1. GARUDA: Achieving Effective Reliability for Downstream Communication in Wireless Sensor Networks Seung-Jong Park et al IEEE Transactions on mobile computing Feb, 2008 presented by jae-hong Kim

2. 2 / 31 Contents  Transport Layer Issues for ad hoc WSN  Reliable bi-directional transport protocol  Characteristics of GARUDA  Pulsing based solution  Virtual infrastructure called core  Two-Phase Loss Recovery  Multiple Reliability Semantics  Evaluation  Discussion

3. 3 / 31 Transport Layer Issues for ad hoc WSN  Vision Statement  Reliable and Robust bi-directional (sink to sensors and sensors to sink) transport protocol for Ad-hoc Wireless Sensor Networks

4. 4 / 31 To the knowledge …  Up to this point Reliability and Robustness has been ignored;  Possible reason:  WSN is low-cost;  Not necessary (due to redundant data)  And also difficult  But …  We require reliability … Disaster Recovery Military Applications etc

5. 5 / 31 Focus  To achieve reliability  Reliable Transport Layer  No packet loss  Bi-directional Reliability Figure from Akyildiz et al, “Wireless Sensor Networks: A Survey”, Computer Networks, 38(4):393-422, 2002.

6. 6 / 31 Is it challenging?  Limitations of sensor nodes  Application specific requirements  Objectives  Reliable Transport  Flow Control  Congestion Control  Self Configuration  Energy Awareness

7. 7 / 31 Types of Data  Single Packet  Block of packets  Stream of Packets

8. 8 / 31 Today’s Situation  Downstream Reliability: from sink to Sensors  Reliability semantics are different  PSFQ (Block of packets data)  MOAP (block of packets data)  GARUDA (Block of packets data) (Single Packet)

9. 9 / 31 Introduction  Reliable downstream point-to-multipoint data delivery  The need for the reliability is dependent on the type of applications. Ex) security application  Reliability in multihop wireless networks vs Reliability in wireless sensor networks  Environment considerations Limited life time, bandwidth, energy, size of the network  Message considerations In a sensor networks, small-sized queries  Reliability considerations Dependent on reliability semantics

10. 10 / 31 GARUDA  GARUDA is a large mythical bird or bird-like creature th at appears in both Hindu a nd Buddhist mythology  Transport reliably

11. 11 / 31 Characteristics of GARUDA 1. An efficient pulsing-based solution for reliable short message delivery 2. A virtual infrastructure called the core, which approximates an optimal assignment of local designated servers 3. A two-stage negative acknowledgment (NACK) based recovery process and out-of- sequence forwarding 4. A simple candidacy based solution to support the different notions of reliability

12. 12 / 31 ACK / NACK Paradox (1)  NACKs  Well established as an effective loss advertisement in multi-hop wireless networks In case loss probabilities are not inordinately high  Not for single-packet delivery or all packets are lost It cannot possibly advertise a NACK to request retransmissions

13. 13 / 31 ACK / NACK Paradox (2)  ACK implosion

14. 14 / 31 Pulsing based solution (1)  It incorporates an efficient pulsing based solution, which informs the sensor nodes about an impending reliable short-message delivery by transmitting a specific series of pulses at a certain amplitude and period  Amplitude : at least 3dB larger Much larger than that of a regular data transmission  Reliability of pulsing mechanism?  Proved by “A Power Control MAC Protocol for Ad Hoc Networks”.

15. 15 / 31 Pulsing based solution (2)  WFP (Wait-for-First-Packet) pulses  Used only for first packet reliability  Short duration pulses  Single radio  Advertisement of incoming packets  Negative ACK  Simple energy detection  Different types of WFP  Forced pulses  Carrier sensing pulses  Piggybacked pulses

16. 16 / 31 Pulsing based solution (3)  A sink sends WFP pulses periodically  Before it sends the first packet  For a deterministic period  A sensor sends WFP pulses periodically  After it receives WFP pulses  Until it receives the first packet  WFP merits  Prevents ACK implosion with small overhead  Addresses the single or all packet lost problem  Less energy consumption  Robust to wireless errors or contentions

17. 17 / 31 Pulsing based solution (4)

18. 18 / 31 Pulsing based solution (5) Implicit NACK

19. 19 / 31 Pulsing based solution (6)  3 modes in delivery procedure for single/first packet  1. the advertisement that notifies the ensuring single/first packet to all nodes with the forced WFP pulses  2. the delivery that sends the single/first packet through simple forwarding (for (ex)CSMA)  3. the recovery that sends NACKs using WFP pulses to request for the retransmission of the single/first packet

20. 20 / 31 Virtual infrastructure called core (1)  The Core  An approximation of the minimum dominating set (MDS) of the network sub graph to which the reliable message delivery is desired. the set of local designated loss recovery servers that help in the loss recovery process.  Constructing the core during the course of a single packet flood.

21. 21 / 31 Virtual infrastructure called core (2)  Principle  The retransmission by neighbor is sufficient to recover the loss of the same packet of all neighbors around it

22. 22 / 31 Virtual infrastructure called core (3)  Instantaneous Core Construction  To approximate the MDS problem, we select a node at 3i hop distance as a core node Approximate number of hops from the sink to the sensor

23. 23 / 31 Two-Phase Loss Recovery (1)  Core recovery – first phase recovery  Out-of-sequence Packet Forwarding with A-map Propagation  Out-of-sequence : NACK implosion  Solve the above problem : uses a scalable A-map (Available Map)  Overhead?  The ratio of the number of core nodes (10 – 30%)  A map request ratio (less than 1 %)

24. 24 / 31 Two-Phase Loss Recovery (2)  Non-core recovery – second phase recovery  Starts only when a noncore node overhears an A-map from the core node indicating that it has received all the packets in a message

25. 25 / 31 Multiple Reliability Semantics (1)

26. 26 / 31 Multiple Reliability Semantics (2)  Involving nodes employing a candidacy check before participating in the core construction algorithm  The candidacy check is where nodes, upon receiving the first packet, determine whether or not they belong in the subset G(s)

27. 27 / 31 Evaluation (1)  For n2-based experiments  100 nodes in 650 m * 650 m square area  Randomly deployed within that area  Sink is located in center  Transmission range of each node is 67 m  Channel capacity is 1Mbps  Each message : 100 packets (25 pkts/ sec)  Size of packet : 1Kbyte

28. 28 / 31 Evaluation (2)  Evaluation of single-packet delivery

29. 29 / 31 Evaluation (3)  Evaluation of multiple-packet delivery

30. 30 / 31 Discussion  Considerations for upstream?  Network model  Sink and sensors static?  There is exactly one sink coordinating the sensors?  Congestion control?  If congestions are appeared, how can GARUDA control them?  Loss recovery for noncore nodes  How can we reduce snooping overheads?

31. 31 / 31 Q & A