Understanding Spatio-Temporal Uncertainty in Medium Access with ALOHA Protocols

1. Understanding Spatio-TemporalUncertainty in Medium Access with ALOHA Protocols Affan Syed, Wei Ye, Bhaskar Krishnamachari, and John Heidemann ACM Proceedings of the second workshop on Underwater networks (WuWNet 2007)

2. Outline • Introduction • Spatio-Temporal Uncertainty • Understanding With ALOHA • Handling Space Uncertainty • Future Work • Conclusion

3. Introduction • Underwater sensor network become an important area of research • Space-time Uncertainty, defined as the two dimensional uncertainty • packet transmit time • propagation latency

4. Space-Time Uncertainty Same Transmission Time Different Transmission Time no collision

5. Evaluation Methodology • ALOHA protocols are the simplest and earliest wireless MAC protocols • It consists of a single receiver that does not transmit • nodes randomly deployed in a circular region

6. vulnerability interval vulnerability interval 2T Data Data T T T ALOHA with Time Uncertainty Slotted ALOHA Pure ALOHA

7. Classical Throughput Analysis

8. ALOHA with Space Uncertainty • Simple ALOHA-constant delay max propagation delay / transmit time

9. ALOHA with Space Uncertainty • Slotted ALOHA – any propagation latency

10. ALOHA with Space Uncertainty • Slotted ALOHA – any propagation latency

11. Slotted ALOHA with Guard Bands β= 1 βtmax A Data farthest A’s Data R

12. Choosing β • β= 1, no overlap at the receiver unless packets send at same time • increase packet transmission latency and bandwidth overhead • β< 1, some node pair remains overlap • lowers band width overhead, increases collision probability • tradeoff

13. Parameters β (1)

14. Parameters β (2)

15. Analysis with Guard Bands • Throughput • Assume a > 0 and βa ≦1 • expected number of collision-free packets per slot is independent of a each transmits a packet in a given slot

16. Varying Guard Bands Length

17. Varying Maximum Delay

18. Simple ALOHA with Guard Bands

19. Future Work • Plan to rigorously analyze the throughput capacity • removing the assumption regarding single and equidistant receiver • Provide a framework to analyze other wireless protocols • CSMA • increase understanding of the characteristics of MAC in UWSN.

20. Conclusion • While designing MAC protocols under high latency environment • it is necessary to consider both space and time uncertainty • Smaller ratio of propagation delay to packet length result get better • short-range communication