Wireless Sensor Networks

Puzzle Three bulbs inside a room Three switches outside the room Room initially locked You can initially play with the switches You then need to enter the room and be able to match switches to their respective bulbs Devise a strategy

Puzzle Turn switch 1 ON Wait for 10 minutes Turn switch 1 OFF Enter room Lit bulb: switch 2 Unlit bulb that is warm: switch 1 Unlit bulb that is cold: switch 3

Grades Exam 1: 10 points Exam 2: 15 points Final exam: 40 points Assignments: 18 points Project: 12 points Class participation: 5 points

Exam 2 Max = 15 Min = 6 Average = 12.96 Stdev = 2.24

Directed Diffusion A node requests data by sending interests for named data The request “How many pedestrians do you observe in region X” is broadcasted to region X Data matching the interest is then “drawn” down towards the node When a node in region X receives the request, it activates its sensors, and returns sensed information along reverse path of interest propagation Intermediate nodes can cache, or transform data Combine reports from multiple sensors to more accurately pinpoint pedestrian’s location

Elements of Directed Diffusion Interests Query of what the user wants Data messages Collected or processed information of a physical phenomenon Gradients Direction state created in each node that receives the interest Reinforcements Of one or a small number of the available paths

Naming Attribute-value pairs Example: Vehicle detection task (query) (Type=wheeled, interval=20ms, duration=10seconds, rect=[-100,100,200,400]) VDT (response) (type=wheeled,instance=truck,location=[125,220],intensity=0.6,confidence=0.85,timestamp=01:20:40)

Interests Can be initiated by the sink Exploratory interest with a large interval, followed by reinforcements e.g. to detect any wheeled vehicles Soft-state refreshing of interests reliability & overheads Each node maintains one entry per interest in an interest-cache

Interests (contd.) Each interest entry contains a gradient (neighbor, report rate, and lifetime) Interest entry possibly created upon receipt of interest Interest possibly forwarded to a sub-set of neighbors e.g. based on cached data

Gradient Establishment A generic notion Can be implemented in several ways: binary values, probabilistic forwarding, load balancing Gradients might be set-up differently for different tasks

Data Propagation Nodes in “rect” sense data Propagates data according to the gradients to the corresponding interest entry If an intermediate node receives data, but finds no interest entry, it drops the data Gradients can change as data is being forwarded e.g. down-sampling : 100 events/second to 50 events/second

Reinforcement Exploratory gradients vs. data gradients Sink reinforces one (or a subset) of the neighbors reporting back exploratory events Data gradients can have a higher reporting rate – positive reinforcement Allows sink to reinforce selective paths and reduce multi-path routing for the real heavy data

Other Issues MAC Topology control (with sensing reliability) Sensor placement Reliable transport Congestion control

CtS

Puzzle A bridge can carry at most two people at a time It is dark and a flashlight is required to cross the bridge Four people A, B, C, D at one end of the bridge want to cross the bridge Have only one flash light A will take 10 minutes to cross the bridge, B will take 5 minutes, C will take 2 minutes, and D will take 1 minute If two people cross the bridge together, time taken is determined by the slowest of the two What is the minimum amount of time in which all four can cross the bridge?