1. REAL TIME COMMUNICATION IN WIRELESS SENSOR NETWORKS BY ZILLE HUMA KAMAL

2. CS 691 - WMU - WSNR WHAT IS A REAL TIME SYSTEM (RTS) “A real time system is one in which the correctness of the computations not only depends on their logical correctness, but also on the time at which the result is produced” [S]

3. CS 691 - WMU - WSNR CLASSIFICATION OF RTS 2 Categories of RTS: –A Hard RTS is one in which one or more activities must never miss a deadline or timing constraints, otherwise the system fails or results in catastrophe. [S] –A Soft RTS is one that has timing constraints, but occasionally missing them has negligible effects, as application requirements as a whole continue to be met. [S]

4. TERM AND DEFINITIONS Task – executable entity Job – instance of a task Release Time – time at which task becomes ready to run and job is released Period – time between releases of two instances of the same task Deadline – relative time at which a job should complete execution Execution Time/ Run Time – time taken to complete execution without interruption Frame – discrete unit of time [CZSB]

5. CS 691 - WMU - WSNR WIRELESS SENSOR NETWORKS CHARACHTERISTICS –An instance of MANET –Resource constraint – energy and storage capacity –Limited range for communication and sensing –Frequent network topology changes –Individual entities are not critical, aggregation of results is necessary for effectiveness and accuracy

6. CS 691 - WMU - WSNR RTS IN WSN Two types of communication groups are inherently formed –Local Coordination – to aggregate results –Sensor-Base Communication – to send results to base station This introduces contention on the communication channel, thus the main schedulable resource is the communication channel

7. CS 691 - WMU - WSNR RAP A Real-Time communication architecture Sensing/Control Application Coordination Service Query/Event Service Location Addressed Protocol Geographic Forwarding Velocity Monotonic Scheduling Prioritize MAC Query/Event Service APIs RAP

8. CS 691 - WMU - WSNR APIs Issue Query - query name - attribute list - area - timing constraints, e.g. period, deadline - querier location

9. CS 691 - WMU - WSNR APIs Event Registration - event name - area - query

10. CS 691 - WMU - WSNR Example register_event{ virus_found(0,0,100,100), query{ virus.count, area=(X event-1,Y event-1,X event+1,Y event+1 ), period=1.5, deadline=5, base=(100,100) } };

11. CS 691 - WMU - WSNR LAP Location Addressed Protocol - transport layer - connectionless - no IP/ID addressing, location based addressing - three types of communication »unicast »area multicast »area anycast

12. CS 691 - WMU - WSNR LAP Unicast Message is delivered to node closest to destination, e.g when sensors send query results back to base station Area Multicast Message is delivered to every node in a specified area, e.g when base station sends query to an area, or for local coordination Area Anycast Message is delivered to at least one node in the specified area, e.g when base station wants to send a query to an area, the node which receives it can start the initiation process

13. CS 691 - WMU - WSNR GF Greedy algorithm A packet is forwarded to a neighbor only if: (1) the neighbor node has the shortest distance to the packet’s destination among all immediate neighbors AND (2) the neighbor node is closer to the destination than the forwarding node If these conditions not satisfied, GPSR is used instead of GF

14. CS 691 - WMU - WSNR VMS Deadline aware Distance aware Deadline aware Distance aware Packet scheduling policy –2 types of packet scheduling policies –Static Velocity Monotonic –Dynamic Velocity Monotonic

15. CS 691 - WMU - WSNR VMS SVM –Requested velocity is fixed at each hop V = dis(x 0, y 0, x d, y d )/D DVM –Requested velocity changes at each hop and reflects the time the packet has spent in the network v i = dis(x 0, y 0, x d, y d )/(D-T i ) v 0 = dis(x 0, y 0, x d, y d )/D

16. CS 691 - WMU - WSNR Priority Queues various FIFO queues, one for each priority Advantage – per packet overhead decreases, ordering of each packet is not required Disadvantage – more storage capacity required single FIFO queue, with priority ordering Advantage – reflects order of packets requested Disadvantage – greater number of packets lost

17. CS 691 - WMU - WSNR MAC PRIORITIZATION Extensions to 802.11 –Initial wait time after idle –Backoff Increase Function Initial wait time after idle DIFS = BASE_DIFS * PRIORITY Backoff Increase Function CW = CW * (2+(PRIORITY-1)/MAX_PRIORITY)

18. CS 691 - WMU - WSNR EXPERIMENTATION Overall deadline miss ratio of DSR and GF with deadlines (5,10)

19. CS 691 - WMU - WSNR EXPERMENTATION Overall deadline miss ratio

20. CS 691 - WMU - WSNR EXPERIMENTATION Miss ratio vs distance between source and destination (Deadline: (5:10) s; Rates: (0.8, 0.36)/s)

21. CS 691 - WMU - WSNR REFERENCES [CZSB] M Caccamo, L.Y Zhang, L Sha, G Buttazzo, “An Implicit Access Protocol for Wireless Sensor Networks,”Proceedings of IEEE Real-Time Systems Symposium, Austin, TX, Dec 2002. [LBASH] C Lu, B.M Blum, T.F Abdelzaher, J.A Stankovic, T He, “RAP: A Real-Time Communication Architecture For Large-Scale Wireless Sensor Networks,” Department of Computer Science, University of Virginia www.cs.virginia.edu/~stankovic/psfiles/rtas02-rap.pdf

22. CS 691 - WMU - WSNR REFERENCES [P] T. F Piatkowski, “Citation and acknowledgment guide,” Department of Computer Science, Western Michigan University, Aug, 2000 www.cs.wmich.edu/~piat/citationAckGuide.pdf [S] D.B Stewart, “Introduction to Real Time,” Embedded.com, Nov 1, 2001. www.embedded.com/story/OEG20011016S0120 www.embedded.com/story/OEG20011016S0120