1. Net 435: Wireless sensor network (WSN)
Networks and Communication Department
Lecture 8: time synchronization in WSN

2. Synchronization
Synchronization is the coordination of events to operate a system in unison.
The familiar conductor of an orchestra serves to keep the orchestra in time.
Systems operating with all their parts in synchrony are said to be synchronous or in sync.
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Networks and Communication Department

3. Synchronization in Communication systems
In electrical engineering for digital logic and data transfer, a synchronous circuit requires a clock signal.
However, the use of the word "clock" in this sense is different from the typical sense of a clock as a device that keeps track of time-of-day;
the clock signal simply signals the start and/or end of some events.
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5. Motivation for Time Synchronization
Most applications require some synchronization accuracy
Fire and flood tracking
Animal movement
Vehicle movement
Gunshot detection

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7. Existing Synchronization Solutions
Global Positioning System (GPS)
Power-hungry
Network Time Protocol (NTP)
Computationally infeasible for wireless sensors
Timing-sync Protocol for Sensor Networks (TPSN)
Transmitter-receiver synchronization

8. GPS
The Global Positioning System (GPS) is a space- based navigation system that provides location and time information in all weather conditions, anywhere on or near the Earth where there is a line of sight to four or more GPS satellites.
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Networks and Communication Department

9. Definitions
Local clocks are maintained at designated time servers, which are timekeeping systems belonging to a synchronization subnet, in which each server measures the offsets between its local clock and the clocks of its neighbor servers or peers in the subnet.
stability of a clock is how well it can maintain a constant frequency,
the accuracy is how well its time compares with national standards and
the precision is how precisely time can be resolved in a particular timekeeping system.
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Networks and Communication Department

10. UTC
Coordinated Universal Time is the primary time standard by which the world regulates clocks and time.
A time standard is a specification for measuring time: the rate at which time passes
It is one of several closely related successors to Greenwich Mean Time (GMT).
For most purposes, UTC is considered interchangeable with GMT, but GMT is no longer precisely defined by the scientific community.
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Networks and Communication Department

11. to synchronize frequency means to adjust the clocks in the subnet to run at the same frequency,
to synchronize time means to set them to agree at a particular epoch with respect to Coordinated Universal Time (UTC), as provided by national standards.
to synchronize clocks means to synchronize them in both frequency and time.
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Networks and Communication Department

12. NTP
Network Time Protocol (NTP) is a networking protocol for clock synchronization between computer systems over packet-switched, variable-latency data networks.
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Networks and Communication Department

13. Historical facts
Developed in 1985 by David Mills at the University of Delaware, USA.
David is still maintaining NTP.
NTP is one of the oldest Internet protocols still in use.
Latest version is NTP version 4 (version 5 is under development)
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Networks and Communication Department

14. Differences to Cristian‘s method and the Berkley algorithm
Cristian‘s method(CM) and the Berkley algorithm(BA) are both designed for primarily use in intranets.
An intranet is a network based on TCP/IP protocols (an internet) belonging to an organization, usually a corporation, accessible only by the organization's members, employees, or others with authorization.
The Network Time Protocol was designed for use in the Internet (or other unreliable networks) right up from the start.
CM and BA both synchronize against one time server.
NTP synchronizes against many time servers.
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Networks and Communication Department

15. Design aims of NTP
Adjust system clock as close to UTC as possible over the Internet. (statistic techniques for filtering timing data)
Handle bad connectivity. (redundant paths, reconfigurable servers)
Enable sufficiently frequently resynchronizations. (scaling well on large numbers of clients and servers)
Security. (distrust server’s time, authentication)
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Networks and Communication Department

16. Hierarchical structure
The NTP service is provided by a network of servers located across the Internet.
Primary servers are connected directly to a time source. (e.g. a radio clock receiving UTC, GPS).
Secondary servers are synchronized with primary servers.
The servers are connected in a logical hierarchy called a synchronization subnet.
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Networks and Communication Department

17. Synchronization subnet
Each level of the synchronization subnet is called stratum. (e.g. primary servers are stratum 1, secondary stratum 2 and so on)
Lowest-level (leaf) servers execute in users‘ workstations. (e.g. ntp under Linux)
Servers with high stratum numbers are liable to have less accurate clocks than those with lower stratum numbers.
The synchronization subnet can reconfigure as servers become unreachable or failures occur.
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Networks and Communication Department

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19. Server synchronization
There are three possible synchronization modes:
multicast (sometimes called broadcast)
procedure-call (sometimes called client)
symmetric (sometimes called peer)
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20. Multicast mode
One or more servers periodically multicasts the time to the servers in the network.
Receivers set their clock assuming a small delay.
Receivers don‘t reply.
Problem:
Relative low accuracy.
Due to hardware limitations this mode only works in IP multicast enabled networks like LAN.
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Networks and Communication Department

21. procedure-call mode One server accepts requests from other computers.
The server replies with its timestamp.
Higher accuracy.
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22. symmetric mode
Intended to be used on higher levels of the synchronization subnet.
A pair of servers exchange messages to improve the accuracy of their synchronization over time. (reduction of the synchronization dispersionتشتت)
Highest accuracy.
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29. Timing-sync Protocol for Sensor Networks
Timing-sync Protocol for Sensor Networks (TPSN) that works on the conventional approach of sender receiver synchronization.
We argue that for sensor networks, the classical approach of doing a handshake between a pair of nodes is a better approach than synchronizing a set of receivers.
Traditional handshake approach
Timestamp at the MAC layer
Two stages
Level Discovery Phase (Flooding)
Synchronization Phase

30. TPSN Model – Level Discovery Phase
Assign root (level 0) node
Broadcast level_discovery packet
Nodes 1 hop away assigned to level 1
Ignore all subsequent level_discovery packets
Broadcast level_discovery packet …

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34. TPSN Model – Synchronization Phase
Each node (A) broadcasts synchronization_pulse
Timestamped at T1
Node B receives pulse at T2, broadcasts ack at T3
Node A receives ack at T4
Δ is clock drift
d is propagation delay

35. COMPUTER CLOCK AS AN APPROXIMATION OF REAL TIME
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Networks and Communication Department

36. TPSN Energy Usage Transmissions and receptions grow as O(n)
Large energy savings over RBS for large n
Less efficient for small n

37. Thank You
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Networks and Communication Department