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
Outline
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Introduction
Concept of Traditional Time Synchronization
Concept of Reference Broadcast Synchronization
Kind of latency in TTS and RBS
RBS algorithm for:
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Single Broadcast Network
Multi-Hop Network
Analysis of RBS algorithms
Advantages and Limitations of RBS
Introduction
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Time synchronization is highly critical in sensor
networks for purposes such as:
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Data Diffusion
Coordinated Actuation
Object Tracking
Purpose
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To Synchronize all the nodes in the sensor
network using a method that:
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Eliminates error efficiently
Energy conservative
Provides tight synchronization
Applications of Time Synchronization
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Secure cryptographic schemes
Coordination of future action
Ordering logged events during system debugging
Concept of TTSTraditional Time Synchronization
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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)
Illustration of TTS
S
R
(a) latency is small compared to desired accuracy
S
R
(b) latency is large compared to desired accuracy
Concept of RBS –
Reference-Broadcast Synchronization
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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)
Illustration of RBS
1
2
A
3
4
RBS vs. TTS
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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
RBS vs. TTS
TTS
Example: NTP
(Network Time Protocol)
RBS
Types of errors that TTS should detect and
eliminate
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Send Time Latency
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Access Time Latency
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time spent at the sender to wait for access to transmit the
message
Prorogation Time Latency
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time spent at the sender to construct the message
time spent by the message in traveling from the sender to the
receiver
Receive Time Latency
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time spent at the receiver to receive the message from the
channel and to notify the host
Types of errors that RBS should detect and
eliminate
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Phase error
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due to nodes’ clock that contains different times
Clock skew
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due to nodes’ clock that run at different rate
Therefore, We go for RBS!!!
RBS algorithm for single broadcast domain
(assuming no clock skew)
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Basic idea to estimate phase offset:
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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
RBS algorithm for single broadcast domain
(assuming no clock skew)
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Basic idea to estimate phase offset for nondeterministic receivers:
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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
RBS algorithm for single broadcast domain
(assuming no clock skew)
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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
Analysis of RBS algorithm for single
broadcast domain (no clock skew)
2-D view:
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Mean group dispersion from the average of 1000 simulated
trials for:
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20-receiver group (top)
2-receiver group (bottom)
Analysis of RBS algorithm for single
broadcast domain (no clock skew)
3-D view:
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Mean group dispersion from the average of 1000 simulated
trials for the same data set, from 2 to 20 receivers
(inclusive)
RBS algorithm for single broadcast domain
(with clock skew)
A MATHEMATICAL APPROACH
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The phase offset with the clock skew is estimated
by:
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Least-squares linear regression graph
From the best-fit line of the graph, following can be
inferred:
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Slope of the line : Clock skew of the nodes’ clock
Intercept of the line : Phase of the nodes’ clock
RBS algorithm for single broadcast domain
(with clock skew)
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Basic idea to estimate phase offset and clock skew
for non-deterministic receivers:
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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
RBS algorithm for single broadcast domain
(with clock skew)
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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
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Diagonal line drawn through the points represents the best
linear fit to the data
RBS algorithm for single broadcast domain
(with clock skew)
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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
Fit error (usec)
Phase offset (usec)
Analysis of RBS algorithm for single
broadcast domain (with clock skew)
Time (sec)
Synchronization of the Mote’s internal clock
Vertical impulses show the distance of each point from the best-fit line – RMS error
Phase offset (usec)
Analysis of RBS algorithm for single
broadcast domain (with clock skew)
Time (sec)
Synchronization of clocks on PC104-compatible single board computers using Mote as NIC
Why RBS is the best?
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Comparison of RBS with NTP and NTP-Offset:
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Hardware implementation
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RBS as a UNIX daemon
UDP datagrams as Motes
Testbed:
- StrongARM-based Compaq IPAQs
- Lucent Technologies 11 Mbit 802.11 wireless Ethernet
adapters
- All Ethernet adapters connected to a wireless 802.11 base
station
Why RBS is the best?
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Test implemented in two different scenarios:
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Light network load
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Minimal load generated by synchronization scheme
Heavy network load
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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
Test Results
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Light traffic scenario:
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RBS performed more than 8 times better than NTP and
NTP-Offset
RBS : average of 6.29  6.45 sec error
NTP : average of 51.18  53.30 sec error
RBS : 95% of trails : 20.53 sec error
NTP : 95% of trails : 131.20 sec error
Test Results
For Light traffic:
Test Results
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Heavy traffic scenario:
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RBS – almost completely unaffected
NTP – suffered a 30 fold degradation
RBS : 95% of trails : 28.53 sec error
NTP : 95% of trails : 3,889 sec error
Test Results
For Heavy traffic:
Working of RBS in multi hop network
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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
Illustration of Multi-Hop Synchronization
Mathematical conversion obtained through the
common node 4
Algorithm for Calculating Phase Offset in
Multi-Hop Network
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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
Analysis of Multi-Hop RBS
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Same test applied to Multi-Hop as the Single-Hop
Test Results:
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If average per-hop error = s
Hop path = n
Average path error of n-hop = s . Sqrt(n)
Advantages of RBS
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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
How RBS is energy conservative?
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Nodes stay in sleep mode until an event of interest
occurs – post-facto sync
Limitations of RBS
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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)
Applications
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Acoustic Motes: Acoustic Ranging implemented
in Berkeley Motes
Collaborative Signal Detection
References
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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 : http://lecs.cs.ucla.edu/~jelson/talks/timesync/RBSOSDI-2002-Dec9.ppt
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