1. Network and Systems Laboratory nslab.ee.ntu.edu.tw Prabal Dutta, Stephen Dawson-Haggerty, Yin Chen, Chieh-Jan (Mike) Liang, and Andreas Terzis Sensys 2010 Presenter: SY

2. Network and Systems Laboratory nslab.ee.ntu.edu.tw Outline Introduction A-MAC primitive Implementation Backcast evaluation High level evaluation Drawbacks Conclusion

3. Network and Systems Laboratory nslab.ee.ntu.edu.tw 3 A sender-initiated MAC: Sender triggers communications by transmitting a data Receiver Sender ListenD D

4. Network and Systems Laboratory nslab.ee.ntu.edu.tw 4 Low-power listening (LPL) with a sender-initiated MAC PreambleD Sender Receiver D T listen Noise Overhearing/noise adds significant unpredictability to node lifetime

5. Network and Systems Laboratory nslab.ee.ntu.edu.tw 5 A receiver-initiated MAC: Receiver triggers exchange by transmitting a probe Receiver Sender PListenD PD

6. Network and Systems Laboratory nslab.ee.ntu.edu.tw 6 Receiver-initiated services -- benefits Handle hidden terminals better than sender-initiated ones Support asynchronous communication w/o long-preambles Support extremely low duty cycles or high data rates Support many low-power services Wakeup (“LPP”, Musaloiu-E. et al., IPSN’08) Discovery (“Disco”, Dutta et al., Sensys’08) Unicast (“RI-MAC”, Sun et al., Sensys’08) Broadcast (“ADB”, Sun et al., Sensys’09) Pollcast (“Pollcast”, Demirbas et al., INFOCOM’08) Anycast (“Backcast”, Dutta et al., HotNets’08)

7. Network and Systems Laboratory nslab.ee.ntu.edu.tw 7 Receiver-initiated services -- drawbacks Probe (LPP) is more expensive than channel sample (LPL)  Baseline power is higher Frequent probe transmissions  Could congest channel & increase latency  Could disrupt ongoing communications  Channel usage scales with node density rather than traffic Services use incompatible probe semantics  Makes concurrent use of services difficult  Supporting multiple, incompatible probes increases power

8. Network and Systems Laboratory nslab.ee.ntu.edu.tw 8 The probe incompatibility mess Probes use hardware acknowledgements Probes do not use hardware acknowledgements Probes include only receiver-specific data Probes include sender-specific data too Probes include contention windows Probes do not include contention windows Pollcast RI-MACLPP Backcast

9. Network and Systems Laboratory nslab.ee.ntu.edu.tw Is it possible to design a general-purpose, yet efficient, receiver-initiated link layer? 9

10. Network and Systems Laboratory nslab.ee.ntu.edu.tw Most consequential decision a low-power MAC makes: stay awake or go to sleep? PreambleD TX RX D T listen Noise TX RX P P DATA Listen Sender-Initiated: Channel Sampling Receiver-Initiated: Channel Probing P 10

11. Network and Systems Laboratory nslab.ee.ntu.edu.tw 11 Solving the synchronization problem with Backcast A link-layer frame exchange in which: A single radio PROBE frame transmission Triggers zero or more identical ACK frames Transmitted with tight timing tolerance So there is minimum inter-symbol interference And ACKs collide non-destructively at the receiver PA TX RX PA PA TX You should be skeptical that this idea might work P. Dutta, R. Musaloiu-E., I. Stoica, A. Terzis, “Wireless ACK Collisions Not Considered Harmful”, HotNets-VII, October, 2008, Alberta, BC, Canada

12. Network and Systems Laboratory nslab.ee.ntu.edu.tw Outline Introduction A-MAC primitive Implementation Backcast evaluation High level evaluation Drawbacks Conclusion

13. Network and Systems Laboratory nslab.ee.ntu.edu.tw 13 A-MAC: An 802.15.4 receiver-initiated link layer PA Sender Receiver PA DATA Max data packet 4.256 ms ACK transmission time 352 µs RXTX turnaround time: 192 µs P P Listen

14. Network and Systems Laboratory nslab.ee.ntu.edu.tw 14 A-MAC’s contention mechanism Receiver Sender PAListenDP-CW PAListenDP-CW PAD D BO D frame collision Backcast

15. Network and Systems Laboratory nslab.ee.ntu.edu.tw 15 A-MAC’s parallel multichannel data transfers use control, data (1), and data (2) channels PA Sender 1 Receiver 1 PA DATA P P Listen PA Sender 2 Receiver 2 PA DATA P P Listen

16. Network and Systems Laboratory nslab.ee.ntu.edu.tw Outline Introduction A-MAC primitive Implementation Unicast Broadcast Pollcast Wakeup Backcast evaluation High layer evaluation Conclusion

17. Network and Systems Laboratory nslab.ee.ntu.edu.tw 17 Unicast PA Node 2 (Receiver) Node 3 (Sender) PA Node 1 (Sender) Listen D DST=0x8002 SRC=0x0002 DP PL MAC=0x8002 PAListenDP-CW MAC=0x8002 PAListenDP-CW MAC=0x8002 PADP-CWD BO D DST=0x8002 SRC=0x0002 ACK=0x0023 FRM=0x0001 DST=0x0002 SRC=0x0001 SEQ=0x23 frame collision Backcast

18. Network and Systems Laboratory nslab.ee.ntu.edu.tw 18 Broadcast TX 1 RX 2 RX 3 RX 4 ListenP A P A PA PA Backcast auto-ack=on addr-recog=off D D P P D D P P P A D P PAD P DST=0x8002 SRC=0x0002 DST=0x8002 SRC=0x0002 ACK=0x0023 FRM=0x0001 DST=0x0002 SRC=0x0001 SEQ=0x23 off on off Backcast

19. Network and Systems Laboratory nslab.ee.ntu.edu.tw 19 Wakeup PA Node 2 Node 3 Node 4 PA Node 1 Node 5 Listen P A P A PA PA PA PA P P A A Backcast DST=0xFFFF SRC=0x0002

20. Network and Systems Laboratory nslab.ee.ntu.edu.tw 20 Pollcast Node 2 (Receiver) Node 3 (Sender) Node 1 (Sender) PredEvent PredEvent Pred M. Demirbas, O. Soysal, and M. Hussain, “A Single-Hop Collaborative Feedback Primitive for Wireless Sensor Networks”, INFOCOM’08, April, 2008, Phoenix, AZ Listen MAC=0x8765 Listen MAC=0x8765 Listen PA PA PA Backcast DST=0xFFFF SRC=0x0002 PRED=elephant MAC=0x8765 Event DST=0x8765 P+Pred DST=0xFFFF SRC=0x0002 PRED=elephant A A A

21. Network and Systems Laboratory nslab.ee.ntu.edu.tw Outline Introduction A-MAC primitive Implementation Backcast evaluation High level evaluation Drawbacks Conclusion

22. Network and Systems Laboratory nslab.ee.ntu.edu.tw 22 Evaluating Backcast: equal-power, equal-path delay Setup Methodology Platform: Telos mote Protocol: IEEE 802.15.4 Radio: Texas Instruments CC2420 Experiment 8 responder nodes Connect w/ splitter/attenuator Turned on sequentially Transmit 100 packets 125 ms inter-packet interval Log RSSI: received signal strength LQI: chip correlation rate ARR: ACK reception rate

23. Network and Systems Laboratory nslab.ee.ntu.edu.tw 23 As the number of colliding ACKs goes from one to eight… But, for two nodes, LQI exhibits outliers and a lower median Median RSSI increases logarithmically Median LQI remains nearly constant but is more left-tailed ACK reception rate stays practically constant

24. Network and Systems Laboratory nslab.ee.ntu.edu.tw Backcast scales to a large number of nodes 24

25. Network and Systems Laboratory nslab.ee.ntu.edu.tw Energy cost of A-MAC primitives: probe, receive, transmit, and listen 25

26. Network and Systems Laboratory nslab.ee.ntu.edu.tw Idle listening cost: under heavy interference 26

27. Network and Systems Laboratory nslab.ee.ntu.edu.tw Outline Introduction A-MAC primitive Implementation Backcast evaluation High level evaluation Drawbacks Conclusion

28. Network and Systems Laboratory nslab.ee.ntu.edu.tw A-MAC offers modest unicast performance 28 R SS SS Collision Domain

29. Network and Systems Laboratory nslab.ee.ntu.edu.tw A-MAC supports multiple parallel unicast flows 29 R S R S R S Collision Domain

30. Network and Systems Laboratory nslab.ee.ntu.edu.tw A-MAC wakes up the network faster and more efficiently than LPL (Flash) flooding 30 Faster Wakeup Fewer Packets A-MAC LPL (Flash) A-MAC LPL (Flash)

31. Network and Systems Laboratory nslab.ee.ntu.edu.tw A-MAC wakeup works well at low duty cycles 31 “Wakeup Latency” is normalized to the probe interval T probe = 4,000 ms P avg = 63 µW I avg = 21 µA N = 59

32. Network and Systems Laboratory nslab.ee.ntu.edu.tw Supporting the Collection Tree Protocol (CTP), A-MAC beats LPL on nearly every figure of merit 32 R S S SS SS S S N = 59 T data = 60 s T probe = 500 ms

33. Network and Systems Laboratory nslab.ee.ntu.edu.tw The effect of interference on idle listening: Sampling, Probing, and Backcast 33 Sampling Channel 18Channel 26 Probing Backcast Sampling Probing Backcast Background noise/traffic/noise in an office environment

34. Network and Systems Laboratory nslab.ee.ntu.edu.tw Outline Introduction A-MAC primitive Implementation Backcast evaluation High level evaluation Drawbacks Conclusion

35. Network and Systems Laboratory nslab.ee.ntu.edu.tw 35 Primitive PA Sender Receiver PA DATA Max data packet 4.256 ms ACK transmission time 352 µs RXTX turnaround time: 192 µs

36. Network and Systems Laboratory nslab.ee.ntu.edu.tw Backcast degrades when path delay differences exceeds approximately 500 ns (500 ft free space) 36

37. Network and Systems Laboratory nslab.ee.ntu.edu.tw A-MAC single channel PDR degrades with high node density and high probe frequency 37 S R S S Collision Domain SS SSSS S SSSS SS SS S

38. Network and Systems Laboratory nslab.ee.ntu.edu.tw 38 Conclusion Backcast provides a new synchronization primitive Can be implemented using a DATA/ACK frame exchange Works even with a 8, 12, 94 colliding ACK frames Faster, more efficient, and more robust than LPL, LPP A-MAC augments Backcast to implement Unicast Broadcast Network wakeup Robust pollcast Results show Higher packet delivery ratios Lower duty cycles Better throughput (and min/max fairness) Faster network wakeup Higher channel efficiency