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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

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

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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

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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

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Roadmap 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 ij cc c2c2 c1c1

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Sources of Loss: Collisions and External Interference 802.11 802.15.4

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Sources of Loss: Non-line of site communication

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

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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

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Current Claim: Multichannel diversity is required 1 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 1D. Sexton, M. Mahony, M. Lapinski, and J. Werb. Radio channel quality in industrial wireless sensor networks. In SICON 05 Sensors for Industry Conference, 2005.

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Experimental methodology 1 1D. Sexton, M. Mahony, M. Lapinski, and J. Werb. Radio channel quality in industrial wireless sensor networks. In SICON 05 Sensors for Industry Conference, 2005. 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)

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No single channel provide best set of links 1 1D. Sexton, M. Mahony, M. Lapinski, and J. Werb. Radio channel quality in industrial wireless sensor networks. In SICON 05 Sensors for Industry Conference, 2005.

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

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

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

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and from this they conclude 1 1D. Sexton, M. Mahony, M. Lapinski, and J. Werb. Radio channel quality in industrial wireless sensor networks. In SICON 05 Sensors for Industry Conference, 2005. 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

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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 ij cc c2c2 c1c1

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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. c 1 c 2 and c 1 c 3 Note path from i to j when c 2 = c 3 Routing can be used as an alternative to multichannel communication

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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

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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

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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

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

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Environmental Comparison Loss pattern observed similar but less narrow May affect multichannels opportunity to find an alternative frequency Industrial Environment Testbed Environment

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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

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The key question 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 ij cc c2c2 c1c1 c important for preventing network partition Which tradeoff do you want to live with?

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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|

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MCT Count: Industrial Machine room 6-hop network diameter Maximum occurrence rate is

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

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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

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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