Wireless Sensor Networks (WSN) Introduction M. Schölzel
Difference to existing wireless networks Infrastructure-based networks e.g., GSM, UMTS, Base stations connected to a wired backbone network Mobile entities communicate wirelessly to these base stations Traffic between different mobile entities is relayed by base stations and wired backbone Infrastructure-free networks Try to construct a network without infrastructure, using networking abilities of the participants This is an ad hoc network a network constructed for a special purpose Simplest example: Laptops in a conference room a single-hop ad hoc network Gateways IP backbone Server Router
A Wireless Sensor Network Internet, LAN, GSM- Network, wireless communication wired communication Network is embedded in environment Nodes in the network are equipped with sensing and actuation to measure/influence environment Nodes process information and communicate it wirelessly Sensornet
Roles of Participants in WSN sources of data: Measure data, report them somewhere Typically equip with different kinds of actual sensors sinks of data: Interested in receiving data from WSN May be part of the WSN or external entity, PDA, gateway, actuators: Control some device based on data, usually also a sink 4
Goal: Collecting data WSN Application Examples Environmental Monitoring Each node measures temperature Derive a temperature map Intelligent buildings, bridges, or machines Measuring vibrations Control of leakages in chemical plants Monitor mechanical stress (e.g. during earthquakes) Predictive maintenance 5
Example: Parking Space Management Each parking space is equipped with a sensor node AMR sensor senses disturbances on the magnetic field of the earth (resistance changes, if a car is above the sensor) Hardware: 8-bit microcontroller RF transceiver: CC1120 with 4kbps data rate 3.6V Li-ion battery Rather simple network topology: Single hop from sensor node to sink.
Other WSN Application Domains Precision agriculture Bring out fertilizer/pesticides/irrigation only where needed Medicine and health care Wearable sensor nodes monitoring movement of patients Logistics Equip goods (parcels, containers) with a sensor node Track their whereabouts total asset management E.g. Bosch already provides commercial solutions Telematics Provide better traffic control by obtaining finer-grained information about traffic conditions Intelligent roadside Cars as the sensor nodes 7
Examples Car2Car and Car2X communication Communication Standard WLAN-p already exists Cars have on-bord-units, infrastructure has roadside-units Cohda-Box commercially available 32-bit ARM-like processor runs a Linux system up to 27 mbps Cohda OBU Cars and infrastructure can send various kinds of messages CAMs: position, direction, speed, etc. DENMs: Detected hazards SPATs: State of the traffic light 8
WSN Application Examples Goal: Building control loops (sensing -> computing -> acting) Communication in industrial environments Communication between mobile machines Requirement real time demands Wireless heart standard much higher reliability of communication required probability of packet error rate ~10-9 typical bit error rate in wireless channels: 10-3 to 10-4
wireless ad-hoc networks: problems Without a central infrastructure, things become much more difficult Problems are due to Lack of central entity for organization available Limited range of wireless communication Battery-operated entities Mobility of participants 10
Problem: Lack of Central Entity Without a central entity (like a base station), participants must organize themselves into a network (self-organization) Pertains to (among others): Medium access control no base station can assign transmission resources, must be decided in a distributed fashion Finding a route from one participant to another 11
Problem: Limited Range For many scenarios, communication with peers outside immediate communication range is required Direct communication limited because of distance, obstacles, Solution: multi-hop network? 12
Problem: Battery-Operated Entities Often (not always!), participants in an ad hoc network draw energy from batteries Desirable: long run time for Individual devices Network as a whole Required: Energy-efficient networking protocols E.g., use multi-hop routes with low energy consumption (energy/bit) E.g., take available battery capacity of devices into account How to resolve conflicts between different optimizations? 13
Problem: Mobility In many (not all!) ad hoc network applications, participants move around (-> mobile ad hoc networks (MANET)) Car2Car-communication Moving robots in industrial environments In cellular network: simply hand over to another base station In MANET Mobility changes neighborhood relationship Routes needs adaptation Complicated by scale Large number of such nodes difficult to support 14
Requirements for WSN (user perspective) Quality of Service (QoS) Detect events and deliver data reliable events may be detected by multiple nodes reliability of a single node doesn t matter, system reliability is important! Deliver data in within a specified delay Lifetime The network should fulfill its task as long as possible definition depends on application Lifetime of individual nodes relatively unimportant Fault tolerance Be robust against faults node failures (often permanent): running out of energy, physical destruction link failures (often temporarily): weather, moving obstacles, 15
Requirements for WSN (administrator perspective) Deployment Simple deployment even for thousands of nodes (self-organization) Scalability Support large number of nodes (several thousands) in existing practical applications < 100 Maintainability WSN has to adapt to changes, self-monitoring, adapt operation Incorporate possible additional resources, e.g., newly deployed nodes Programmability Re-programming of nodes in the field might be necessary, improve flexibility, fixing bugs 16
Mechanisms to Overcome Problems and to Meet Requirements Multi-hop wireless communication to overcome limited communication range requires self-organization mechanisms Energy-efficient operation For communication, computation, sensing, actuating Very typical: computation far less energy hungry than communication Collaboration & in-network processing Nodes in the network collaborate towards a joint goal Pre-processing data in network (as opposed to at the edge) can greatly improve efficiency Locality Do things locally (on node or among nearby neighbors) as far as possible Data centric networking Focusing network design on data, not on node identifies (id-centric networking) To improve efficiency 17
Mechanisms to meet requirements Auto-configuration Manual configuration just not an option Right after deployment But also periodically during operation to overcome link and node failures mobility problems Exploit tradeoffs during the design phase E.g., between invested energy and accuracy Design-space-exploration 18
Design Space Due to the many options for tackling problems, WSNs offer a very large design space Trade-offs between many design space parameters must be found in order to meet the requirements Design space parameters: Form Factor Connectivity Communication Modality Heterogeneity Network Topology Radio Coverage Sensor Coverage
Design Space Form factor of the mote classes: brick, matchbox, grain, dust Depends on application (microscopic to shoebox) Costs may range from cents to hundreds of euro's Impacts life time (e.g. size of battery), processing resources, costs Heterogeneity of the WSN classes: homogeneous/heterogeneous First approach: identical nodes only In practice: a variety of nodes can be very useful e.g. cluster heads with more resources special capabilities only required for some nodes (e.g. GPS) gateways to external networks (GSM, satellite, Internet) Impacts Complexity of software 20
Design Space Communication Modality classes: radio (169MHZ, 434MHz, 868MHz, ), visual light communication (VLC), ultrasound, How do nodes communicate? most common: radio waves, usually sub-gigahertz bands light beams or laser: smaller, more energy efficient (cf. Smart Dust) Impacts data rate, life time, communication range Connectivity classes: connected / intermittent / sporadic Nodes always connected or only sometimes (regularly or sporadic)? Impacts Protocols, data gathering, life time 21
Design Space Sensor-Coverage classes: sparse / dense / redundant sensors could cover only part of area of interest, or all, or the same area is covered multiple times Impacts: observational accuracy, number of nodes, costs Radio-Coverage classes: from sparse to dense / static /dynamically How many other nodes will be in the communication range? May be adapted at runtime Impacts: reliability, energy consumption due to collisions, transmission power, etc., number of nodes 22
Design Space Network Topology classes: infrastructure or ad-hoc (single-hop / star / networked stars / tree / graph) infrastructure-based: motes communicate via base stations only often costly ad-hoc: direct communication between nodes no expensive infrastructure diameter is the max number of hops between any two nodes single hop (d=1), infrastructure based (d=2), multi-hop (d big for ad-hoc networks) Impacts: Communication delay, protocol complexity 23
Development Flow and Design Space Exploration Start with requirements Models Implementation (+Software) Testbed (+Hardware) Deployment (+Topology) Evaluation Simulation Test/Debug Test/Debug Models used for an early evaluation of possible design parameters Simulations: Evaluate the prediction of the model in the full dynamic of the network Test and debug to find software bugs Not all aspects of the hardware are accurately reflected Testbed uses real hardware Test the software on real hardware Test and Debug in a distributed system difficult Systematic test of fault-tolerance techniques not easy (often the code is instrumented) real sensors and actors maybe not available virtualization Deployment test in a real setup is required Not always possible Test-bed infrastructure not available anymore Over-the-air update power measurement 24
Required Knowledge for Developing WSN Applications Knowledge of Models and Simulators Microprocessor architecture and programming Communication with peripherals Usage of operating systems Power saving techniques Architecture and Design of Protocols Power saving techniques Fault Tolerance Forward- and backward-error correction