1. Time Synchronization - using Reference-Broadcast Synchronization
Fine-Grained Network Time Synchronization using Reference Broadcasts by
Jeremy Elson, Lewis Girod and Deborah Estrin
Presentation by
Vivek Vaidyanathan
CS 691, Winter 2003

2. Outline Introduction Concept of Traditional Time Synchronization
Concept of Reference Broadcast Synchronization
Kind of latency in TTS and RBS
RBS algorithm for:
Single Broadcast Network
Multi-Hop Network
Analysis of RBS algorithms
Advantages and Limitations of RBS

3. Introduction
Time synchronization is highly critical in sensor networks for purposes such as:
Data Diffusion
Coordinated Actuation
Object Tracking

4. Purpose
To Synchronize all the nodes in the sensor network using a method that:
Eliminates error efficiently
Energy conservative
Provides tight synchronization

5. What is Data Diffusion?
Merging individual sensor readings into a high level sensing result

6. Applications of Time Synchronization
Secure cryptographic schemes
Coordination of future action
Ordering logged events during system debugging

7. Concept of TTS- Traditional Time Synchronization
The sender periodically sends a message with its current clock as a timestamp to the receiver
Receiver then synchronizes with the sender by changing its clock to the timestamp of the message it has received from the sender (if the latency is small compared to the desired accuracy)
Sender calculates the phase error by measuring the total round trip-time by sending and receiving the respective response from the receiver (if the latency is large compared to the desired accuracy)

8. Illustration of TTS S R S R
(a) latency is small compared to desired accuracy S R
(b) latency is large compared to desired accuracy

9. Concept of RBS – Reference-Broadcast Synchronization
Reference broadcasts do not have an explicit timestamp
Receivers use reference broadcast’s arrival time as a point of reference for comparing nodes’ clocks
Receivers synchronizes with one another using the message’s timestamp (which is different from one receiver to another)

10. Illustration of RBS A 1 3 2 4

11. RBS vs. TTS RBS - Synchronizes a set of receivers with one another
Traditional - Senders synchronizes with receivers
RBS – Supports both single hop and multi hop networks
Traditional – mostly supports only single hop networks

12. RBS vs. TTS
TTS RBS
Example: NTP
(Network Time Protocol)

13. Types of errors that TTS should detect and eliminate
Send Time Latency
time spent at the sender to construct the message
Access Time Latency
time spent at the sender to wait for access to transmit the message
Prorogation Time Latency
time spent by the message in traveling from the sender to the receiver
Receive Time Latency
time spent at the receiver to receive the message from the channel and to notify the host

14. Types of errors that RBS should detect and eliminate
Phase error
due to nodes’ clock that contains different times
Clock skew
due to nodes’ clock that run at different rate
Therefore, We go for RBS!!!

15. RBS algorithm for single broadcast domain (assuming no clock skew)
Basic idea to estimate phase offset:
Transmitter broadcasts a reference packet to two receivers
Each receiver records the time that the reference was received, according to its local clock
The receivers exchange their observations

16. RBS algorithm for single broadcast domain (assuming no clock skew)
Basic idea to estimate phase offset for non-deterministic receivers:
Transmitter broadcasts m reference packets
Each of the n receivers records the time that the reference was received, according to its local clock
The receivers exchange their observation
Each Receiver i can compute its phase offset to any other receiver j

17. RBS algorithm for single broadcast domain (assuming no clock skew)
Formula for calculating the phase offset of receiver i with other receiver j:
n : number of receivers
m : number of reference broadcasts
Tr,b : r’s clock when it received broadcast b {r  n, b  m} m
 in, jn : Offset[i,j] = 1/m k=1 (Tj,k – Ti,k)
Then the receiver changes its clock by the calculated phase offset

18. Analysis of RBS algorithm for single broadcast domain (no clock skew)
2-D view:
Mean group dispersion from the average of 1000 simulated trials for:
20-receiver group (top)
2-receiver group (bottom)

19. Analysis of RBS algorithm for single broadcast domain (no clock skew)
3-D view:
Mean group dispersion from the average of 1000 simulated trials for the same data set, from 2 to 20 receivers (inclusive)

20. RBS algorithm for single broadcast domain (with clock skew)
A MATHEMATICAL APPROACH
The phase offset with the clock skew is estimated by:
Least-squares linear regression graph
From the best-fit line of the graph, following can be inferred:
Slope of the line : Clock skew of the nodes’ clock
Intercept of the line : Phase of the nodes’ clock

21. RBS algorithm for single broadcast domain (with clock skew)
Basic idea to estimate phase offset and clock skew for non-deterministic receivers:
Transmitter broadcasts m reference packets
Each of the n receivers records the time that the reference was received, according to its local clock
The receivers exchange their observation
Each Receiver i can compute its phase offset to any other receiver j

22. RBS algorithm for single broadcast domain (with clock skew)
Formula for calculating the phase offset and clock skew of receiver r1 with other receiver r2:
Tr,b : r’s clock when it received broadcast b,
for each pulse k that was received by receivers r1 and r2 ,
we plot a graph :
x = Tr1, k
y = Tr2,k – Tr1,k
Diagonal line drawn through the points represents the best linear fit to the data

23. RBS algorithm for single broadcast domain (with clock skew)
Diagonal line minimizes the residual error (RMS).
Therefore, we go for calculating the slope and intercept of the diagonal line
Time value of r1 is converted to time value of r2 by combining the slope and intercept data obtained

24. Synchronization of the Mote’s internal clock
Analysis of RBS algorithm for single broadcast domain (with clock skew)
Phase offset (usec)
Fit error (usec)
Time (sec)
Synchronization of the Mote’s internal clock
Vertical impulses show the distance of each point from the best-fit line – RMS error

25. Analysis of RBS algorithm for single broadcast domain (with clock skew)
Phase offset (usec)
Time (sec)
Synchronization of clocks on PC104-compatible single board computers using Mote as NIC

26. Why RBS is the best? Comparison of RBS with NTP and NTP-Offset:
Hardware implementation
RBS as a UNIX daemon
UDP datagrams as Motes
Testbed:
StrongARM-based Compaq IPAQs
Lucent Technologies 11 Mbit wireless Ethernet adapters
All Ethernet adapters connected to a wireless base station

27. Why RBS is the best? Test implemented in two different scenarios:
Light network load
Minimal load generated by synchronization scheme
Heavy network load
Two additional IPAQs configured as traffic generators
Each IPAQ sent randomly sized UDP datagrams of 500 to 15,000 bytes
Inter-packet delay: 10 msec

28. Test Results Light traffic scenario:
RBS performed more than 8 times better than NTP and NTP-Offset
RBS : average of 6.29  6.45 sec error
NTP : average of  sec error
RBS : 95% of trails : sec error
NTP : 95% of trails : sec error

29. Test Results
For Light traffic:

30. Test Results Heavy traffic scenario:
RBS – almost completely unaffected
NTP – suffered a 30 fold degradation
RBS : 95% of trails : sec error
NTP : 95% of trails : 3,889 sec error

31. Test Results
For Heavy traffic:

32. Working of RBS in multi hop network
Obtained by mathematical conversion of output obtained in available single hop networks in the multi-hop network.
Least square linear regression graph – used to synchronize all the single hop networks in the multi-hop network
The values are then formulated and converted accordingly for all the nodes in the multi-hop network

33. Illustration of Multi-Hop Synchronization
Mathematical conversion obtained through the common node 4

34. Algorithm for Calculating Phase Offset in Multi-Hop Network
Events E1 and E7 – observed by R1 and R7 respectively
Best-fit line calculated by R4 using A’s broadcast
E1(R4) => E1(R1) : R1 synchronized with R4 by A
Best-fit line calculated by R4 using B’s broadcast
E1(R4) => E1(R7) : R4 synchronized with R7 by B
R1 synchronizes with R7 using R4
All nodes in Multi-hop are synchronized similarly

35. Analysis of Multi-Hop RBS
Same test applied to Multi-Hop as the Single-Hop
Test Results:
If average per-hop error = s
Hop path = n
Average path error of n-hop = s . Sqrt(n)

36. Advantages of RBS Can be used without external timescales
Energy conservative
Does not require tight coupling between sender and its network interface
Covers much wider area
Applicable in both wired and wireless networks
Largest resources of latency (that exists in TTS) is removed from critical path
Allows tighter synchronization

37. How RBS is energy conservative?
Nodes stay in sleep mode until an event of interest occurs – post-facto sync

38. Limitations of RBS Works only with broadband communication
Does not support point to point communication
(as time synchronization is done among a set of receivers. In point-to-point – only one receiver exists)

39. Applications
Acoustic Motes: Acoustic Ranging implemented in Berkeley Motes
Collaborative Signal Detection

40. References
Fine-Grained Network Time Synchronization using Reference Broadcasts. Jeremy Elson, Lewis Girod and Deborah Estrin, UCLA
Power point presentation on Fine-Grained Network Time Synchronization using Reference Broadcasts. Jeremy Elson, Lewis Girod and Deborah Estrin, UCLA.
Available at :
Wireless Sensor Networks: A New Regime for Time Synchronization. Jeremy Elson and Kay Romer, UCLA
Time Synchronization for Wireless Sensor Networks. Jeremy Elson and Deborah Estrin, UCLA