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Clock Synchronization in Sensor Networks* Introduction Sensor nodes maintain local clocks Typically lower precision clocks (crystal oscillators) Clocks need to be synchronized to determine Exact timing of events Chronology of events Clock synchronization Global : all nodes are synchronized to a single clock Example: NTP (often used in LANS) Local : a set of “ localized” nodes are synchronized Reference Broadcast Synchronization (RBS) Post-facto Synchronization Challenges Scalability Must be scalable to a large number of nodes Energy Efficiency Limited messaging GPS technology cannot be employed Requirements are more stringent Microsecond precision desirable Local : a set of “ localized” nodes are synchronize Clock skew is large Two node clocks might drift 1–100μsec per second Clock Synchronization Models Maintain relative notion of time Generate the right chronology of events Clocks need not be synchronized Relative Clocks Nodes maintain individual clocks Maintain relative drift information with other clocks RBS “Always On” Model Every clock is synchronized to a reference clock Superset of the above models TSPN Today’s Talk TSPN : Time-Sync Protocol for Sensor Networks Sender-Receiver synchronization (classical approach) needs MAC level integration Always-On RBS : Reference Broadcast Synchronization Receiver-Receiver synchronization Could be implemented at user-level Relative Sync Packet Delay (Uncertainty) Model Sender Uncertainty Send, Access, Transmission Propagation Uncertainty Negligible Speed thru medium = 1foot/nanosecond = c Receiver Uncertainty Packet reception Packet processing TSPN Clock Sync Algorithm involves 2 steps Level Discovery phase Synchronization phase TSPN makes the following assumptions Sensor nodes have unique identifiers Node is aware of its neighbors Bi-directional links Creating the hierarchical tree is not exclusive to TSPN TSPN: Level Discovery Algorithm Root node is assigned level i = 0 broadcasts “level discov