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REAL TIME COMMUNICATION IN WIRELESS SENSOR NETWORKS

REAL TIME COMMUNICATION IN WIRELESS SENSOR NETWORKS. BY ZILLE HUMA KAMAL. 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” [St].

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REAL TIME COMMUNICATION IN WIRELESS SENSOR NETWORKS

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  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” [St] 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. [St] • 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. [St] 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. REAL TIME COMMUNICATIONS IN WIRELESS SENSOR NETWORK NOW PRESENTING SPEED BY Zille Huma Kamal

  22. UNFAVORABLE • Despite the simplicity of RAP and the high miss deadline ratio it serves, RAP does not guarantee for soft or hard real time communication systems. • Therefore, our search for a Real Time Communication protocol is unsatisfied. CS 691 - WMU - WSNR

  23. TO END THE SEARCH • SPEED is a real time communication protocol which guarantees end to end soft real time communication • We will discuss the components of SPEED and then relate SPEED to other existing protocols for MANETS, ad-hoc networks and real-time communication systems. CS 691 - WMU - WSNR

  24. COMPONENTS OF SPEED • API • Neighbor Beacon Exchange • Delay Estimation • Stateless Non-deterministic Geographic Forwarding(SNGF) • Neighborhood Feedback Loop(NFL) • Backpressure Rerouting • Void Avoidance • Last mile processing CS 691 - WMU - WSNR

  25. PacketType Global_ID DestinationArea TTL Payload API & PACKET FORMAT • UnicatSend(Global_ID, packet) • AreaMulticastSend(position, radius, packet) • AreaAnycastSend(position, radius, packet) • SpeedReceive( ) • SPEED packet format: CS 691 - WMU - WSNR

  26. Neighbor Beacon Exchange • Periodic beacons – exchange location information • In static or slow moving sensor networks – very low beaconing rate • Further reduce overhead – piggybacking, include ID on data packets, so that you are using the existing packets and not introducing more traffic CS 691 - WMU - WSNR

  27. Neighbor_ID Position SendToDelay ExpireTime … … … … … … … … … … … … NEIGHBOR TABLE • Through beaconing each node is capable of maintaining a Neighbor Table (NT) • In addition to location beacons, you have delay estimation beacons and backpressure rerouting beacons CS 691 - WMU - WSNR

  28. DELAY ESTIMATION • Single Hop Delay – delay across one router • Sender - timestamps when packet leaves node and then waits for acknowledgement from receiver. • Receiver – in acknowledgment packet sends the time taken to process the acknowledgment • Sender – after receiving the acknowledgment, calculates round trip time as timestamp – ACK time – ACK processing time CS 691 - WMU - WSNR

  29. DELAY ESTIMATION • This round trip delay time is aggregate with previous delay times via EWMA • Since delay estimation expensive – SPEED only invokes delay estimation when round trip delay for an individual case exceeds a predetermined threshold value CS 691 - WMU - WSNR

  30. ID Destination AvgSendToDelay BACK-PRESSURE REROUTING • Routing layer adaptation to congestion • Beacon format • When congestion occurs, node sends back-pressure beacon to sender with AvgSendToDelay equal to infinity CS 691 - WMU - WSNR

  31. SNGF - TERMINOLOGY • Nsi = {all nodes within radio range of nodei} • FSi(destination) = {x | x  Nsiand it is closer to the destination than nodei} • Relay Speed CS 691 - WMU - WSNR

  32. SNGF – FORWARDING CONDITIONS • Only if node belongs to FSi(destination) • FSi(destination) into 2 categories: • FS1i(destination)of nodes with relay speed > Ssetpoint • FS2i(destination) of nodes with relay speed < Ssetpoint • Forwarding node is always from FS1i(destination) • If no node in FS1i(destination) then call Neighborhood Feedback Loop (NFL) and decide whether to drop packet or not CS 691 - WMU - WSNR

  33. NFL - TERMINOLOGY • Miss = when packet delivered at neighbor with relay speed < Ssetpointor any packet loss due to collision • Miss ratio calculation CS 691 - WMU - WSNR

  34. NFL • MAC layer adaptation to avoid congestion CS 691 - WMU - WSNR

  35. VOID AVOIDANCE • By using backpressure rerouting • Only guarantees to find a path if a greedy path exists CS 691 - WMU - WSNR

  36. LAST MILE PROCESS • For AreaMulticast and AreaAnycast – TTL manipulation • For Unicast CS 691 - WMU - WSNR

  37. EXPERIMENTATION - CONGESTION CS 691 - WMU - WSNR

  38. EXPERIMENTATION - CONGESTION CS 691 - WMU - WSNR

  39. EXPERIMENTATION – E2E DEADLINE MISS RATIO CS 691 - WMU - WSNR

  40. EXPERIMENTATION – E2E DEADLINE MISS RATIO CS 691 - WMU - WSNR

  41. EXPERIMENTATION - COST CS 691 - WMU - WSNR

  42. EXPERIMENTATION - COST CS 691 - WMU - WSNR

  43. EXPERIMENTATION – ENERGY CONSUMPTION CS 691 - WMU - WSNR

  44. EXPERIMENTATION – TRAFFIC BALANCING CS 691 - WMU - WSNR

  45. 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. http://www.cs.wustl.edu/~venkita/publications/class/implicitedf.pdf CS 691 - WMU - WSNR

  46. REFERENCES • [HSLA] T He, J.A Stankovic, C Lu, T Abdelzaher, “SPEED: A Stateless Protocol for Real-Time Communication in Sensor Networks,” Department of Computer Science, University of Virginia and Department of Computer Science and Engineering, Washington University in St Louis http://www.cs.virginia.edu/~stankovic/psfiles/SPEED_ICDCS.pdf CS 691 - WMU - WSNR

  47. REFERENCES • [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 • [P] T. F Piatkowski, “Citation and acknowledgment guide,” Department of Computer Science, Western Michigan University, Aug, 2000 www.cs.wmich.edu/~piat/citationAckGuide.pdf CS 691 - WMU - WSNR

  48. REFERENCES • [Sp] “Delay Analysis,” Sprint, 2003 http://ipmon.sprintlabs.com/delaystat/ • [St] D.B Stewart, “Introduction to Real Time,” Embedded.com, Nov 1, 2001 www.embedded.com/story/OEG20011016S0120 CS 691 - WMU - WSNR

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