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

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

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

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 cαcα c2c2 c1c1

Sources of Loss: Collisions and External Interference

Sources of Loss: Non-line of site communication

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

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

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.

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, 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)

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.

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, Links (1,2),(2,1)

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, Links (1,2),(2,1) Channel 13, 25 High loss Channel 25, ~30% loss Channel 13, ~70% loss 21

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, 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

…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, “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

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 cαcα c2c2 c1c1

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

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

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 channels probed Multiple experimental runs

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

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

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

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

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 cαcα c2c2 c1c1 c α important for preventing network partition Which tradeoff do you want to live with?

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|

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

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

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!

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

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)

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

Thank You Contact information: Dataset Publicly Available ▫ php?dataset=sodahttp://wsn.eecs.berkeley.edu/connectivity/about. php?dataset=soda Questions?

Extra slides

Misc Comparisons Multichannel protocols are larger in code size and more complex Passive listening yields high listening-cost overhead ProtocolROMRAM PracMac B-MAC B-MAC w/ACK B-MAC w/LPL + ACK

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

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

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

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

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

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