GARUDA: Achieving Effective Reliability for Downstream Communication in Wireless Sensor Networks

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. 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. 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. 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. Focus • To achieve reliability • Reliable Transport Layer • No packet loss • Bi-directional Reliability Figure from Akyildizet al, “Wireless Sensor Networks: A Survey”, Computer Networks, 38(4):393-422, 2002.

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

7. Types of Data • Single Packet • Block of packets • Stream of Packets

8. 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. 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 multihopwireless 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. GARUDA • GARUDA is a large mythical bird or bird-like creature that appears in both Hindu and Buddhist mythology • Transport reliably

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

12. 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. ACK / NACK Paradox (2) • ACK implosion

14. 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. 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. 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. Pulsing based solution (4)

18. Pulsing based solution (5) Implicit NACK

19. 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. 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. 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. 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. 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. 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. Multiple Reliability Semantics (1)

26. 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. 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. Evaluation (2) • Evaluation of single-packet delivery

29. Evaluation (3) • Evaluation of multiple-packet delivery

30. 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. Q & A