Qingjiang Tian and Edward J. Coyle Center for Wireless Systems and Applications ( CWSA )

1. SOSBRA: A MAC-Layer Retransmission Algorithm Designed for thePhysical-Layer Characteristics of Clustered Sensor Networks Qingjiang Tian and Edward J. Coyle Center for Wireless Systems and Applications (CWSA) School of Electrical and Computer Engineering Purdue University {tianq,coyle}@ecn.purdue.edu

2. Outline • Background • SOSBRA Approach for Clustered Sensor Networks • Numerical Results • Optimal Contention Window • Conclusions

3. Introduction Application Network Energy Efficiency MAC Physical • Design for Energy Efficiency Through All Layers of the Protocol Stack • Cross-Layer Design to Improve Performance • Need to avoid fragility • My Work: Physical-MAC Layer Interface • Small Propagation delay in sensor net applications • Opportunity to redesign Retransmission algorithms

4. Background • General – 802.11 MAC Layer • CSMA/CA Collision Avoidance • Binary Exponential Backoff • Homogeneous peer-to-peer • Designed for hidden nodes (RTS-CTS Handshake) • V. Bharghavan, “MACAW: A Media Access Protocol for Wireless LANS” • All nodes can hear each other • Y. Kwon,etc, “A Novel MAC Protocol with Fast Collision Resolution for Wireless LANs” • multiplicative-increase, linear-decrease • C. Wang,etc, “A new collision resolution mechanism to enhance the performance of IEEE 802.11 DCF,” • contention window size is halved after c consecutive successful transmission

5. Motivation for My Work • IEEE 802.11 Distributed Coordination Function (DCF) • Called WiFi • Homogeneous, peer-to-peer Communications • Binary exponential backoff & cross-stage collisions

6. Motivation for My Work SENSOR SENSOR SENSOR SENSOR SENSOR SENSOR SENSOR fd fd fd fd fd fd fd • Clustering in Sensor Networks • Clusterhead: central control, broadcasting, synchronization of other nodes • Energy efficiency is a goal • Increase throughput on the channel • Minimize collisions and idle time • Very Small Propagation Delay 100m

7. SOSBRA: Synchronized, One-Stage-Backoff Retransmission Algorithm • Assumptions • One-hop cluster considered • Traffic model: collect one packet from each node within the cluster • We ignore the small propagation delay between sensor nodes and CH • All nodes within one cluster can be synchronized to within 1 microsecond • Synchronization beam – similar to ZigBee – starts “rounds” or retransmissions on the channel • Nodes can sense each other’s activity

8. SOSBRA Approach • Each node that needs to either transmit or retransmit at the beginning of a round will chose a slot at random in a contention window of size W for its retransmission. • Nodes that transmit without collision are done. • Nodes in collisions in the current round will reschedule transmissions in the next round of W slots. • W is the same for every round.

9. SOSBRA vs 802.11 DCF A B C A A Standard 802.11 DCF C B NewRound A B A C B SOSBRA-based 802.11 DCF 1 2…………………W 1 2 ……………. W Window 2 Window 1

10. PerformanceAnalysis N: Total non-CH nodes within the cluster W: fixed one stage contention window :Total time required to collect one packet from each node : The duration of a RTS collision : The duration of a data packet transmission

11. PerformanceAnalysis • No collisions (1) (2)

12. PerformanceAnalysis • N1 nodes succeed in the first round and all of remaining N2 nodes succeed in the second round,C1 collisions in the first round (3) (4)

13. PerformanceAnalysis (5) • General Case (6)

14. Numerical And Simulation Results Fig.1Numerical results for the probability mass function of , the total time to empty the cluster, for the SOSBRA-based 802.11 protocol. Here, N =50 nodes and W =120.

15. Numerical And Simulation Results Fig. 2 Simulations for the SOSBRA-based 802.11 protocol that show during empty the cluster for different contention window sizes. is the number of nodes in the cluster.

16. Numerical And Simulation Results Fig.3Simulations for the SOSBRA-based 802.11 protocol that show the average channel throughput during the emptying the cluster for different contention window sizes. N is the number of nodes in the cluster.

17. Numerical And Simulation Results Fig. 4 Simulations determining the optimal contention window size for different for the SOSBRA-based 802.11 protocol

18. Numerical And Simulation Results Fig. 5Simulations determining the minimum , for different cluster sizes for the SOSBRA-based 802.11 protocol.

19. Numerical And Simulation Results Fig. 7. :Simulations comparing the wasted-time before the cluster is emptied for the SOSBRA-based 802.11, Standard 802.11 DCF, and ZigBee with and without GTS.

20. Numerical And Simulation Results Fig. 8. : Simulations comparing total energy consumption to empty the cluster for the SOSBRA-based 802.11, Standard 802.11 DCF, and ZigBee with/without GTS. The energy consumption ratios used was idle:receive:send=1:2:2.5 11

21. Numerical And Simulation Results Fig. 9. Comparison between SOSBRA and TDMA-based approaches. Here, and a slot time is 10 microsecond in SOSBRA.

22. Optimal Contention Window Size • Probabilistic Approach • Cost Function • Cost results from two sources • The first is from the total idle slot W • The other one comes from possible collisions • (7)

23. Optimal Contention Window Fig.10.Numerical Results showing Cost Function Vs 1/W

24. Optimal Contention Window Fig.11.Comparison between simulation and analytical results

25. Optimal Contention Window Fig. 12. Average Total time obtained with from both simulation and analysis.

26. Large Number of Nodes if for very large N, We may approximate the total cost to be (11)

27. Large Number of Nodes Define (12) (13) (14)

28. Conclusions • SOSBRA provides better performance in term of both time and energy compare to 802.11 DCF • Help minimize the multi-access interference (collisions) in design of physical access scheme, especially for CDMA approach • Our future work includes • analysis of cross layer designs for wireless sensors with directional transmission capability • physical layer improvements, including adaptive modulation schemes • synchronization across a sensor network • CDMA based optimization of PHY/MAC design

29. Derivations of Formulas • N: Total non-CH nodes within the cluster • W: fixed one stage contention window • :Total time required to collect one packet from each node • : The duration of a RTS collision • : The duration of a data packet transmission

30. Derivations of Formulas

31. Derivations of Formulas (7) (8) (9)

32. Derivations of Formulas • Given N nodes and length W contention window, for each of the W Slots: • No nodes choose this slot………………………. • Only one node chooses this slot……………...... • More than one nodes choose this slot………… • Cost Function: • Cost results from two sources: total # of empty slots and possible collisions • Minimize the Cost Function: (10)

33. Derivations of Formulas if for very large N, We may approximate the total cost to be (11)

34. Derivations of Formulas Define (12) (13) (14)