Clock Synchronization Clock Synchronization Chapter 9 Ad Hoc and Sensor Networks – Ad Hoc and Sensor Networks – Roger Wattenhofer – Roger Wattenhofer – 9/1 Rating Overview • • Area maturity Motivation • Clock Sources & Hardware First steps Text book • Single-Hop Clock Synchronization • Clock Synchronization in Networks • • Practical importance Protocols: RBS, TPSN, FTSP, GTSP • Theory of Clock Synchronization No apps Mission critical • Protocol: PulseSync • Theory appeal Boooooooring Exciting Ad Hoc and Sensor Networks – Ad Hoc and Sensor Networks – Roger Wattenhofer – Roger Wattenhofer – Ad Hoc and Sensor Networks – Roger Wattenhofer – 9/3 9/4
Motivation Properties of Clock Synchronization Algorithms • • Synchronizing time is essential for many applications External versus internal synchronization – – External sync: Nodes synchronize with an external clock source (UTC) Coordination of wake-up and sleeping times (energy efficiency) – TDMA schedules – Internal sync: Nodes synchronize to a common time – Ordering of collected sensor data/events – to a leader, to an averaged time, or to anything else – Co-operation of multiple sensor nodes – Estimation of position information (e.g. shooter detection) • One-shot versus continuous synchronization – Periodic synchronization required to compensate clock drift • Goals of clock synchronization – Compensate offset* between clocks – • Compensate drift* between clocks A-priori versus a-posteriori – A-posteriori clock synchronization triggered by an event *terms are explained on following slides Localization • Global versus local synchronization (explained later) Sensing Duty- Cycling TDMA • Accuracy versus convergence time, Byzantine nodes, … Time Synchronization Ad Hoc and Sensor Networks – Roger Wattenhofer – 9/6 Clock Sources Clock Sources (2) • • Radio Clock Signal: AC power lines: – Clock signal from a reference source (atomic clock) – Use the magnetic field radiating from electric AC power lines is transmitted over a long wave radio signal – AC power line oscillations are extremely stable (10 -8 ppm) – DCF77 station near Frankfurt, Germany transmits at 77.5 kHz with a transmission range of up to 2000 km – Power efficient, consumes only 58 μ W – Accuracy limited by the distance to the sender, – Single communication round required to correct Frankfurt-Zurich is about 1ms. phase offset after initialization – Special antenna/receiver hardware required • Sunlight: • Global Positioning System (GPS): – Using a light sensor to measure the length of a day – Satellites continuously transmit own position and – Offline algorithm for reconstructing global time code timestamps by correlating annual solar patterns – Line of sight between satellite and receiver required (no communication required) – Special antenna/receiver hardware required
Clock Devices in Sensor Nodes Clock Drift • • Structure Accuracy – External oscillator with a nominal frequency (e.g. 32 kHz or 7.37 MHz) – Clock drift: random deviation from the nominal rate dependent on power supply, temperature, etc. – Counter register which is incremented with oscillator pulses – Works also when CPU is in sleep state rate This is a drift of up to 7.37 MHz quartz 1+ ² 50 μ s per second 1 or 0.18s per hour 1- ² 32 kHz quartz t – E.g. TinyNodes have a maximum drift of 30-50 ppm at room temperature Mica2 TinyNode 32 kHz quartz Ad Hoc and Sensor Networks – Roger Wattenhofer – 9/9 Sender/Receiver Synchronization Messages Experience Jitter in the Delay • • Round-Trip Time (RTT) based synchronization Problem: Jitter in the message delay Various sources of errors (deterministic and non-deterministic) Time accor- B t 2 t 3 ding to B 0-100 ms 1-10 ms 0-500 ms Answer Request from A from B Send Access Transmission Time accor- t 1 A t 4 ding to A Reception Receive 0-100 ms • Receiver synchronizes to the sender‘s clock t • Propagation delay � and clock offset � can be calculated • Solution: Timestamping packets at the MAC layer (Maróti et al.) (t � t ) � (t � t ) → Jitter in the message delay is reduced to a few clock ticks 4 1 3 2 δ = 2 (t � (t + δ)) � (t � (t + δ)) (t � t ) + (t � t ) 2 1 4 3 2 1 3 4 θ = = 2 2 Ad Hoc and Sensor Networks – Roger Wattenhofer – 9/11 Ad Hoc and Sensor Networks – Roger Wattenhofer – 9/12
Some Details Symmetric Errors • • Many protocols don’t even handle single -hop clock synchronization Different radio chips use different paradigms: – Left is a CC1000 radio chip which generates an interrupt with each byte. well. On the left figures we see the absolute synchronization errors of TPSN and RBS, respectively. The figure on the right presents a – Right is a CC2420 radio chip that generates a single interrupt for the single-hop synchronization protocol minimizing systematic errors. packet after the start frame delimiter is received. • In sensor networks propagation can be ignored (<1 ¹ s for 300m). • Even perfectly symmetric errors will sum up over multiple hops. • Still there is quite some variance – In a chain of n nodes with a standard deviation ¾ on each hop, the in transmission delay because of expected error between head and tail of the chain is in the order of ¾ √n . latencies in interrupt handling (picture right). Ad Hoc and Sensor Networks – Ad Hoc and Sensor Networks – Roger Wattenhofer – 9/14 Roger Wattenhofer – Reference-Broadcast Synchronization (RBS) Time-sync Protocol for Sensor Networks (TPSN) • A sender synchronizes a set of receivers with one another • Traditional sender-receiver synchronization (RTT-based) • Point of reference: beacon’s arrival time • Initialization phase: Breadth-first-search flooding t – Root node at level 0 sends out a level discovery packet 2 A � � � � � – Receiving nodes which have not yet an assigned level set their level t t S A P R 2 1 S S S , A A � to +1 and start a random timer � � � � � t t S A P R 3 1 S S S , B B – After the timer is expired, a new level discovery packet will be sent S B � � � � � � � t t ( P P ) ( R R ) – When a new node is deployed, it sends out a level request packet after 2 3 S , A S , B A B t t 1 3 a random timeout • Only sensitive to the difference in propagation and reception time • Time stamping at the interrupt time when a beacon is received 0 • Why this random timer? After a beacon is sent, all receivers exchange their reception times to 1 1 calculate their clock offset 1 2 • 2 Post-synchronization possible 2 2 • E.g., least-square linear regression to tackle clock drifts • Multi-hop? Ad Hoc and Sensor Networks – Roger Wattenhofer – 9/15 Ad Hoc and Sensor Networks – Roger Wattenhofer – 9/16
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