REAL TIME COMMUNICATION IN WIRELESS SENSOR NETWORKS

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

2. 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] CS 691 - WMU - WSNR

3. 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] CS 691 - WMU - WSNR

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. 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 CS 691 - WMU - WSNR

6. 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 CS 691 - WMU - WSNR

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

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

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

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

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

12. 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 CS 691 - WMU - WSNR

13. 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 CS 691 - WMU - WSNR

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

15. VMS • SVM • Requested velocity is fixed at each hop V = dis(x0, y0,xd, yd)/D • DVM • Requested velocity changes at each hop and reflects the time the packet has spent in the network vi = dis(x0, y0,xd, yd)/(D-Ti) v0 = dis(x0, y0,xd, yd)/D CS 691 - WMU - WSNR

16. 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 CS 691 - WMU - WSNR

17. 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) CS 691 - WMU - WSNR

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

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

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

21. 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 CS 691 - WMU - WSNR

22. 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 CS 691 - WMU - WSNR