Clock Synchronization for
Wireless Sensor Networks:
A Survey
Bharath Sundararaman, Ugo Buy, and
Ajay D. Kshemkalyani
Department of Computer Science
University of Illinois at Chicago
1
Outline
 I.
Introduction
 II. Synchronization protocols
 III. Comparison
 IV. Conclusion & comments
2
I. Introduction
3
Why synchronization?
 The
time of the day at which an event
happened.
 The time interval between two events.
 The relative ordering of events.
4
Requirements
 Cope
with unreliable network transmission
and unbounded message latencies.
 Able to estimate the local time on the other
node’s clock.
 Time must never run backward.
 Should not degrade system performance.
5
Several issues in synchronization (1/5)
 Master-slave
v.s. peer-to-peer
synchronization
Master-slave. The slave node consider the
clock reading of the master as the reference
time and attempt to synchronize with the master.
Peer-to-peer. Any node can communicate
directly with every node in the network. They
are more flexible but are also more difficult to
control.
6
Several issues in synchronization (2/5)
 Clock
correction versus untethered clocks
Clock correction. Correcting the local clock in
each node to run on par with a global time scale
or an atomic clock.
Untethered clock. Build a table of parameters
that relate the local clock of each node to the
local clock of every other node in the network.
When timestamps are exchanged between
nodes, they are transformed to the local clock
values of the receiving node.
7
Several issues in synchronization (3/5)
 Internal
synchronization v.s. external
synchronization
Internal synchronization. The goal is to
minimize the maximum difference between the
readings of local clocks of the sensors.
External synchronization. A standard source of
time is provided.
8
Several issues in synchronization (4/5)
 Probabilistic
v.s. deterministic
synchronization
Probabilistic synchronization. Provide a
probabilistic guarantee on the maximum clock
offset with a failure probability that can be
bounded or determined.
Deterministic synchronization. Guarantee an
upper bound on the clock offset with certainty.
9
Several issues in synchronization (5/5)

Sender-to-receiver v.s. receiver-to-receiver
synchronization
 Sender-to-receiver synchronization. The receiver
synchronizes with the sender using the time stamps
received. Message delay is calculated by measuring
the round-trip delay.
 Receiver-to-receiver synchronization. If any two
receivers receives the same message in single-hop
transmission, they receive it at approximately the same
time. The receivers exchange the time at which they
received the same message and compute their offset
based on the difference in reception time.
10
Terminology (1/3)
 Time:
The time of a clock in a machine p is
given by the function Cp(t), where Cp(t) = t
for a perfect clock.
 Frequency : Frequency is the rate at which
a clock progresses. The frequency at time t
of clock Ca is C’a(t).
 Offset: Clock offset is the difference
between the time reported by a clock and
the real time.
11
Terminology (2/3)
Skew: The skew of a clock is the difference in the
frequencies of the clock and the perfect clock.
The skew of a clock Ca relative to clock Cb at time
t is (C’a(t) − C’b(t)).
 Drift (rate): The drift of clock Ca is the second
derivative of the clock value with respect to time,
namely C’’a (t). The drift of clock Ca relative to
clock Cb at time t is (C’’a (t) − C’’b (t)).

12
Terminology (3/3)

A timer is said to be working within its
specification if
where constant ρ is the maximum skew rate
specified by the manufacturer.
13
II. Synchronization protocols
14
Remote clock reading method
The client then sets its time to Stime (accurate time
from the server) + (T1-T0)/2 (time required to
transmit the message).
 The time for any message to be sent is highly
variable due to network traffic and message
routing.

15
Time transmission method (1/2)
M
is the source node and S is the target
node.
 M sends a series of synchronization
messages to S. The ith message is sent at
time Ti of M’s clock and received at time Ri
of S’s clock.
Ti
M
Ri
S
16
Time transmission method (2/2)
δ: the offset between clock S
and M
d: message delay
17
Set-valued estimation method (1/3)

We assume that the local times ti and tj on
processors Pi and Pj ,respectively, can be related
by the linear equation:
 ti = aijtj + bij
 where aij and bij represent the relative skew and offset
between the two hardware clocks.

Time-stamped triples.
18
Set-valued estimation method (2/3)
19
Set-valued estimation method (3/3)
20
Reference broadcast
synchronization (1/5)
 RBS
seeks to reduce nondeterministic
latency using receiver-to-receiver
synchronization and to conserve energy via
post-facto synchronization.
21
Reference broadcast
synchronization (2/5)
Time critical path: nondeterministic delay
22
Reference broadcast
synchronization (3/5)

Receiver j will compute its offset relative to any
other receiver i as the average of clock
differences for each packet received by nodes i
and j.
23
Reference broadcast
synchronization (4/5)
24
Reference broadcast
synchronization (5/5)
 The
largest sources of error are removed.
 Require O(n2) message exchanges for a
network of n nodes.
25
Romer’s protocol (1/4)
 Uses
innovative time transformation
algorithm for achieving clock
synchronization
 Assumptions:
There is a maximum skew ρ of computer clocks
Whenever a message is exchanged between
two nodes, the connection remains long enough
for the two nodes to exchange one additional
message.
26
Romer’s protocol (2/4)
Real time difference Δt
 Computer clock difference ΔC1, ΔC2
 Skew upper bound for node 1 and node 2 are ρ 1
and ρ2,, respectively.

C
 1   
t
1  1 t  C1  1  1 t
1    
C1
C1
 t 
1  1
1  1
1  2 t  C2  1  2 t
1  2
1  2
C1  C2 
C1
1  1
1  1
27
Romer’s protocol (3/4)

The message delay between two node is
estimated by bounding it within interval
[0, rtt]
28
Romer’s protocol (4/4)
Require low resource and message overhead.
 The synchronization error increases with the
number of hops along the path of the message
containing the timestamp.

29
Timing-sync Protocol for sensor
networks (1/3)
 A self-configuring
hierarchical structure.
 A node in this structure can simultaneous
act as a synchronization server to a number
of client nodes and as a synchronization
client to another node.
30
Timing-sync Protocol for sensor
networks (2/3)

Two phase.
 Level discovery phase. It is based on constrained flooding. The
root node is assigned level 0; The receiver assign themselves a
level that is one greater than the level in the packet received.
 Synchronization phase. T2=T1+δ+d and δ represents the clock
offset between two nodes and d represents the propagation
delay.
T4  T3    d
T2  T1    d

T2  T1   T4  T3  ; d  T2  T1   T4  T3 
2
2
31
Timing-sync Protocol for sensor
networks (3/3)
 The
protocol is scalable and the accuracy
does not degrade significantly as the size of
the network is increased.
 The protocol requires a hierarchical
infrastructure which makes it unsuitable for
highly mobile nodes.
32
III. Comparison
33
Quantitatively evaluation (1/2)


Synchronization precision
Piggybacking
 Reduction of message traffic

Computational complexity
 Run time and memory requirements
 Number of messages exchanged

Convergence time
 Total time required to synchronize a network


Network size
Compatibility with sleep mode
 Synchronize and active only when application demands it.
34
Quantitatively evaluation (2/2)
Protocols
Precision
Piggybacking
Complexity
Convergence
time
Network
size
Sleep
mode
RBS [19]
1.85
± 1.28 μs
N/A
High
N/A
2-20 Nodes
Yes
Romer [56]
3 ms
Yes
Low
N/A
Unknown
Yes
TSPN [25]
16.9 μs
No
Low
Unknown
150-300
Nodes
Yes
35
Qualitatively evaluation (1/2)
 Energy
efficiency
 Accuracy
How well the time maintained within the network
is true to the standard time
 Scalability
 Overall
complexity
 Fault tolerance
Poor reliability of message delivery
36
Qualitatively evaluation (2/2)
Protocols
Accuracy
Energy
Efficiency
Overall
Complexity
Scalability
Fault
Tolerance
RBS[19]
High
High
High
Good
No
Romer [56] Low
High
Low
Poor
No
TSPN[25]
Average
Low
Good
Yes
High
37
IV. Conclusion & comments
38
Conclusion
 The
design considerations presented will
help designers in building successful
synchronization scheme, best tailored to his
application.
39
Comments

Pros
 It is a good start to begin studying synchronization.
 Design tradeoffs are discussed and these help us in
designing synchronization protocols.

Cons
 Some protocols’ description are too rough to be useful.
 The authors didn’t conduct any experiment to verify the
claimed results.
40