Clock Synchronization in Wireless Sensor Networks: Local vs' Global

1
Clock Synchronization in Wireless Sensor
Networks Local vs. Global

• Philipp Sommer
• Roger Wattenhofer

2
Time in Sensor Networks

• Synchronized clocks are essential for many
  applications

Hardware Clock
3
Clock Synchronization in Practice

• Many different approaches for clock
  synchronization

Global Positioning System (GPS) high
accuracy - outdoor use only - high energy
consumption

• Radio Clock Signal
• indoor use possible
• low cost, low energy
• limited accuracy
• bulky antenna

• AC-power line radiation
• low accuracy
• low energy consumption
• indoor use only
• bulky device

Synchronization messages high accuracy
indoor/outdoor low energy no additonal
hardware
4
Hardware Clocks of Sensor Nodes

• Counter register of the microcontroller
• Sourced by an external crystal (32kHz, 7.37 MHz)
• Clock drift
• Random deviation from the nominal rate dependent
  on ambient temperature, power supply, etc.
  (30-100 ppm)

Mica2
5
Message Delay in Wireless Sensor Networks

• Problem Jitter in the message delay
• Various sources of errors (deterministic and
  non-deterministic)
• Solution Timestamping packets at the MAC layer
  (Maróti et al.)
• ? Jitter in the message delay is reduced to a few
  clock ticks

1-10 ms
0-100 ms
0-500 ms
Send
Access
Transmission
Reception
Receive
0-100 ms
t
Expected delay T
Jitter J
6
Bounds on the Synchronization Accuracy

• Two nodes u and v cannot be synchronized
  perfectly
• Messages between two neighboring nodes may be
  fast in one direction and slow in the other, or
  vice versa.
• Error increases as the square-root of the
  distance from the reference node

7
Clock Synchronization Local vs. Global

• Global property Minimize clock error between any
  two nodes
• Local (gradient) property Small clock error
  between two nodes if the distance between the
  nodes is small.

Flooding Time Synchronization Protocol (FTSP)
Gradient Time SynchronizationProtocol (GTSP)
8
Gradient Time Synchronization Protocol (GTSP)
Sommer et al., IPSN09

• Synchronize clocks with all neighboring nodes
• No reference (root) node necessary
• No tree or pre-established topology
• Averaging clock value/rate of all neighbors
  (including node itself)

9
Experimental Evaluation

• Testbed of 20 Crossbow Mica2 sensor nodes
• Global clock synchronization error
• Pair-wise synchronization error between any nodes
  in the network
• Local clock synchronization error
• Pair-wise synchronization error between
  neighboring nodes

10
Experimental Results

• Global clock synchronization error
• 7.7 µs with FTSP, 14.0 µs with GTSP
• FTSP needs more time to synchronize all nodes
  after startup

11
Experimental Results (2)

• Local clock synchronization error
• 5.3 µs with FTSP, 4.0 µs with GTSP
• GTSP takes slightly more time to stabilize

FTSP
GTSP
12
Neighbor Synchronization Error FTSP vs. GTSP

• FTSP has a large clock error for neighbors with
  large stretch in the tree (Node 8 and Node 15)

GTSP
FTSP
13
Time in Sensor Networks (Revisited)

• Synchronized clocks are essential for many
  applications

Local
Global
Local
Global
Hardware Clock
14
Conclusion and Future Work

• Gradient Time Synchronization Protocol (GTSP)
• Distributed time synchronization algorithm (no
  leader)
• Improves the synchronization error between
  neighboring nodes while still providing precise
  network-wide synchronization
• Is there a perfect clock synchronization
  protocol?
• Goal Minimizing local and global error at the
  same time

15
Questions?

• Philipp Sommer ltsommer_at_tik.ee.ethz.chgt

16
Multi-Hop Time Synchronization in Practice

• Is gradient clock synchronization relevant in
  practice?
• Ring topology of 20 nodes seems to be
  artificial!?
• Finding a tree-embedding with low stretch is hard
• In a n mm grid you will always havetwo
  neighbors with a stretch of at least
• Example FTSP on a 5x4 grid topology
• Node 2 and 7 have a distance of 13 hops!

17
Simulation Results

• Simulation of GTSP for larger network topologies
• Network error of 1 ms for 100 nodes in a line
  topology
• Neighbor error below 100 µs for the same topology

18
Time Synchronization in (Sensor) Networks

• Time, Clocks, and the Ordering of Events in a
  Distributed SystemL. Lamport, Communications of
  the ACM, 1978.
• Internet Time Synchronization The Network Time
  ProtocolD. Mills, IEEE Transactions on
  Communications, 1991
• Reference Broadcast Synchronization (RBS)J.
  Elson, L. Girod and D. Estrin, OSDI'02
• Timing-sync Protocol for Sensor Networks
  (TPSN)S. Ganeriwal, R. Kumar and M. Srivastava,
  SenSys'03
• Flooding Time Synchronization Protocol (FTSP)M.
  Maróti, B. Kusy, G. Simon and Á. Lédeczi,
  SenSys'04
• and many more ...

State-of-the-art time sync protocol for wireless
sensor networks
19
Open Problems

• Fault-Tolerance
• What happens when node crash or have Byzantine
  failures?
• Impact of high message loss
• New applications
• Can we use principles of clock synchronization in
  other domains?(e.g. synchronize movement of a
  group of robots)