Timing for IoTapplications
Chris Marshall
1 May 2019
Introduction
IoT devices
absolute and relative timing
Sources of timing
wireless provision of time
location and time
Some hybrid timing examples
u-blox AG2
Timing for Internet of Things applications
u-blox AG3
Internet of ThingsApplications
Hours Hours-Minutes Sec-mSec µSec and better
1st Gen 2nd Gen 3rd Gen 4th Gen
Mechanization,
water power,
steam power
Mass production,
assembly line,
electricity
Computer and
automation
Cyber physical
systems
• Lots of devices, sensors
• Connected (e.g. wirelessly)
• Often static
• Often indoors
• Application from shared information
• Measurements and data have value through knowledge of its time
• Instructions and control require coordination of actuators
The relevance of timing in industrial revolutions
u-blox AG4
What’s a nanosecond between friends?What are IoT applications looking for
Time Performance
Class
Accuracy Purpose
T1 ±1 ms Time tagging of events
T2 ± 100 µs Time tagging of zero crossings and
of data for the distributed
synchrocheck. Time tags to
support point on wave switching.
T3 ±25 µsInstrument transformer
synchronization (sampled values)T4 ±4 µs
T5 ±1 µs
• “Accuracy of ±1ms”
• for comparison of events
• for slow measurements
• “Accuracy of ±1µs”
• for comparison of waveforms
Time performance requirements laid out in IEC 61850
u-blox AG5
Accuracy and precisionA reminder about terminology
• Precision is about repeatability
• affected by
• jitter
• signal level
• bandwidth of the signal being measured
• Accuracy is about bias
• affected by propagation delays
• and with what we are comparing our measurement
• … relative timing
A high quality watch set to the wrong time may be precise, but inaccurate
PrecisionHigh Low
Acc
ura
cyH
igh
Low
u-blox AG6
A framework for thinking about applications Relative and absolute timing
• Time
• how precisely?
• relative to what?
• and how accurately?
A framework for expressing IoT device timing requirements
Group of devices
Single device
Absolute time
Relative time
u-blox AG7
Sources of timing
u-blox AG8
Sources of timing informationFor a wireless IoT device
• Local clock
• Wireless signal
• Messages from a server
• GNSS
u-blox AG9
Local clockProviding on-board timekeeping
Local timer LTE signal Network timing GNSS
Elapsed time locally Relative time everywhere Absolute time everywhere Absolute and relative time
outdoors
Pros
• Always available
provided sufficient
power
• Widely available, even
underground
• Excellent short term
stability
• Widely available for
connected devices
• Excellent accuracy
• Excellent long term
stability
Cons
• Applicability limited by
drift, temperature and
aging
• Absolute timing may be
undefined
• Observed time depends
on signal propagation
distance
• Limited by variable
latency, depending on
the network, which
affects timing accuracy
• Limited to the cases
where signal reception
is not obstructed (not
indoors and not
underground)
• Commonly at the heart of a device
• Compromises between performance and power consumption
• Subject to temperature drift and aging
Time sources available for cellular modem IoT applications
u-blox AG10
Wireless signalsTerrestrial transmissions that are available
Local timer LTE signal Network timing GNSS
Elapsed time locally Relative time everywhere Absolute time everywhere Absolute and relative time
outdoors
Pros
• Always available
provided sufficient
power
• Widely available, even
underground
• Excellent short term
stability
• Widely available for
connected devices
• Excellent accuracy
• Excellent long term
stability
Cons
• Applicability limited by
drift, temperature and
aging
• Absolute timing may be
undefined
• Observed time depends
on signal propagation
distance
• Limited by variable
latency, depending on
the network, which
affects timing accuracy
• Limited to the cases
where signal reception
is not obstructed (not
indoors and not
underground)
• Broadcast transmitters for frequency and time
• Cellular transmitters (LTE) plentiful
• of increasingly good quality
• but uncontrolled absolute timing
• signal anyway received in IoT modem, so measurement “for free”
• Good for relative timing
• Accuracy is affected by signal propagation
Time sources available for cellular modem IoT applications
u-blox AG11
Providing time in different locationsWireless distribution affected by propagation delay
• Relative delay between devices∆𝑡= Τ𝑑𝐴 − 𝑑𝐵 𝑐
• The location of the devices might not be known
• Location of the transmitter might not be known
• might be nearer 𝐴, or nearer 𝐵
• Give an uncertainty (bias) in the relative accuracy
• though for many applications this is fine100m → ±0.3s
Cellular base station
Unknown clock
modem on sensor𝐴
𝐵
𝑑𝐵
𝑑𝐴
u-blox AG12
Network timing servicesMessage from the cloud
Local timer LTE signal Network timing GNSS
Elapsed time locally Relative time everywhere Absolute time everywhere Absolute and relative time
outdoors
Pros
• Always available
provided sufficient
power
• Widely available, even
underground
• Excellent short term
stability
• Widely available for
connected devices
• Excellent accuracy
• Excellent long term
stability
Cons
• Applicability limited by
drift, temperature and
aging
• Absolute timing may be
undefined
• Observed time depends
on signal propagation
distance
• Limited by variable
latency, depending on
the network, which
affects timing accuracy
• Limited to the cases
where signal reception
is not obstructed (not
indoors and not
underground)
Time sources available for cellular modem IoT applications
• Time messages sent over the network
• Available to anything that’s connected
• Affected by the network delays
• Excellent for general orientation
u-blox AG13
GNSSTime and position
Local timer LTE signal Network timing GNSS
Elapsed time locally Relative time everywhere Absolute time everywhere Absolute and relative time
outdoors
Pros
• Always available
provided sufficient
power
• Widely available, even
underground
• Excellent short term
stability
• Widely available for
connected devices
• Excellent accuracy
• Excellent long term
stability
Cons
• Applicability limited by
drift, temperature and
aging
• Absolute timing may be
undefined
• Observed time depends
on signal propagation
distance
• Limited by variable
latency, depending on
the network, which
affects timing accuracy
• Limited to the cases
where signal reception
is not obstructed (not
indoors and not
underground)
• Available world-wide
• Perfect for interfacing to third parties
• Performance much improved with Assistance provided by the connectivity
• But the signals are very weak –indoor operation is tricky
• Outstanding performance, with excellent accuracy
Time sources available for cellular modem IoT applications
u-blox AG14
Solving for position as well as timeTo obtain the propagation delay
• GNSS is a positioning system
• the transmitter locations are known
• the receiver (sensor) locations are solved
• Needs to receive signals from 4 satellites
• to find (latitude, longitude, altitude, time)
• Difficult indoors
Obtaining position and time from four satellites
Unknown clock
Receiver
x
y
z
t
u-blox AG15
Solving for position as well as timeTo obtain the propagation delay
• GNSS is a positioning system
• the transmitter locations are known
• the receiver (sensor) locations are solved
• Needs to receive signals from 4 satellites
• to find (latitude, longitude, altitude, time)
• Difficult indoors
• If have an estimate position of receiver, this becomes a known
• then1 satellite is sufficient to find time
• giving improved coverage and reliability
Obtaining time from a single satellite, with the device in an estimated location.
Unknown clock
Receiver
t
u-blox AG16
Hybrid timingIn practice combine sources, depending on circumstances
• To meet application requirements for accuracy, precision, jitter, robustness…
cellular modem
absolute timeeverywhere
relative timing
everywhere
accurate time
outdoors
elapsed time locally
u-blox AG17
Examples
u-blox AG18
MetersSubstantially autonomous devices
• Each device acts on its own
• Task is to measure intervals of time
• Absolute timing accuracy is not particularly important
• Important are cost, coverage, reliability
• Time set and updated over the network
• Operational timing maintained by a local clock providing autonomy
• For high precision, could use LTE signals for measuring timing intervals
Group of devices
Single device
Absolute time
Relative time
Local clockWireless signalNetwork messageGNSS
u-blox AG19
Factory of the future, and signal handlingMultiple devices in a locality, acting in concert
• Multiple devices (often indoors, and usually fairly close together)
• Synchronised processing of signals or information in a distributed application
• Examples
• Fault finding
• Average speed measurement
• Audio streaming
• Cellular signal can provide a common shared time base for measurements
• Network timing to time-stamp UTC of events
Group of devices
Single device
Absolute time
Relative time
Local clockWireless signalNetwork messageGNSS
u-blox AG20
Wide area sensor networkLarge scale, wide area deployment
• Outdoors
• Examples
• Seismology
• Forest fire detection
• Utilities
• Financial transactions
• GNSS
• Local clock for low power
Group of devices
Single device
Absolute time
Relative time
Local clockWireless signalNetwork messageGNSS
u-blox AG21
Distributing timing indoorsWith reference to a wireless signal
• Wireless signal measured by a GNSS receiver in a good location
• Wireless signal timing then used nearby
𝑑𝐵
𝑑𝐴
• Accuracy determined by difference in propagation distances
Group of devices
Single device
Absolute time
Relative time
Local clockWireless signalNetwork messageGNSS
u-blox AG22
Maintaining timing for robustnessWhen an interruption in the primary source
• Wireless signal measured by a GNSS receiver when conditions are good
• Wireless signal timing then used when GNSS signal lost
• Accuracy determined by drift in wireless signal transmitter
GPS LTE
Timing drift over 1 hour after disconnection of GPS antenna
1s
-1s
Group of devices
Single device
Absolute time
Relative time
Local clockWireless signalNetwork messageGNSS
u-blox AG, For internal use23
Relative timing for indoor sensorsExperiment comparing the time of events at separate modems
• Measure the speed of a «hotwheels» car:
• two separate modem application boards with optical sensor detect the car passing
• each has a module with CellTimeTM, they keep time using LTE and time-stamp events
• the modems send time stamps to a server,application calculates and outputs speed of car
• Illustrates
• hybrid timing, with initial calibration by GNSS
• timing indoors using the LTE signal
• relative timing between two cellular modems
Relative timing between two modems
8.37km/h
u-blox AG24
ConclusionsTiming for IoT applications
• Think about the application
• precision
• relative timing accuracy
• absolute timing accuracy
• usage model and environment
• Robustness, power consumption, cost…
• Address with hybrid timing, using
• the local clock
• wireless signals
• network messages
• GNSS
For further discussion, see https://www.u-blox.com/en/white-papers
u-blox AG25
Thank youfor your attention