivations Possible approaches A proposed solution in the paper Evaluations Conclusions & Comments. Motivation. Real time applications are ever more popular

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Real-Time Traffic over the IEEE 802.11 Medium Access Control Layer

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  1. Real-Time Traffic over the IEEE 802.11 Medium Access Control Layer J. Sobrinho and A. krishnakumar Tian He

  2. Outline • Motivations • Possible approaches • A proposed solution in the paper • Evaluations • Conclusions & Comments

  3. Motivation • Real time applications are ever more popular • VOIP Market $5b by 2005. • Myriad of streaming services: VOD, Video Phone, D-Sharing. Videoconferencing. • 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. 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. • VOIP specification require average e2e delay 145ms. • Effect of lost packets is strongly codec dependant.

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

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

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

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

  10. Key ideas 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 real-time station wins for each contention phase.

  11. Assumption Every node can sense each other’s transmissions (no hidden/exposed terminal problem). No RTS/CTS is used. Real-time stations periodically send out packets at same rate.

  12. Definitions

  13. Access procedures • Single data station access procedure • Interactions among data stations • Single real-time station access procedure • Interaction among real-time stations • Interaction between data stations and real-time stations

  14. Busy Medium Tlong DATA Tshort ACK t 1. Single data station access • CSMA/CA as access procedure. Tlong Contention Window Tshort A DATA Backoff-Window A

  15. Difference from 802.11 Standard • Data stations keep sensing the channel even no packets ready for transmission. 802.11 only senses the channel when need. • It use the pervious channel status to decide whether back-off or transmit immediately.802.11 needs DIFS delay before make a decision. This scheme consumes more energy , but has shorter delay.

  16. Busy Medium Busy Medium t 2. Interactions among data stations • CSMA/CA as access procedure. Tlong Contention Window Tshort A DATA Backoff-Window Tlong Contention Window Tshort B Backoff-Window

  17. Busy Medium Tobs 3. Single real-time station access Tlong Contention Window Tmed Tlong Tshort A DATA Tmed Tobs Tshort A DATA ACK

  18. Busy Busy Busy Busy Busy Busy Tmed Tmed Tobs Tobs Data Tmed Tmed Tobs Data 4. Interaction among real-time stations • Round robin access among real-time stations A B

  19. Busy Medium Busy Medium 4.Interaction among real-time stations • How to set Tobs. • Tobs must be shorter than a black burst slot, otherwise we station A will not back off. • Tobs must be shorter than Tmed , so that no real-time station will access channel during observation. A Tobs Tmed Tlong Data B Schedule Time Tmed Tobs Tlong Data

  20. 5.Interaction between real-time and data stations

  21. Negative Acknowledgement • Positive acknowledgement has an efficiency penalty. • When receiver gets a packet from sender at time T, it expects another packet at time: T+ tsch+tobs. • When receiver do not receive the packet at expected time interval, it sends out a negative acknowledgement.

  22. Theoretical Analysis: Stability • Definition 1: • The system is stable if and only if whatever the initial conditions is, there is an L >=0, such that the access delay for real-time station is zero after L rounds (converge) • Definition 2: • The system is unconditionally stable if and only if it is stable no matter the magnitude of the perturbation T (Overshoot independent)

  23. Stability (contd) DATA Delay Delay

  24. Stability conditions • The system is unconditionally stable if and only if following inequality holds N is #real-time station (1) • In addition if , the system is stable if and only if following inequality holds T is initial disturbance (2)

  25. Nominal values

  26. Simulation Result (1)

  27. Simulation Result (2)

  28. Simulation Result (3)

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

  30. Critical comments • Possible data contention between real-time stations,even assume no hidden & exposed channel problem. RTS/CTS is desired. • Fix channel access interval for real-time stations ( round robin), which is inefficient. • Only consider initial disturbance T. No stable analysis for periodic or sporadic disturbance. • Evaluations on how data stations impact the performance of real-time stations are more desired: converge(settling) time vs initial disturbance

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