RxLayer: Adaptive Retransmission Layer for Low Power Wireless

1. RxLayer: Adaptive Retransmission Layer for Low Power Wireless Daibo Liu1,Zhichao Cao2, Jiliang Wang2 Mengshu Hou1 and Yunjun Li1 1University of Electronic Science and Technology of China 2Tsinghua University

2. Wireless data transmission • Communication over unreliable wireless links Packet loss ACK Data Data Retransmission & update link quality • Countermeasures: • Retransmission • Link quality update • Change next-hop Data Data Data ACK Change next-hop

3. Link burstiness However.. • Many factors have effect on link burstiness Appearance of Obstacles Link burstiness brings about consecutive retransmission failures Receivers' PRR The change of RSSI at receiver

4. However.. Link correlation R3 R2 R1 Link correlation brings about ill-advised next-hop change, e.g., replacing R1 with R2. Three receivers' PRR Different concurrent interference Interference(dBm)

5. Serious situation Complicated external circumstance aggravates link burstiness and correlation

6. Consequence.. Ill-advised retransmission S is source node. A is S’ parent node. B, C, and D are S’ candidate next-hop. Quick decrease of <S, A>' link quality <S, D> is not the optimal link now <S, D> is relatively stable Quick decrease of <S, B>' link quality

7. Consequence.. Ill-advised retransmission S is source node. A is S’ parent node. B, C, and D are S’ candidate next hop node. Consecutive retransmission strategy is inefficient when link is severely degraded. How to accurately perceive the link burstiness? It is ineffectual to change next-hop node only according to link quality. How to select the optimal candidate receiver? Consecutive retransmission strategy misleads link estimator in the presence of link burstiness and correlation.

8. Efficiency of retransmission • Quantify the conditional probability of immediate retransmission • Conditional packet delivery functions (CPDF) CPDF(i) is the probability that the ith retransmission successes after i-1 consecutive failures. Ni is the cumulative count that packets are retransmitted no less than i times.

9. Efficiency of retransmission • Quantifying the conditional probability of immediate retransmission • Conditional packet delivery functions (CPDF) CPDF quickly slips down to 0.2. Pause consecutive retransmissions Consecutive retransmission (CR) is inefficient. A interrupt point of CR is needed .

10. However.. Outdoor Indoor Dynamic feature of CPDF • Link CPDF • Different scenarios • Different time Link burstiness is time-varying and spatial-varying. Online capturing link burstiness is needed. Two hours later

11. Solution is the cumulated CPDFi. is the updated CPDFi. Online model for link burstiness • Update link burstiness by moving average • Value of α: Making a tradeoff between the adaptability to network dynamics and accuracy. Long-term trace data could learn an appropriate value.

12. Correlation of link pair • Correlation between link pair: P(i, j) 1 Source node 16 Receivers 1000 Packets P(i, j) is the probability that a packet transmission will success in link j while failed in link i. A high quality link is not always an optimal candidate for a severely degraded link. High PRR and low P(0,6) indicating link 6 is not good when link 0 fails. The correlation between each pair of links should be captured with low overhead. Low PRR and high P(0,9) indicating link 9 is a good substitution for link 0

13. Solution P(A,B) P(A,C) P(B,A) P(B,C) P(C,A) P(C,B) Capturing link correlation • Bitmap and uniform broadcast sequence number(BSN) However, bitmap size is limited, e.g., 2 bytes.

14. Solution : the probability, when S transmits, that a packet succeed on link Sj given that it failed on link Si. : is the probability of packets failed on link Si. is the accumulated correlation between link i and j. is the computed correlation using the latest BSN set. Online model for link correlation • Model for capturing link correlation: ω • Update by using moving average

15. Solution Online model for link correlation • Correlation update • Value of θ ωi is the correlation calculated by moving average by hearing the ith routing beacon. Miis the computed correlation using collected BSNs from 0 to i..

16. Solution RxLayer: Decision maker rules • Exploit link burstiness and correlation 1. Transmission failure/success Transmission model Link burstiness model 2. Immediate retransmission 3. Pause consecutive retransmissions Link correlation model 4. Change next-hop node Link burstiness model: update CPDF, pause consecutive retransmission. Link correlation model: select the optimal candidate receiver. Transmission model: transmit packet and report result to network layer.

17. Solution RxLayer in protocol stack • Integrate RxLayer into protocol stack Beneath network communication layer; Above MAC layer; Connecting with link estimator.

18. Evaluation • Implementation: -Integrating with CTP built upon LPL in TinyOS 2.1.1 • Goals • High energy efficiency • Improvement on forwarding delay, network reliability • Scenarios • Indoor Testbed: 22 Telosb nodes • Outdoor Scenario: 30 Telosb nodes

19. Network reliability Indoor Outdoor Indoor, the average PRR CTP+LPL+RxLayer with 1.53% improvement Than CTP+LPL Outdoor, the average PRR CTP+LPL+RxLayer with 7.82% improvement than CTP+LPL

20. Transmission efficiency Indoor Outdoor Indoor, the avg. tx count CTP+LPL+RxLayer with 24.7% improvement than CTP+LPL Outdoor, the avg. tx count CTP+LPL+RxLayer with 36.3% improvement than CTP+LPL

21. Energy consumption Indoor Outdoor Indoor, the avg. radio duty cycle Using RxLayer, radio duty cycle is reduced by about 3.5%. Outdoor, the avg. duty cycle Using RxLayer, duty cycle is decreased from 19.3% to 10.4%

22. Key design -Online link burstiness model - Online link correlation model Evaluation - Indoor and outdoor experiments -Improvements on network efficiency Future works - Large testbed - Dyanmic forwarding Conclusions • RxLayer is a ready-to-use module for existing protocol stack

23. Thank you! Q&A