Wireless Sensor Networks: Clustering, Routing, Localization, Time Synchronization Maurizio Bocca, M.Sc. Control Engineering Research Group Automation and Systems Technology Department maurizio.bocca@tkk.fi http://autsys.tkk.fi/en/mauriziobocca
Outline Introduction Clustering in WSNs Routing in WSNs Localization in WSNs Time Synchronization in WSNs 2
Introduction ROUTING LOCALIZATION CLUSTERING TIME-SYNC 3
Clustering in WSNs DEFINITION: the process of grouping two or more nodes of the network in such a way that they behave like a single (more powerful) one Involved issues: CLUSTERS: number, intra-cluster topology, inter-cluster connectivity, stability CLUSTER-HEADS: capabilities, functionality, load balancing CLUSTERING PROCESS: selection of the cluster head, objective of node grouping (e.g. load balancing, fault tolerance, network connectivity, etc.), methodology (centralized, distributed, hybrid) 4
Routing in WSNs Routing: the process of selecting paths in a network along which to send traffic The choice of the suitable routing protocol is closely related to the architecture of the network Fixed vs. moving nodes Static events (reactive reporting, less traffic) vs. dynamic events (periodic reporting, more traffic) Deployment of the nodes: deterministic (fixed paths to the sink) vs. self-organizing (ad-hoc routing infrastructure) Energy consumption: multi-hop routing introduces significant overhead for topology management and MAC Data delivery model: continuous, event-driven, query-driven, hybrid Node capabilities: homogeneous vs. heterogeneous Presence of data aggregation/fusion nodes in the network 5
Routing in WSNs - Comparison 6
Routing in WSNs LEACH LEACH (Low-Energy Adaptive Clustering Hierarchy) Suitable for periodically reporting applications Clusters formation is based on RSSI measurements The cluster heads (CHs) are used as routers to the sink node The optimal number of cluster heads is estimated to be 5% of the total number of nodes 7
Routing in WSNs LEACH The CHs perform data aggregation/fusion The CHs automatically change randomly over time to balance the energy consumption of the nodes The node selects a random number in [0,1] The node becomes a CH for the current round if the chosen random number is less than a threshold T p = desired percentage of cluster heads (optimal = 0.05) r = current round G = set of nodes that have not been CHs in the last 1/p rounds 8
Routing in WSNs LEACH After the election, the CHs broadcast an advertisement message to the rest of the nodes of the network These nodes receive the advertisements and decide which cluster they want to join depending on the RSSI measurements The nodes inform the selected CH that they will be a member of the cluster Based on the number of nodes in its cluster, the CH creates a TDMA schedule The TDMA schedule is broadcasted to all the nodes of the cluster 9
Routing in WSNs LEACH PLUS: It increases the lifetime of the network Completely distributed (no global knowledge of the network is required) MINUS: It uses single-hop routing (each node is supposed to be able to transmit directly to the CH or to the sink) It is not applicable to networks deployed in large regions Dynamic clustering formation introduces extra overhead (advertisement, joining, TDMA messages...) 10
Routing in WSNs Location-Based Protocols The nodes are addressed by means of their locations The distance between neighbouring nodes is estimated on the basis of RSSI measurements The relative coordinates of neighbouring nodes are obtained by exchanging information between neighbours Once the topology of the network is known, optimized routing can be performed 11
Localization in WSNs Localization: techniques and methods to measure the spatial relationships between the nodes of the network and the environment in which they operate Why? To increase the value of the collected information (data make more sense when you know WHERE they were acquired!) Example: temperature measurements without location information can not help in creating a temperature map of the monitored area Localization can be useful for routing, clustering, or some applications (e.g. tracking) 12
Localization in WSNs Key Factors Different applications motivate different solutions Granularity and scale of the measurements what is the smallest and largest measurable distance? Accuracy and reliability of the measurements Relation to an established coordinate system absolute positions or relative coordinate system? Dynamics of the network are the nodes fixed or moving? What refresh rate of the estimates is needed? Cost in terms of power consumption and hardware of the nodes Communication requirements (overhead) Environment how sensitive is the chosen technique to environmental factors? Target cooperation does the target play a cooperative role in the system? Is it passive? 13
Ad-Hoc Localization The localization problem involves: The definition of a coordinate system Estimation of the distances and angles between the nodes of the network (ranging) Ad-hoc localization: No infrastructure (as in e.g. GPS) No central processing Sparse ANCHOR NODES (which know their position a priori) The other nodes determine their position using the anchor nodes positions and the distance measurements 14
Ranging Technologies RSSI (Radio Signal Strength Indicator): based on the fact that RSS decreases as a function of distance The path loss model is highly dependent on environmental factors, reducing the accuracy of the distance estimation Multipath fading is a change of the RSS caused by constructive or destructive interference of reflected paths (heavily affected by the environment) ToF: requires a very accurate time synchronization, which tends to be expensive in terms of power consumption, communication overhead, and hardware 15
Localization in WSNs - Ranging Direction of Arrival (DoA): The same signal is received at a node equipped with an array of antennae and the direction of arrival is estimated Time Difference of Arrival (TDoA): uses two different signals with different propagation speeds (e.g. radio and ultrasound) The difference between arrival times is measured and used to compute the distance estimate Problems: calibration of the hardware, time resolution 16
Time Synchronization in WSNs Why is time synchronization critical? To understand and record WHEN an event has been observed To know HOW LONG and HOW FAST a phenomenon was For ENERGY EFFICIENCY: switch off and wake up the radio of the nodes in a coordinated fashion For LOCALIZATION: estimation of distances measuring the signal propagation time For DATA ACQUISITION: some data processing algorithms (e.g. PCA, ICA,...) rely on an accurate periodical sampling For RADIO COMMUNICATIONS: nodes can switch simultaneously to a new channel if they sense the current one has strong interference 17
Time Synchronization in WSNs Internal: all the nodes of the network agree on a common time scale and adjust their clocks at the same time according to this External: one external node provides the reference, all the nodes of the network synchronize to this 18
Time Synchronization in WSNs Hardware issues: Each node counts the time in ITS OWN way (drift from the ideal time) The performance of the oscillators varies depending on e.g. temperature and age Accurate clocks (e.g. atomic ones) are expensive and powerhungry 19
Time Synchronization in WSNs Communication issues: When two nodes are synchronizing, the variation in the latency of the packets (jitter) decreases the accuracy MAC TIMESTAMPING: Read the clock just before sending / after receiving the first bit of the packet 20
Time Synchronization in WSNs ONE-SHOT SYNCHRONIZATION: The reference node sens a packet containing h R The client adjust its clock: h c = h; It can adjust multiple nodes when the reference node broadcasts the packet TWO-WAYS SYNCHRONIZATION: The client sends a request of synchronization The reference replies The client adjusts its clock: h c = (h 1 +h 2 )/2 Large communication overhead with multiple clients 21
Time Synchronization - References RBS: Reference Broadcast Synchronization J. Elson, UCLA, 2002 TPSN: Timing Sync Protocol for Sensor Networks S. Ganeriwal, UCLA, 2003 FTSP: Flooding Time Synchronization M. Maroti, Vanderbilt, 2004 TSS: Time-Stamp Synchronization K. Römer, ETH, 2001, 2007 22
Conclusions Clustering, routing, localization, and time synchronization are important: For the EFFICIENT functioning of the network For the quality, amount, and information content of the data For the survival of the network to its changes and to the changes in the environment around it BUT When designing your protocol, keep in mind that no simulation will ever take into consideration the real-world deployment problems and the limited resources of the nodes 23