Marginal Value of Multiple Channels in Real World WSNs Jorge Ortiz and David Culler CS262B Final Project

Talk Overview  Motivation  Reliability  Channel Quality in Industrial Environments  Multichannel MAC Standardization  Limitations  Sexton Links and Sexton Triangles  Experimental Setup and Methodology  Machine room, computer room, testbed  Analysis of Traces  Results  Link characteristics in our environments  Sexton Links in practice  Sexton Triangles in practice

Communication in Sensornets  Sensornets subject to tight resource constrains  Small memory, little energy  Communication is expensive  Wireless Communication is hard  Signal propagation unpredictable  Non-Line-of-Site communication (NLOS)  Interference  Internal, External, Fading, Multipath  Layered Diversity  Time (retries)  Space (Multiple antennas, routes)  Frequency (DSSS, FHSS, Multichannel)

Reliability  Reliability is about reachability  High latency not a factor  Low throughput not a factor  Reachability is about connectivity graph formation  Is there a path from me to you at this time?  Graphs change over time  Wireless landscape changes over time and space  Link population change

Industrial Environments  Metal and concrete surfaces  Multipath-induced narrowband fading  LOS and NLOS communication  Reliability requirement amplified

Dan Sexton’s Study  Motivated (in part) the formation of Multichannel Standards Bodies  SP100A  Wireless HART  e  Examined industrial environment RF connectivity for low- power radios  Radio (CC2420)  Signal delay spread  Time-of-arrival differences in signal  Large signal delay spread problematic for radios without equalization (such as CC2420). Anything > 50 nanoseconds is a problem  Study measures average delay spread at 34.4 nanoseconds but cites similar studies that measure up to 500 nanosecond delay-spread

Sexton’s Assertions Connectivity Assertion  “[In the first experiment] there was no channel that allowed for reliable communications over all paths for all units throughout the entire test period. [In the second experiment], only channel 15 was clear for all paths. 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... ” How much more reliability does frequency agility offer in the context of routing in a mesh network?

Asymmetric Links and Sexton links “None of the paths were very symmetric for all channels. …a frequency agile approach might be more robust than a single channel approach…” ij c1c1 c2c2 Single Channel Asymmetric Links  Link usability threshold, T ij c1c1 c2c2 Construct bidirectional path c1c2c1c2 ij cαcα Some channel α where Bidirectional link exists

Multihop Connectivity  Only single-hop paths examined  Most deployments over multihop meshes  Routing must be considered “…there was no channel that allowed for reliable communications over all paths for all units throughout the entire test period…“ i j c3c3 c2c2 k c1c1 C 1 ≠ C 2 AND C 1 ≠ C 3 If c 2 = c 3 Sexton Triangle

Live Network Experiments  We can test how often this occurs in live networks  Our experiments took place in various environments  Industrial machine room  Computer room with various computer racks, AC units  Testbed with interference in office environment

Experimental Setup  Motes with CC2420 Radio  Each mote sends 100 packets and 20 millisecond inter- packet interval  Each mote that’s not sending logs received packets flash  Once all motes send out 100 packets, network switches to the next channel  Data collected after each mote has transmitted 100 packets on each channel  Multiple channels probed  Multiple experimental runs

Log Analysis  Transmitted packets include sender and sequence number  Receiver logs both plus their own id and channel  Link packet reception rate (PRR) extracted for each pair of nodes from the logs  Connectivity graph constructed on each channel (16 channels) for each experimental run  Sexton Link Locator  Find all links that either do not exist or exist unidirectionally while it exists bidirectionally on another channel

Log Analysis (2)  Sexton Triangle Locator  Construct Single-channel triangle set (S)  Distinct nodes (i,j,k) such that they share a pairwise bidirectional link on a single channel  Construct Multichannel triangle set (M)  Distinct nodes (i,j,k) such that they share a pairwise bidirectional link on a any channel  Sexton Triangle Set (M u )  All triples in M that are not in set S  Distinct nodes (i,j,k) such that they shared share a pairwise bidirectional link on any channel, but there is no channel where they can all communicate

Sexton Links  Considered all environments  All channels, all thresholds (between T=1 to T=90)  Asymmetric links are common  18-46% of the links in each environment are asymmetric  Large difference in distribution of asymmetric links across channels  Only between 2-6% of all links are Sexton Links Computer Room

Triangle Sets in Machine room  4 orders of magnitude difference between the size of set M (Multichannel Triangle Set) and M u (Sexton Triangle Set).  ~20 parts per million  Note: M uK denotes the Sexton Triangle set with a K-hop solution All Sexton Triangles found have a 2-hop routing solution

Triangle Sets in Computer Room  Also 4 orders of magnitude difference between M and M u  Sexton Triangles only present in first run  Each has two-hop routing solution  None found in 2 nd and 3 rd runs

Triangle Sets on Testbed  4 orders of magnitude difference  ~50 parts per million  Only 4 runs had 2-hop routing solution for all Sexton Triangles found  Two with a routing solution > 3 hops  2 of the 72 Sexton Triangles found in run 9  2 of the 74 Sexton Triangles found in run 10

Conclusion  Multichannel reliability argument can be reduced to existence and prevalence of Sexton Links and Sexton Triangles  Sexton Links are rare in practice  2-6% of all links are Sexton Links  Sexton Triangles are extremely rare in practice  ppm rate across all our environments  Every Sexton Triangle has a single channel routing solution  Multichannel unnecessary for reliability in well-connected networks  More analysis and results in paper!

Thanks  Questions?

Sexton Link definition  Asymmetric Link useful to multichannel protocol  If i and j are distinct nodes in the network and (i, j) is a link from node i to node j, then it is a Sexton Link if there is are distinct channels c 1 and c 2 and a useability threshold, T, such that PRR((i,j)) c1 > T and PRR((j,i)) c1 T and PRR((i,j)) c2 T and PRR((i,j)) c α > T  A link that is unidirectional on some channel and bidirectional on another  A link that is non-existent on some channel and bidirectional on another

Sexton Triangle Definition  A Sexton Triangle consists of a 3-tuple of distinct nodes (i,j,k) such that (i,j) share a bi-directional link on channel c 1, (i,k) share a bi-directional link on channel c 2, and (j,k) share a bi-directional link on channel c 3, but c 1 ≠c 2 and c 1 ≠c 3 and there is no channel where all three can communicate

Sexton Triangles in Machine Room  Multichannel Triangles in Industrial Environment  554,578 Multichannel Triangles found  Sexton Triangles where each link on different channel  11 Sexton Triangles found

Sexton Triangles in Computer room  Multichannel Triangles in computer room  512,357 Multichannel Triangles found  Sexton Triangles where each link on different channel  2 Sexton Triangles found

Sexton Triangles on Testbed  Multichannel Triangles in computer room  3,199,286 Multichannel Triangles found  Sexton Triangles where each link on different channel  176 Sexton Triangles found