Architectures and Applications for Wireless Sensor Networks (01204525) Network Architecture Chaiporn Jaikaeo chaiporn.j@ku.ac.th Department of Computer Engineering Kasetsart University Materials taken from lecture slides by Karl and Willig

Outline Network scenarios Optimization goals Design principles Service interface Gateway concepts

Typical Views of WSN Self-organizing mobile ad hoc networks (MANETs) Peer-to-peer networks Multi/mobile agent systems and swarm intellegence

Sensor Network Scenarios Sources: Any entity that provides data/measurements Sinks: Nodes where information is required Sink Source Sink Source Source Internet Sink

Single-Hop vs. Multi-hop Multi-hop networks Send packets to an intermediate node Intermediate node forwards packet to its destination Store-and-forward multi-hop network Store & forward multi-hopping NOT the only possible solution E.g., collaborative networking, network coding Source Sink Obstacle

Multi-hopping Always Efficient? Obvious idea: Multi-hopping is more energy-efficient than direct communication Suppose we put a relay at distance d/2 Energy for distance d is reduced from cd to 2c(d/2) c - some constant  - path loss coefficient ( 2) Usually wrong, or over-simplified Need to take constant offsets for powering transmitter, receiver into account

Multiple Sinks, Multiple sources

Different sources of mobility Node mobility Sink mobility Event mobility

Sink Mobility Request Propagation of answers Movement direction

WSN Event Mobility

Outline Network scenarios Optimization goals Design principles Service interface Gateway concepts

Goal: Quality of Service QoS in WSN is more complicated (compared to MANET) Event detection/reporting probability Event classification error, detection delay Probability of missing a periodic report Approximation accuracy (e.g, when WSN constructs a temperature map) Tracking accuracy (e.g., difference between true and conjectured position of the pink elephant) Related goal: robustness Network should withstand failure of some nodes

Goal: Energy efficiency Many definitions Energy per correctly received bit Energy per reported (unique) event Delay/energy tradeoffs Network lifetime Time to first node failure Network half-life (how long until 50% of the nodes died?) Time to partition Time to loss of coverage Time to failure of first event notification

Sharpening the Drop Sacrifice long lifetimes in return for an improvement in short lifetimes

Goal: Scalability Network should be operational regardless of number of nodes At high efficiency Typical node numbers difficult to guess MANETs: 10s to 100s WSNs: 10s to 1000s, maybe more Requiring to scale to large node numbers has serious consequences for network architecture Might not result in the most efficient solutions for small networks! Carefully consider actual application needs before looking for scalable solutions

Outline Network scenarios Optimization goals Design principles Service interface Gateway concepts

Distributed Organization WSN participants should cooperate in organizing the network Centralized approach usually not feasible Potential shortcomings Not clear whether distributed or centralized organization achieves better energy efficiency Option: “limited centralized” solution Elect nodes for local coordination/control Perhaps rotate this function over time

In-Network Processing WSNs are expected to provide information Gives additional options E.g., manipulate or process the data in the network Main example: aggregation Apply aggregation functions to a collection tree in a network Typical functions: minimum, maximum, average, sum, … Not amenable functions: median

Aggregation Example 1 3 6

Signal Processing Another form of in-network processing E.g., Edge detection Tracking/angle detection of signal source Exploit temporal and spatial correlation Observed signals might vary only slowly in time Signals of neighboring nodes are often quite similar Compressive sensing

Adaptive Fidelity Adapt data processing effort based on required accuracy/fidelity E.g., event detection When event occurs, increase rate of message exchanges E.g., temperature When temperature is in acceptable range, only send temperature values at low resolution When temperature becomes high, increase resolution and thus message length

Data Centric Networking Interactions in typical networks are addressed to the identities of nodes Known as node-centric or address-centric networking paradigm In WSN, specific source of events might not be important Several nodes can observe the same area Focus on data/results instead  Data-centric networking Principal design change

Implementation Options Publish/subscribe (NDN – Named Data Networking) Nodes can publish data, can subscribe to any particular kind of data Once data of a certain type has been published, it is delivered to all subscribers Databases SQL-based request

Other Design Principles Exploit location information Exploit activity patterns Exploit heterogeneity By construction By evolution Cross-layer optimization of protocol stacks for WSN Goes against grain of standard networking Promises big performance gains

Outline Network scenarios Optimization goals Design principles Service interface Gateway concepts

Interfaces to Protocol Stacks The world’s all-purpose network interface: sockets Good for transmitting data from one sender to one receiver Not well matched to WSN needs (ok for ad hoc networks) App as another component Well-designed interface

Outline Network scenarios Optimization goals Design principles Service interface Gateway concepts

Gateways in WSN/MANET Allow remote access to/from the WSN Bridge the gap between different interaction semantics E.g., data vs. address-centric networking Need support for different radios/protocols

WSN to Internet Communication E.g., deliver an alarm message to an Internet host Issues Need to find a gateway (integrates routing & service discovery) Choose “best” gateway if several are available How to find Alice or Alice’s IP?

Internet to WSN communication How to find the right WSN to answer a need? How to translate from IP protocols to WSN protocols, semantics? Gateway nodes Remote requester Internet Gateway

WSN tunneling Use the Internet to “tunnel” WSN packets between two remote WSNs Gateway nodes Internet Gateway

6LoWPAN IPv6 over Low-power Wireless Personal Area Networks Nodes communicate using IPv6 packets An IPv6 packet is carried in the payload of IEEE 802.15.4 data frames

Example 6LoWPAN Systems

Summary Network architectures for WSNs look quite different from typical networks in many aspects Data-centric paradigm opens new possibilities for protocol design