1. Frame counter: Achieving Accurate and Real-Time Link Estimation in Low Power Wireless Sensor Networks Daibo Liu, Zhichao Cao, Mengshu Hou and Yi Zhang IPSN 2016

2. Link Estimation in Low Power Wireless Sensor Networks Accurately count arrived data frame for each active link Monitor all routing/non-routing links in real-time during active state Wireless sensor networksGeneral network structureLow Power Listening-based X-MAC Ideal Approach in Low Power WSNs

3. State-of-the Art Passive Estimation & Active Estimation Ignored receiving information at non-routing links Ignoring the lost frame in routing link Ignoring the lost frame Treating it as one frame loss

4. Non Real- Time Root cause: Low frequency of active beacon Results: Routing selection based on outdated estimation Actions: Except for routing beacon, ignoring neighbors’ data frames; Inaccurately counting beacon frame. Defect of State-of-the-art Empirical Study of Low Power WSNs Inaccuracy Root cause: chance to hear multiple same frames Results: Overestimation Actions: Ignoring all lost frames if at least one frame is received; Treat no frame reception as one loss.

5. New Challenges Duty cycle and asynchronous radio work mode Accurate and Real-Time Link Estimation Data transmission is organized as repeated data frames Count all arrived frames (decoded or not decoded) Several nodes may successively transmit Monitor all neighboring links in real-time As a neighbor, it is difficult to know which frame is corrupted During active state, the transmitted frames by any neighbor should be counted Sender information of not decoded data frames is unknown

6. Capture Frames with RSSI Features Observation 1: The RSSI of corrupted frames can be captured in real-time. Observation 2: It is feasible to distinguish ZigBee from other 2.4GHz technologies with RSSI features. Observation

7. Overview the Basic Idea Tx Neighbor Rx Decoded data frame #1 #2 #3 #1 #2 #3 Lost data frame Sampled RSSI sequence Repeat data frame transmission until be ACKed 1 Frames arrive at receiver (decoded or lost) 2 Sampled received signal strength in time domain 4 Decoded frames match corresponding RSSIs 5 #1#3#3 #3#3 Neighbor also overhears arriving frames 3

8. Necessary Information for Accuracy Estimation Total arrived frames How many frames have arrived during the node’s active state? Decoded frames How many frames have been successfully decoded by the node? Solution: Using RSSI sequence to count Solution: Counting it according to frame receiving event Ongoing Sender Which node is transmitting data frame? Solution 1: Extracting ID from decoded frame Solution 2: Using RSSI Feature to infer

9. Utilization of RSSI Sequence Frame Transmission by sender Sampled RSSI Sequence at Receiver/Neighbor Noise floor Signal strength Translated to Step Pulse Signal Low level High level Rising edge Falling edge Procedure Counting the detected pulses to represent the number of arrived frames

10. Impacts on Accurate Data Frame Counting Coexistent Interference in 2.4GHz ISM Band Distinguish ZigBee Data Frame and ACK Frame Frame TypeOn-air timeInterval Data Frame[576, 4256] μsLarger than 512μs ACK Packet352μsBetween 192μs and 512μs

11. ZigBee Frame Identification Shorter on-air time longer on-air time Feature #1: On-air time Valid range of on-air time Feature #2: Frame interval Shorter packet interval Fixed frame interval Longer packet interval

12. ZigBee Frame Identification Feature #3: PAPR (Peak-to-Average Ratio)Feature #4: RSSI < Noise floor Flat sequence Large variation TRUE FALSE ZigBee

13. Determination of Transmitter Extracting transmitter ID from decoded frame Exploiting low power RSSI features Fixed inter- frame interval Inter-frame interval (T ifi ) is fixed; System congestion backoff > T ifi. Check |T ifi (k) - T ifi |≤ δ Valid RSSI bias, 1dBm Determine whether two successive frames are transmitted by the same sender. Parameter for frame segment Computing average frame RSSI R avg ; Comparing R avg with each neighbor A’s RSSI (R(A)) by: Steadily averaged RSSI |R avg – R(A)|≤ R δ If passes the check, A is a possible candidate transmitter. If more than one candidate, adopt deferred determination.

14. Deferred Transmitter Determination ….….….…. ….…. ….…. ….…. Neighbor A Neighbor B Neighbor C timeline …….. Time unit, 1 wakeup interval A{1, 0, 1, 0, 0, 0} B{0, 1, 0, 0, 0, 0} C{0, 0, 0, 0, 1, 1} Neighbor Transmitting Time Bitmap Neighbor A Neighbor B Neighbor C Neighbors transmits data frames in different time Different bit corresponds to different time. Using averaged RSSI features and compressed transmitting time to accurately determine the transmitter.

15. Implementation and Evaluation Implementation Evaluation setup TinyOS-2.1.1 Combining with LPL Beneath collection tree protocol (CTP) Multiple-hop networks Indoor & outdoor testbeds Comparing with the state-of-the-art With/without coexisting interference

16. CorrectFalse negativeFalse positive Radio97.3%0.8%1.9% Accuracy Number of arrived frames Determination of transmitter For segment vectors, more than 92.5% with <6% FN, and 89.4% with <8% FP. Overall accuracy About (>) 60% segments are determined by decoded frames, for the remainder: Averaged RSSIDeferred determination Accuracy95.8%98.9% DF Averaged RSSI Deferred determination Decoded frame ≈60%

17. Timeliness Link Quality Update Period 97.9% links can be updated within 200 seconds. Comparing with 4-bit link estimator Estimation window size: 5 frames Indoor & outdoor testbed with multiple-hop data collection networks Trickle controls routing probe Data packet interval: 2 minutes More than 50% links can not be updated one time for 8 minutes by 4-bit

18. Collection Tree Protocol Performance Network reliability, Energy consumption, Delay, and Path length More reliable Less energy Lower delay Less transmission hops

19. Conclusion Existing passive and active link estimators can not achieve accurate and real-time link estimation Implement on configurable indoor/ourdoor testbed Validate performance Resolve accurate and real-time link estimation in Low Power WSNs Using decoded frames to directly determine the transmitter RSSI features for ZigBee frame identification and frame counting Averaged RSSI and deferred determination to accurately infer transmitter

20. Q&A Q&A