Time Synchronization using ReferenceBroadcast Synchronization

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

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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

• (a) latency is small compared to desired accuracy

(b) latency is large compared to desired accuracy
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Concept of RBS Reference-Broadcast
Synchronization

• Reference broadcasts do not have an explicit
  timestamp
• Receivers use reference broadcasts arrival time
  as a point of reference for comparing nodes
  clocks
• Receivers synchronizes with one another using the
  messages timestamp (which is different from one
  receiver to another)

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Illustration of RBS
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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

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RBS vs. TTS

• TTS RBS
• Example NTP
• (Network Time Protocol)

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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

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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!!!

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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

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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

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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 rs clock when it received broadcast b
  r ? n, b ? m
• 
  m
• ? i?n, j?n Offseti,j 1/m ?k1 (Tj,k
  Ti,k)
• Then the receiver changes its clock by the
  calculated phase offset

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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)

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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)

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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

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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 rs 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

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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

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Analysis of RBS algorithm for single broadcast
domain (with clock skew)
Phase offset (usec)
Fit error (usec)
Time (sec)

• Synchronization of the Motes internal clock
• Vertical impulses show the distance of each point
  from the best-fit line RMS error

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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

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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 802.11 wireless
  Ethernet adapters
• All Ethernet adapters connected to a wireless
  802.11 base station

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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

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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 51.18 ? 53.30 ?sec error
• RBS 95 of trails 20.53 ?sec error
• NTP 95 of trails 131.20 ?sec error

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Test Results
For Light traffic
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Test Results

• Heavy traffic scenario
• 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

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Test Results
For Heavy traffic
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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

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Illustration of Multi-Hop Synchronization
Mathematical conversion obtained through the
common node 4
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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 As
  broadcast
• E1(R4) gt E1(R1) R1 synchronized with R4 by A
• Best-fit line calculated by R4 using Bs
  broadcast
• E1(R4) gt E1(R7) R4 synchronized with R7 by B
• R1 synchronizes with R7 using R4
• All nodes in Multi-hop are synchronized similarly

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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)

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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

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How RBS is energy conservative?

• Nodes stay in sleep mode until an event of
  interest occurs post-facto sync

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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)

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Applications

• Acoustic Motes Acoustic Ranging implemented in
  Berkeley Motes
• Collaborative Signal Detection

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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 http//lecs.cs.ucla.edu/jelson/ta
  lks/timesync/RBS-OSDI-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