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Tutorial on Synchronization
Jean-Loup Ferrant, Tim Frost,
Silvana Rodrigues, Stefano Ruffini
25 November 2014
Agenda
Intro on need for sync (S. Ruffini)
Time sync, basic concepts (T. Frost)
Sync distribution (S. Rodrigues)
Q13 status and ITSF/WSTS (J-L
Ferrant)
25 November 2014
2
Need for Sync
25 November 2014
Time and Frequency
Aligning two time clocks (synchronization) implies:
Make frequency B = frequency A (syntonization)
Make phase B = phase A (e.g. roll-over instant of 107 counter)
Make seconds B = seconds A (elapsed time equal; same time
origin)
Choose same formatting convention (and time-zone, etc.)
Clock A
“Time”
Clock B
Time alignment (“local time”)
“Time”
Time alignment (UTC)
Seconds
Counter
Time alignment (equal # of seconds)
1Hz
10,000,000
Counter
10MHz
Seconds
Counter
1Hz
Phase alignment (roll-over coincident)
(equality to within 1 clock cycle of 100ns may suffice)
Frequency alignment (syntonization)
10,000,000
Counter
10MHz
Timing Alignment in Wireless
Df = frequency offset between BSs
DT = time offset between BSs
BS - B
BS - A
Mobile in motion; speed = X m/s
When hand-over occurs, the mobile must reacquire carrier
frequency
Mobile in motion (X m/s) introduces a Doppler shift (X/c)
Loop bandwidth wide enough to handle (Df + X/c +LO) (LO = local oscillator
offset)
Loop bandwidth should be small from a noise rejection viewpoint
Large Df compromises the reliability of hand-over; 50 ppb typical requirement
TDD networks require time/phase alignment between A & B
To control interference between uplink and downlink
Requirement in the microsecond range
LTE-Advanced require DT to be small (microsec) for providing the
more bandwidth intensive features
8
Current Timing Issues
Networks are being migrated to packet switching as
opposed to circuit-switched (i.e. based on TDM)
Significant impact of variable delay (packet delay variation)
Timing requirements remain
Going “IP” does not mean that real-time services or mobile
networks no longer need synchronization!
Transition Phase:
Hybrid Networks (IP/TDM islands)
Circuit Emulation
Timing over Packet Networks (packet-based
methods)
PTP, NTP, adaptive clock recovery
Monitoring and Testing
Metrics for packet-based timing methods (quantifying PDV)
9
Emerging Needs
Increased need of time/phase sync in
Mobile networks
Time sync over various technologies
(microwave, OTN, MPLS, etc.)
Financial Sector
IoT, Network of Sensors
Power Networks
...
25 November 2014
10
Power – the need for Sync
“The Power Grid” is one of the world’s largest infrastructures
High synchronization requirements due to distributed nature of
the grid and the critical balance between power generation and
consumption
Power can’t be stored easily so Grids Generate according to Demand
Need good Comms and Sync to correlate Demand and Generation
Has evolved from seconds to milliseconds and will evolve to
microseconds → Greater Efficiencies
Also enables the Greater Diversity of the Smart Grid
Power Profile – IEEE C37.238-2011 (target: 1 ms accuracy)
Source: NIST
What is Time?
What is Time?
Time is a fundamental physical dimension
Allows ordering and scheduling of events
Enables sharing of resources (e.g. time division muxing)
Passage of time measured by counting a
regularly repeating event
Astronomical events, e.g. day/night, year
Physical events, e.g. pendulum swing,
quartz resonance or atomic transitions
Common Time
Common time requires a reference point
Time at an instant has no meaning without a reference
Need to start counting from a common point, or epoch
Example: the Gregorian calendar counts years from the
birth of Christ
A time reference clock counts at a
constant frequency from a known epoch
Sending time in a message...
Need to know how long the message takes
A letter – might be usable for setting the date
A phone call – could use to set hour/minute/seconds
A packet – millisecond level accuracy
What is Time Error?
Time Error
The time error of a clock is the difference
between the time indicated by that clock
and a reference clock at the same
instant in time
Always relative: has no meaning without a
reference
Defined by ITU-T Recommendation G.810:
x (t )  T(t )  Tref (t )
Time error
Time at measured clock
Time at reference clock
Direction of Time Error – clocks
Reference Clock:
Measured Clock:
Time Error
= 11.55 – 12.00
= – 5 minutes
Time Error
= 12.05 – 12.00
= + 5 minutes
Clock lags reference (slow, delayed): negative time error
Clock leads reference (fast, advanced): positive time error
Direction of Time Error – signals
Reference Clock:
Measured Clock:
negative
Time Error
negative
Time Error
positive
Time Error
Signal lags reference (slow, delayed): negative time error
Signal leads reference (fast, advanced): positive time error
Characterizing Time Error
Time Error Function
Time error varies with time and can be expressed
as a function*:
D  Dref 2  t    ref t 
xt   x0   y0  y0,ref .t 
t 
2
2 nom
Time error
Frequency
offset
Constant time error
Frequency
drift
Random variations
(dynamic time error)
If clocks are locked in phase, frequency offset
and drift are eliminated, and time error reduces
to two components:
Constant time error or offset
Dynamic time error or random variations
* Equation defined in ITU-T Recommendation G.810
Examples of Time Error Functions
Time Error
Clock frequency high; time error increasing
0
Time
Periodic time corrections (not always accurate)
Clock frequency low; time error decreasing
Clock frequency high; time error increasing
Time Error
0
Time
Periodic frequency corrections
Measuring Time Error
Time Error
Max|TE|
cTE
dTE
0
Time
Need an accurate time reference – time error has no
meaning without a reference
Maximum Absolute Time Error (Max|TE|) is the maximum
distance from zero of the time error function
Sign doesn’t matter: excursions may be positive or negative
Constant Time Error (cTE) is the mean of the time error fn.
Period over it is measured is not specified; depends on signal
Dynamic Time Error (dTE) is the change of the time error fn.
Phase or time wander, analyzed using MTIE and TDEV
Sync Distribution
Slides for this section provided by Christian Farrow, Chronos
Technology Ltd (except for the last slide)
25 November 2014
How Are “Bits” Represented..?
The value of a Bit (0 or 1) can be represented by different
modulations of a carrier signal examples are:
1 10101
– Fibre Optics
• The presence or absence of a
light pulse
• Different frequencies of light
– Radio/Microwaves - Mobile
phone, satellite comms, WiFi, etc.
• Changes in phase, frequency or
amplitude of the electromagnetic
waves
– Electrical Cabling - Coaxial,
twisted pair, etc
• Voltage levels on the wire
+2.5v
0v
-2.5v
The bits arrive at regular intervals, represented here
24
Sync Trail Architecture Rules
•
•
PRC Level
Standards
•
EN 300 462-2-1
•
ITU G.803
EN 300 462-6-1
ITU G.811
PRC
SEC
•
•
SEC
N x SECs (N<=20)
N <= 60 for entire chain
EN 300 462-5-1
ITU G.813
SEC
SEC
•
•
1st SSU
SSU-T
EN 300 462-4-1
ITU G.812
SEC
SEC
SEC
N x SECs (N<=20)
SEC
SEC
SSU-T
•
•
Kth SSU, (K<=10)
N x SECs (N<=20)
EN 300 462-7-1
ITU G.812
SSU-L
26
SyncE Overview
How is SyncE different from normal Ethernet?
Existing Ethernet PHY (Physical Layer)
TX
RX
IEEE 802.3 defines Ethernet PHY
Rx uses incoming line timing. Tx uses free-running 100ppm
oscillator.
No relationship between the Rx & Tx.
SyncE PHY (Physical Layer)
Rx disciplines the internal oscillator
Tx uses the traceable clock reference, creating end-to-end
scheme.
PRC can provide the reference. SSUs filter jitter/wander.
SyncE and asynchronous switches cannot be mixed.
TX
Traceable
TX
RX
100 ppm
TX
TX
RX
RX
TX TX
4.6 ppm
Inaccurate
RX RX
RX
RX
100 ppm
Ext.Sync
4.6 ppm
SyncE Element
TX
Asynchronous
Element
From clocks to packets
Packet “clocks” can be thought of in the same
way as physical layer clocks…
CES Packets do have a regular rhythm – E1 =
1mS
NTP/PTP Packets may not arrive regularly, but
timestamps within the packets themselves
mean time information can be extracted
Time and timing can be distributed from point
A to point B
Packets
time
(header, payload and footer)
F
Payload
4
H
F
Payload
3
H
F
Payload
2
H
F
Payload
1
H
28
Significant instants
IEEE 1588-2008 PTPv2 Overview
The Grandmaster “reference clock” sends a series of time-stamped messages
to slaves.
Slaves eliminate the round-trip delay & synchronize to the Grandmaster.
Frequency is recovered from an accurate time of day reference.
Accuracy is enhanced by:
Frequent packet send rate (up to 128 per second)
Hardware time-stamping (eliminate software processing delays)
Best Master Clock Algorithm (optional, “best” master voted by nodes)
Embedded
Slave
1588
Grandmaster
(Server)
1588 Packets
External
Slave
PTPv2 Slave clocks
can be either standalone or embedded
in network
equipment
29
Combination Operation
• SyncE as “frequency assistance” to 1588
PRC
UTC
1588
GM
PTP
SyncE Node
Stream
1588 Packet Stream
SyncE Physical Layer
PSN
PTP
SyncE Node
Stream
1588
Client
PRC freq
• Gives immediate “frequency lock” to 1588 client
• SyncE & 1588 functionality may be in the same
node/element
31
G.8271.1 Architecture
G.8273.2
T-BC1
Class A
G.8272
PRTC
T-BC2
Class A
T-BC9
Class A
T-BC10/
T-TSC
Class A
End App.
T-GM
100ns
50ns
1.5us
T-BC1
Class B
PRTC
T-BC2
Class B
T-BC19/
Class B
End App.
T-GM
100ns
T-BC20/
T-TSC
Class B
20ns
1.5us
Note: The network limit of 1.5us also accounts for other sources of noise (e.g. holdover, link asymmetries, syncE
rearrangements)
PRTC = Primary Reference Time Clocks
T-GM = Telecom Grand Master
Q13 status
transport of timing through telecom data
networks
-transport of frequency
-SyncE
-1588
done for IP networks, through NEs not processing 1588
messages
- Pure frequency T-GM ongoing: G.8266
-transport of phase and time
-ongoing
- difficult as propagation delay corrupts time
-need a two way transport
-asymmetry of 2 directions
-Testing sync transport over packets
- Need for new metrics and news methods
33
Overview of recommendations
Definitions /
terminology
Basics
G.8260
(Definition & metrics)
Frequency: G.826x
G.8261
Agreed
G.8271
Network
requirements
Clock
Time/Phase:G.827x
Full support
G.8271.1
NetwkPDV
SyncE J & W
G.8261.1
G.8262
(SyncE)
G.8272
PRTC
G.8263
(slave clock)
G.8273
Ongoing
assisted
partial support
G.8271.2
73.1-GM
73.2 BC
& slave
73.2 BC
& slave
73.3 TC
G.8273.4
A-PTS ck
G.8266
GM for F
G.8264
Methods
G.8265
()
Profiles
G.8265.1
G.8265.m
G.8275
G.8275.1
PTPprofile1
G.8275.2
34PTPprofile2
I-6 Time profiles
First profile
Full timing support from the network
It means that all NEs process the PTP messages
PTP messages mapped in Ethernet (G.8275.1)
Completed with
G.8272 (PRTC)
G.8273.2 (BC and slave clock)
Will be upgraded with
Stand alone T-GM G.8273.1
Transparent clocks G.8273.3
35
I-7 Second time profile
Partial Timing Support profile (PTS)
Work item created in July 2013
General architectural view (G.8275)
PTP unaware networks separated by T-BCs
PTP messages mapped in IP
PRTC
•
two-way packet timing signals
time reference, Tin
Packet Master
Clock
Boundary Clock
Packet Slave
Clock
Tout + 
36
I-8 Assisted PTS profile (A-PTS)
New ideas brought in October 2013
eNode B be synchronized with GPS receivers in
priority
PTP will be used only as a backup in case of GPS
failure
Operators request A-PTS for end 2014
Seemed difficult to achieve on time
37
new PTS profile- « nonA-PTS »
New architecture agreed in September 2014
simple PTP over IP based distributed deployment
model
a grandmaster is deployed inside a building to
provide frequency and time synchronization to the
small cells within the building or to nearby
buildings.
Only a few PTP-unaware nodes between the local
GM and the slaves on the small cells.
38
Partial Timing Support profile
New recommendations required
G.8275
Add the « 1588unaware » equipments in the
network architectures for A-PTS & « nonA »-PTS
G.8271.2(A-PTS), G.8271.x(PTS)?
Define the network limits and HRMs
G.8273.2
Define a new Boundary Clock if needed
G.8273.4(A-PTS), G.8273.y(PTS)?
Defines new clocks if needed
G.8275.2, G.8275.x??
both A-PTS & PTS based on IP
39
Future of recommendations?
Definitions /
terminology
G.8260
(Definition & metrics)
Full support
F
Basics
R
G.8271
E
Network
Time/Phase:G.827x
G.8271.1
NetwkPDV
Q
Agreed
assisted
partial support
G.8271.2
Ongoing
TBD
partial support
G.8271.y
requirements U
E
Clock
N
G.8272
PRTC
G.8273
73.1-GM
73.2 BC
& slave
Profiles
?
G.8273.4
A-PTS ck
Y
:
G
.
8
2
6
X
73.y
73.3 TC
C
Methods
73.2 BC
& slave
G.8275
G.8275.1
PTPprofile1
G.8275.2
PTPprofile2
G.8275.y
PTPprofile?
40
?
Links between Q13/15 and fora
Operators, manufacturers, std organisations,universities
and scientists meet once a year at WSTS in USA and at
ITSF in Europe.
During these events several sessions addresses:
-the needs for synchronization
-tutorials on synchronization
-the requirements for synchronization
-news from GNSS systems
-alternative to GNSS: E Loran, ..
-information on future technologies
-information on deployments
-information on sync standards: ITU Q13/15, IEEE1588
-etc…
25 November 2014
41
ITSF
(International Telecom Sync Forum)
ITSF Website: www.telecom-sync.com
Organiser’s Website: www.itsf-conference.com
Meets every year since 2001 in November
Last event: 4-6 Nov 2014 in Budapest
Next event: 3-5 Nov 2015 location tbd
WSTS (Workshop on Synchronization in
Telecommunication Systems)
Depends on NIST-ATIS
http://www.atis.org/wsts/cfp.asp
Meets every year since 1992in spring
Last event: 4-6 June 2014 in San Jose, Ca USA
Next event: 9-12 March 2015 in San Jose, Ca USA
25 November 2014
42
Backup slides
List of main ITU-T recommendations
related to synchronization
(updated November 2014)
43
Where to get the recommendations?
http://www.itu.int/ITU-T/recommendations/index.aspx?ser=G
44
Recommendations for TDM hierarchies

G.803 (03/2000), Architecture of transport networks based on the
synchronous digital hierarchy (SDH)
•
G.803Amd1(06/2005)

G.810 (08/1996), Definitions and terminology for synchronization
networks
• G.810 Corr1(10/2001)

G.811 (09/1997), Timing requirements of primary reference clocks

G.812 (06/2004), Timing requirements of slave clocks suitable for use
as node clocks in synchronization networks

G.813 (03/2003), Timing requirements of SDH equipment slave clocks
(SEC)
• G.813 Corr1(06/2005)
 G.822 (11/1988), Controlled slip rate objectives on an international
digital Connection
 G.823 (03/2000), The control of jitter and wander within digital
networks which are based on the 2048 kbit/s hierarchy

G.824 (03/2000), The control of jitter and wander within digital
networks which are based on the 1544 kbit/s hierarchy

G.825 (03/2000), The control of jitter and wander within digital
networks which are based on the synchronous digital hierarchy (SDH )
• G.825 Amd1 (05/2008)

G.781 (09/2008), Synchronization layer functions
45
Recommendations for timing over packet networks
 G.8260 rev(02/2012), Definitions and terminology for
synchronization in packet networks
•G.8260 Amd1 (8/2013), Amd2 (5/2014)
Recommendations for Synchronous Ethernet





G.781 (09/2008), Synchronization layer functions
G.8261 rev(08/2013), Timing and Synchronization aspects in
Packet Networks
G.8262 (07/2010), Timing characteristics of synchronous Ethernet
Equipment slave clock
• G.8262 Amd1 (02/2012), Amd2 (10/2012)
G.8263 (02/2012), Timing characteristics of packet-based
equipment clocks
• G.8263 Amd1 (08/2013), Amd2 (05/2014)
G.8264 rev(05/2014), Distribution of timing information through
packet networks
46
Recommendations for
phase







the telecom profile for time and
G.8271 (02/2012), Time and phase synchronization aspects of packet
networks
•
G.8271 Amd1 (08/2013)
G.8271.1(08/2013), Network Limits for Time Synchronization in Packet
Networks
•
G.8271.1 Amd1 (5/2014)
G.8272 (10/2012), Timing characteristics of primary reference time
clocks
•
G.8272 Amd1 (8/2013)
G.8273 (08/2013), Framework of phase and time clocks
•
G.8273 Corr1 (5/2014)
G.8273.2 (05/2014) Timing characteristics of telecom boundary clocks
and telecom time slave clocks
G.8275 (11/2013), Architecture and requirements for packet-based time
and phase distribution
G.8275.1 (07/2014), Precision time protocol telecom profile for
phase/time synchronization with full timing support from the network
47
Recommendations for OTN
 G.8251 (09/2010), The control of jitter and wander within the optical
transport network (OTN)
• G.8251 Amd1 (04/2011),Amd2 (02/2012), Amd3 (10/2012)
• G.8251 Corr1 (02/2012)
Recommendations for the telecom profile for frequency only
 G.8261 rev(08/2013), Timing and Synchronization aspects in
Packet Networks
 G.8261.1 (02/2012), Packet Delay Variation Network Limits
applicable to Packet Based Methods (Frequency Synchronization)
• G.8261.1 Amd1 (5/2014)
 G.8263 (02/2012), Timing characterisctics of packet based
equipment clocks
• G.8263 Amd1 (8/2013), Amd2 (5/2014)
 G.8265 (10/2010), Architecture and requirements for packet
based frequency delivery
 G.8265.1 rev(07/2014), Precision time protocol telecom profile
for frequency synchronization
48
Future recommendations ( provisional titles)
G.8266
Timing characteristics of telecom grandmaster clocks for
frequency synchronization
G.8273.1
Timing characteristics of packet master clocks
G.8273.3
Timing characteristics of telecom transparent clocks
G.8273.4
Timing characteristics of assisted partial timing support
slave clocks
G.8275.2
Precision time Protocol Telecom Profile for time/phase
synchronization with partial timing support from the network
49
Recommendation on Jitter and wander tests equipments
O.171 (04/1997), Timing jitter and wander measuring equipment
for digital systems which are based on the plesiochronous digital
hierarchy (PDH)

O.172 (04/2005) , Jitter and wander measuring equipment for
digital systems which are based on the synchronous digital hierarchy
(SDH)

O.173 (02/2012), Jitter measuring equipment for digital systems
which are based on the Optical Transport Network

O.174 (11/2009),
Jitter and wander measuring equipment for
digital system based on synchronous Ethernet network

50