Clock Synchronization:
Overview and Outlook
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Synchronization as Network Service
•Reference clocks
know correct time
•Daemons consult
reference clock and
distribute time to
clients
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How to Achieve Accuracy
•Time is sent from sender to receiver
•Network delay deteriorates result
•Must be added to transmitted
value
•Receivers can tune each other
(internal synchronization)
•Network delay must be measured
•Round-trip delay
•Timestamps are inserted in
messages
•Node knows the communication
delay
•Jitter deteriorates
measurement
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Various Approaches: internal and external synchronization
Clock synchronization accuracy in distributed systems LANs
Internal clock synchronization
External clock synchronization
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What for? –Application Fields
•Distributed measurement systems
•Synchronized data sampling
•Distributed control systems
•Correlation of sensor signals
•Synchronization of actuators
•Reliable data transmission
•Secure data transmission
•Avoidance of replay attacks
•Network access
•Basis for TDMA schemes
•e.g. WirelessHART computes the difference between expected
time and arrival time of the frame (offset)
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Synchronization to measure time-of-flight
• Measuring time-of-flight or round-trip-time allows localization
(if travelling speed is known e.g. 30cm/ns)
• Location range ±10 m ⇒10 ns accuracy
• Ethernet-based networks
20cm/ns and the knowledge of length allows computing the
delay between two nodes or infrastructure points
• Wireless-based networks
Normally round trip time is used for ranging purposes
-Delay of retransmission should be known and fixed
-The time instant of transmission and of reception should be
known
• Synchronization depends on timestamping
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Distributed Measurements
•Detection of power line failures
•Delay analysis of transient signals
•Location range ±10 m ? ⇒ 10 ns accuracy required!
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Automation Network and Control Systems
•Classical usage
•Local control loops
•Configuration over the fieldbus
•Set point values
•More recent usage
•Distributed control systems
•Control loop in corporate fieldbus
•Example: x-by-wire systems
•Synchronization allows to be independent on network delay
•“do that at time …”
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Application Bottlenecks
•Standard protocol stacks do not care about Real Time
•Time required for data provision and processing is
independent of communication (and could be variable)
•OS and application could have their clocks, different
from communication servo clock
•Synchronized clocks can help… but communication, OS
and application should share the same clock
Same
Clock
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RTE: performance indicators
•IEC 61784-2 Industrial communication networks –Profiles– Part 2:
Additional fieldbus profiles for real-time networks based on ISO/IEC 8802-3 suggests some
performance indicators
•Delivery time
Time needed to deliver the ADPU -Application Protocol Data Unit- (measured at the
interface between the application process and the Fieldbus application entity).
•Maximum Delivery time (two cases)
No transmission errors, one lost frame with recovery
•Throughput RTE
Total amount of ADPU data (by octet length) on one link per second
•Non RTE bandwidth (related to Throughput RTE)
% of bandwidth that can be used for non-RTE communication on one link
•Time synchronization accuracy
Maximum deviation between any two node clocks
•Non-time-based synchronization accuracy
Maximum jitter of the cyclic behavior of any two nodes (triggering event for cycle)
•Redundancy recovery time
Maximum time from failure to become fully operational again (single permanent failure)
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Overall performance indicators
• Communication stacks have their own performance indicators
• OS have different indicators and a different way to experimentally
evaluate them
• Applications are normally evaluated by means of I/O
… overall evaluation of performance is difficult
Let’s start with the sense of time!
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Universal Time, UT
•Defined by the earth's rotation
•Astronomical observations
•Motion of GPS satellites
•Slightly irregular
•There are several different definitions of UT, but the
difference between them is always less than about 0.03 s
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Universal Time
•UT0, "raw" uncorrected UT
•Derived from meridian circle observations
•UT1
•UT0 corrected for polar wandering (precession)
•UT2
•UT1 corrected for seasonal variations
• t is the fraction of the year
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International Atomic Time
•Temps Atomique International, TAI
•Weighted average of ~200 atomic clocks
•50 national laboratories
•TAI -UT1 = 0 on 1958 Jan 1
•TAI and UT1 are since drifting apart
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Universal Time Coordinated
•UTC, Coordinated Universal Time
•TAI + integral number of seconds
•Leap seconds to keep UTC -UT < 0.9 s
(Next leap second to be added has been programmed 2012 Jun 30th)
•UTC follows Gregorian Calendar (days, hours, minutes,…) and
support Julian Date (since Jan 1, 4713 BC Greenwich noon=JD0,
29May2012 = Julian day number is 2456076)
•UTC days contain exactly 86,400 SI seconds (60 s per minute).
..but the mean solar day is slightly longer than 86,400 SI seconds,
therefore, occasionally the last minute of a UTC day is adjusted to
have 61 seconds. This extra second is called a leap second.
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Universal Time Coordinated
•Introduced in 1972
- In 1955 the cesium atomic clock replaced astronomic observation
1 January 1972 00:00:00 UTC = 1 January 1972 00:00:10 TAI
(same tick rate)
- UTC Format: 1970-01-01T00:00:00Z (“zulu” format)
- This is “the starting point” for Unix time (Posix time), that counts
the number -32bit- of seconds elapsed (not counting leap seconds)
- Overflow at 03:14:07 UTC 2038-01-19 (231 seconds)
- 32 bit for fractional part (0,4 ns as the resolution)
•Used in Internet and WWW (NTP or SNTP protocol)
•Defined by Int. Telecom. Union Recommendation (ITU-R TF.460-6)
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GPS (Global Positioning System) Time
•Kept by US Naval Observatory
•Reference two cesium and two
rubidium atomic clocks
•is kept as closely as possible to UTC
•Offset to UTC is given in GPS
Navigation Message
•Each GPS satellite continuously
broadcasts a navigation message
(50bits/s) composed of 5 subframes
(37500bits) and sent at a precise time
•Galileo Time
•Will be kept by European agencies
•Control Stations
• Fucino, Oberpfaffenhofen
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GPS
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GPS Time of the last 180 days
UTC vs. GPS Time
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NTP Timescale
Network Time Protocol, NTP
•NTP timescale based on the UTC timescale,
•First Tick: 0n 1 January 1972 (UTC)
•Insertion of leap seconds: conversion between NTP and UTC
has to be modified
•After leap second insertion (stop counting), timescales
resume ticking as if the leap had never happened.
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A Brief History of Clocks
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Servo clock
• Each node (transceiver) has its own quartz oscillator Fosc with a
related counter Cosc (time Tosc=Cosc/Fosc)
• A servo clock Thw can be locked to Tosc
(time Thw=Thw,offs+khw·Tosc where normally khw=1)
• Thw,offs can be adjusted according to the time offset error TOE
TOE = <expected time> - Thw
Thw,offs = Thw,offs + TOE
• khw can be adjusted according to the time drift (rate) error TDE
TDE = (<expected time>i - <expected time>i-1)/(Thwi – Thwi-1 )
khw = khw·TDE
• Other servo clocks can be managed at OS or application level
-> how to lock a servo clock to another servo clock
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Oscillator Model Frequency
Clock1 (Ref)
Rate error
Offset error
Clock2
Clock1 (Ref)
From clock1:
“Now it’s time…”
Clock2 (offset correction)
•Clocks depend on oscillators (temperature, voltage –batteries-, quality,…)
•A good synchronization implies periodic compensation of offset θ and rate π
•If at time t0 Clock1 and Clock2 are synchronized with error ε(t0)
•…then at time t ε(t) = π(t-t0) + ε(t0)
•…and ε(t) can be limited with a correct selection of synchronization interval
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Oscillator Model Frequency
Frequency f(t) as a function of time t
Frequency Offset
Aging Factor
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Environmental Term
Jitter noise
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Short-Term Noise
•Short-term noise is typically normal-distributed, hence
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Quality of Clocks
Problem: Determine the local quality w/o reference clock
•Accuracy
•Precision
•ALLAN deviation for phase errors
x is the time error sampled with τ time interval
•ALLAN deviation takes in account noise that cannot be
compensated with offset and rate correction
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Short-Term Noise
• Allan Variance as a function of τ is the synchronization interval
(constant, typical value τ = few seconds)
QZ = Quartz Crystal Oscillator
RB = Rubidium Gas-Cell
CS = Cesium-beam
HM = Active Hydrogen-Maser
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Finding the best master
• In order to have good external synchronization, the best
clock source should be found
• Who has the best Allan deviation
• A-priori knowledge (“GPS is better than human-tuning”)
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Synchronization protocol stacks
• Wired
• fieldbus (broadcast message)
• Ethernet (NTP, over UDP/IP port 123, 1ms accuracy)
• Real-time Ethernet (PTP or IEEE1588, over Ethernet or
UDP/IP –v2 only-, better than 100ns)
• Powerline (target: 1ms for metering, <1µs for power quality)
• Wireless (target: 1ms, 1ns if localization is required)
• GPS-synchronization
• A GPS-receiver provides a 1pps signal or synchronization
over protocols (e.g. IEEE1588)
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NTP -Network Time Protocol
•Strata structure (up to 256 strata, normally limited to 8-16)
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NTP -Network Time Protocol
•Structure
•Minimum-weight spanning tree of time severs
•Remote clock reading of all peer time servers
•Well engineered statistical algorithms for data
filtering and clock selection
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NTP –Pros and Cons
•Pros
•Available, standardized and field proven
•Cons
•Complex error modes to handle large and unreliable
networks (Internet)
•Behavior in small systems not investigated
•Supports only ms-range accuracy
•Deterministic analysis not available
•Not suitable for provably correct, dependable
systems
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IEEE1588-2002 (PTP Precision Time Protocol)
Master Clock
Tm1
Slave Clock
Tm1+Off
Sync message
Follow_Up with the
Sync transmission
precision_Timestamp
IEEE 1588 assumes network
propagation delay to be
symmetric
Master_to_slave
Delay
Ts1
Messages:
Ts2 - Off
Slave_to_Master
Delay
Ts2
Sync: multicast
Follow_up: multicast
Delay_Req: unicast
Delay_Res: unicast
Delay_Req
Tm2
Delay_Res with the
Delay_Req receiving
precision_Timestamp
Ts1= Tm1 + Off + master_to_slave_delay
Tm2= Ts2 – Off + slave_to_master_delay
Time
(master clock)
Time
(slave clock)
One_way_delay = (master_to_slave_delay+slave_to_master_delay)/2= ((Ts1-Tm1)+(Tm2-Ts2))/2
Off = Ts1-Tm1-One_way_delay
NOTA: Best Master Clock algorithm to select the master
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IEEE1588-2008 (PTP Precision Time Protocol)
•Mapping
• mapping to UDP/IP (not only Ethernet)
•Type length value (TLV) field
• additional fields for arbitrary information (e.g. security)
•Alternate master
• a backup master that runs if the selected master fails
•Security
• authentication, key-distribution
•Transparent clock
• the problem to limit uncertainty in clock propagation
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PTP: Boundary and transparent clock
If several networks share the medium, the “master
clock” synchronizes its own network, while the other
networks can be synchronized by means of some
shared nodes (clock uncertainty propagation)
Boundary clock (IEEE1588-V1)
A shared node acts as a “slave clock” of a network and as a “master clock”
of the other one
This node blocks PTP messages
Transparent clock (IEEE1588-V2)
A shared node corrects timestamping information according to the delays it
introduces (internal switch delays and cable delays)
It does not propagate clock uncertainty!
(its clock error affects the retransmission delay only –short time-)
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IEEE 1588 with HW Timestamping
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The PLC (powerline) communication media
•Compared with COTS Ethernet, fieldbusses
•Noisy, poor data transmission rate
•Communication may be interrupted
•Transmission power is limited due to EMC regulations
•Extensive signal processing due to channel characteristics
•Result: additional, but deterministic signal delay
•Channel adaptation not possible
•Counter measures against reflections, multi-path transmission
not possible
•Every node may also be a repeater, since some nodes are not
reachable by AP, Bridges directly
•Non-deterministic packet delay
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PLC: Network Topology
•Non-stable topology
•Suppliers switch subnets
for load balancing
•Nodes may be reachable
(synchronized) by multiple
Aps (with multiple
accuracies)
•Nodes, subnets may be
switched from one domain
to another
•IEEE1588-v2 compliant
solutions
•The problem of the choice
of synchronization interval
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IEEE1588 for PLC Side
•Drawbacks of IEEE1588 in PLC,
•Limited transmission bandwidth
•Poor reliability
•Non-stable topology
•Asymmetric Delay
•TDMA oriented network
•But with…
•PLC-receiver side generated follow-up packets
•Fixed-Value Delay-Response Packets
•PTP is possible!
•Advantage:
•Full IEEE1588 stack is usable
•Slave may act as fallback-Master
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IEEE1588 for PLC Side
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Synchronization in wireless fieldbus
• Clock synchronization among wireless sensor nodes allows data
sorting, but … s? ms? µs? ns?
• A very good synchronization…
Allows power saving
(transmitter and receiver are turned on only when needed)
Wastes power
(more data and messages must be transmitted)
bad synchronization
Tx
data
data
Rx
good synchronization
Tx
sync
data
sync
Rx
Wasted power
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Synchronization protocols for WSN
• PTP is a complex stack …(not very used)
•Requires IPv6 (6LowPAN) and UDP stack
• There are many synchronization protocols for WSN, trading
accuracy with complexity (power consumption)
• e.g. TPSN is a light version of PTP (round trip time)
• e.g. LTS corrects only offset (no drift, identical nodes)
Flooding Time Synchronization Protocol (FTSP)
Lightweight Tree-based Synchronization (LTS)
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Reference Broadcast Synchronization (RBS)
• Receivers synchronize each other
(optimal for internal synchronization)
i
R
j
Packet reception
interrupt
Receiver uncertainty
Timestamp with
Packet reception
interrupt
Timestamp with
Send
Send
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