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Multichannel Reliability Assessment in Real World WSNs

Multichannel Reliability Assessment in Real World WSNs. Jorge Ortiz and David Culler University of California at Berkeley 9 th Int. Conf. on Information Processing in Sensor Networks (SPOTS Track) April 12-16, 2010 Stockholm, Sweden. Motivation.

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Multichannel Reliability Assessment in Real World WSNs

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  1. Multichannel Reliability Assessment in Real World WSNs Jorge Ortiz and David Culler University of California at Berkeley 9th Int. Conf. on Information Processing in Sensor Networks (SPOTS Track) April 12-16, 2010 Stockholm, Sweden

  2. Motivation • Channel diversity seen as necessary in industrial setting for reliable communication • Standards • 802.15.4e • SP100.11a • WirelessHART

  3. Our results demonstrate the contrary • Opportunities where multichannel provides communication where single-channel multihop routing cannot are rare • Event when those opportunities exists are rarely important • Well-connected network on single channel provides enough diversity for reliable communication

  4. Our contribution • This work formalizes assumptions that motivate multichannel in industrial settings • Evaluate multichannel utility in context of routing • Quantify the opportunity where multichannel necessary • Multichannel often unnecessary for reliable delivery when routing is an alternative

  5. Roadmap c2 • Diversity hides link variability • Standards goals and assumptions • Motivating study • Formalize assumptions • Introduce network facets to test assumptions • Multichannel Links (MCL) and Multichannel Triangles (MCT) • Results • Quantify the MCL and MCT occurrences • Show multichannel rarely helps when there is routing cα i j c1

  6. Sources of Loss:Collisions and External Interference 802.11 802.15.4

  7. Sources of Loss:Non-line of site communication

  8. Diversity Helps • Spatial diversity • Use multiple receivers • Multiple antennas • Multiple next-hop routing choices • Frequency Diversity • Signal modulation • DSSS • Channel hopping • FHSS • Time Diversity

  9. Standards Diversity Recommendations • To address multipath and external interface • Multichannel provides level of immunity against both loss sources • Interference on current channel • The sender has load to offer • Interference spans narrow band • To support end-to-end reliability • Multihop routing • Topology-formation recommendations made

  10. Current Claim:Multichannel diversity is required1 • Partly motivates standards decision to include multichannel • Directly motivates ISA SP100.11a • Evaluates the WSN radio channel quality in industrial environment • Link quality varies substantially over time • Multipath induced narrow-band fading negatively affects link quality • Multichannel necessary for reliability D. Sexton, M. Mahony, M. Lapinski, and J. Werb. Radio channel quality in industrial wireless sensor networks. In SICON ’05 Sensors for Industry Conference, 2005.

  11. Experimental methodology1 • 6 motes with CC2420 Radio • 40 ft x 66 ft (12 m x 20 m) • Round-robin transmission with local logging • 4 hours of continuous probing • Recorded packet reception rates (PRR) D. Sexton, M. Mahony, M. Lapinski, and J. Werb. Radio channel quality in industrial wireless sensor networks. In SICON ’05 Sensors for Industry Conference, 2005.

  12. No single channel provide best set of links1 D. Sexton, M. Mahony, M. Lapinski, and J. Werb. Radio channel quality in industrial wireless sensor networks. In SICON ’05 Sensors for Industry Conference, 2005.

  13. No single channel provide best set of links1 Links (1,2) ,(2,1) 46 26 65 35 15 54 24 63 43 13 52 61 32 21 41 D. Sexton, M. Mahony, M. Lapinski, and J. Werb. Radio channel quality in industrial wireless sensor networks. In SICON ’05 Sensors for Industry Conference, 2005.

  14. No single channel provide best set of links1 Channel 13, 25 High loss Channel 13, ~70% loss Channel 25, ~30% loss Links (1,2) ,(2,1) 21 D. Sexton, M. Mahony, M. Lapinski, and J. Werb. Radio channel quality in industrial wireless sensor networks. In SICON ’05 Sensors for Industry Conference, 2005.

  15. No single channel provide best set of links1 PRR(12, 13)=70% PRR(21, 13)=100% No single channel good for all links Links (1,2) ,(2,1) PRR(12, 25)=30% PRR(21, 25)=100% 21 D. Sexton, M. Mahony, M. Lapinski, and J. Werb. Radio channel quality in industrial wireless sensor networks. In SICON ’05 Sensors for Industry Conference, 2005.

  16. …and from this they conclude1 • “There was no channel that allowed for reliable communications over all paths for all units throughout the entire test period … None of the paths were very symmetric for all channels. The results of these experiments clearly show that a frequency agile approach might be more robust than a single channel approach. . . ” • These implicitly identify instances where multichannel provides reliable delivery where single channel cannot • Conclusion is sound • …but only if we consider one-hop, direct communication D. Sexton, M. Mahony, M. Lapinski, and J. Werb. Radio channel quality in industrial wireless sensor networks. In SICON ’05 Sensors for Industry Conference, 2005.

  17. Formalizing the observations:Multichannel Links (MCL) • What is an asymmetric link? • A link for distinct nodes i and j is asymmetric if the link PRR(i,j) >= T and PRR(j,i) < T for some usability threshold, T. • How can frequency agility improve problems with asymmetric links? • We refer to this link as a Multichannel Link c2 cα i j c1

  18. Formalizing the observation: Multichannel Triangle (MCT) • “There was no channel that allowed for reliable communications over all paths for all units throughout the entire test period .” • Formally: 3 distinct nodes, i, j, and k, can all communicate bi-directionally on some channel, but no channel where all 3 can communicate. • c1≠ c2and c1 ≠ c3 • Note path from i to j when c2 = c3 • Routing can be used as an alternative to multichannel communication

  19. Environments tested • Industrial machine room • 95 ft x 40 ft (28.9 m x12.2 m) • 20 TelosB motes • Computer room • 28 ft x 28 ft (8.5 m x 8.5 m) • 23 TelosB motes • Office setting • 128 ft x 128 ft (39 m x 39 m) • 55-60 MicaZ motes

  20. Experimental Methodology • Motes with CC2420 Radio • Each mote sends 100 packets at 20 millisecond inter-packet interval, round robin, on each channel • Listening motes log packets to flash • Multiple 802.15.4 channels probed • Multiple experimental runs

  21. Log Analysis • Reliability determined through path existence • Logs contain source and sequence number • Connectivity graph constructed on each channel • Link PRR calculated for each observed link • Usability threshold, T, applied in the construction of each graph • MCL and MCT locator processed over every connectivity graph • ~1.7 million packets sent, >3500 graphs examined • 3 runs (12 hours) in machine room, 2 (8 hours) in computer room, 17 in office setting (10 days) continuous probing

  22. Environmental Comparisons • 6 random nodes selected • Similar patterns observed in all 3 environments Computer Room Links Link(36, 22)=70% loss Link(63, 22)=4% loss Link(36, 13)=53% loss Link(63, 13)=2% loss

  23. Environmental Comparison • Loss pattern observed similar but less ‘narrow’ • May affect multichannel’s opportunity to find an alternative frequency Industrial Environment Testbed Environment

  24. Experimental Results: MCL Count • Many unidirectional links found • Varies from 8-70% of the links being unidirectional on some graph • Connectivity still maintained throughout in machine room and computer room, ~95% of time on testbed • 2-6% of links in all graphs for all settings are MCL links

  25. The key question Which tradeoff do you want to live with? • Are these links important? • Our data show: • Never important in machine room • Never important in computer room • 1.8% of occurrences on testbed prevent network partition c2 cα i j c1 cα important for preventing network partition

  26. Experimental Results: MCT Count • Single channel set (SC set) • All distinct node triples connected on a single channel • Multichannel set (MC set) • All distinct node triples connected on any channel • MCT set • All distinct node triple in the multichannel set and not in the single channel set • MCT occurrence rate = |MCT set|/|MC set|

  27. MCT Count: Industrial Machine room • 6-hop network diameter • Maximum occurrence rate is <60 ppm

  28. MCT Count: Computer room • 3-hop network diameter • Fewer samples, same trend • Maximum occurrence rate is 2 ppm

  29. MCT Count: Office Space Setting • 4-hop network diameter • Maximum occurrence rate is 200 ppm • almost 5 times the rate of the industrial machine room!

  30. MCT Routing Solution • Every MCT has a routing solution • One of the channels tested had routing solution for all MCTs • Connectivity graphs connected vast majority of the time • When disconnected, it was disconnect on every channel

  31. Routing Stretch and Transmission Stretch for MCT Route Solutions • Routing Stretch • Ratio of number of hops for the single channel solution to multichannel solution in MCT • Average stretch = 1.03 • Transmission Stretch • Ratio of number of expected transmissions for single channel solution to the multichannel solution in MCT • Average stretch = 0.97 best case (1.22 worst case)

  32. Conclusion • Opportunities where multichannel provides communication where single-channel multihop routing cannot are extremely rare • Event when those opportunities exists are rarely important • Routing solution available • Routing cost comparable to multichannel cost • Well-connected network, single channel communication provides enough diversity (coding, spatial) for reliable communication

  33. Thank You • Contact information: jortiz@cs.berkeley.edu • Dataset Publicly Available • http://wsn.eecs.berkeley.edu/connectivity/about.php?dataset=soda • Questions?

  34. Extra slides

  35. Misc Comparisons • Multichannel protocols are larger in code size and more complex • Passive listening yields high listening-cost overhead

  36. Experiment runtime and path stability • Experiment runtime • Testbed experiment ran for 10 days continuously • Computer room ran for about 12 hours • Industrial machine room 8 hours • How stable were the paths observed on single channel? • Not explicitly in this paper, but we have another paper that does examines this on our testbed using the same methodology and finds that it varies by threshold between 7-9 hours average path stability • The time per experiment is 30 minutes • Channel 26 only • As T increases, the routes are less stable and the stretch increases

  37. More questions • Might multichannel have more stable routes? • Depends on the threshold criteria • If MCL is found • Looser criteria increases path stability • But that’s if the multichannel scheme does not switch from that channel according to the schedule • Something else to consider • Have you considered the effects on communication bandwidth? • No • Still limited by routing tree • What’s the overhead comparison? • Synchronization • Broadcast • Idle listening approach keeps radio on • Join cost is high • Added complexity reflected in code size

  38. Methodology Discussion • Justify methodology • We cannot observe link state without probing • Probes cannot occur simultaneously • Sequential probing must be done to observe state • Similar to routing-protocol topology formation • Many samples over extended periods of time in heterogeneous settings decreases sampling error • Random stalls in experimental runs desynchronizes probe stages • Prevents bias through aliasing

  39. Context and Assumptions • Link-level acknowledgements necessary for reliability through re-transmissions • For reliable delivery link must be bi-directional over a given channel • Only about reliability • Multiple runs, diverse settings, broad timeframe necessary to observe underlying behavior • ~1.7 million packets sent, >3500 graphs examined • 3 runs in machine room, 2 in computer room, 17 in office setting • ~12 days worth of experimental runs

  40. Standards Address Concerns for Industrial Settings • Three main standards bodies formed to address concerns in industrial settings • 802.15.4e • SP100.11a • Wireless HART • General goals • Reliable packet delivery • Long deployment lifetime • Adjustable QoS

  41. SP100.11a Frequency Hopping Simulation • Supports 5 hopping patterns • We ran pattern index 1 • 19, 20, 24, 16, 23, 18, 25, 14, 21, 11, 15, 22, 17, 13, 26 • Connectivity graph much worse without backlisting • Connectivity graph same to remaining on single channel with backlisting • Random nodes selected to transmit on random channel

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