Evolution of Network Synchronization Technologies

Evolution of Network
Synchronization Technologies
Hydro-Quebec Symposium
May 16, 2016
Chuck Perry
cperry@Oscilloquartz.com

© 2015 ADVA Optical Networking. All rights reserved. Confidential.2 © 2015 ADVA Optical Networking. All rights reserved. Confidential.2
Oscilloquartz at a Glance
• Member of the ADVA Optical Networking Group
• Focused offerings for communications, government and enterprise
sync applications
• Longstanding relationship with customers worldwide
• Around 100 sync focused partners in about 80 countries around the
globe
• State-of-the-art time and frequency systems
• End-to-end solutions for all markets
• Timing delivery and assurance Excellency
Innovation leader for timing distribution and assurance

City
Town
Public switch
Television
Answering
machine
PBX
IBM Compatible
Laptop computer
Telephone
Fax
IBM Compatible
Public switch
Answering
machine
PBX Telephone
Laptop computer
Fax
City
Town
… to enable service providers to transport
bits of information within and across network
boundaries without losing any bits of information.
This is accomplished by synchronizing all
transmitted signals to a common stable frequency
source.
The Objective of Synchronization Is …

Common Sync Problems
Voice Audible Click
Fax Illegible text
Email Retransmitted data
Video Corrupted video
Cell/PCS Dropped calls
SONET/TDM Data loss
ATM Corrupted data
VoIP-voice Latency (echo)
VoIP-FAX Dropped calls
QUICK PROBLEMS CHECKLIST
Poor synchronization is the most common, non-obvious, cause of service degradation

Typical Hierarchical Synchronization Plan
Stratum
2
Stratum
2
Stratum
2
Stratum
2
Stratum
1 Stratum
3
Stratum
3
Stratum
3
Stratum
3E
Stratum
1
Stratum
3E
Stratum
3E
Stratum
1
Stratum
3E
Stratum
3EStratum
3E
Distributing a highly accurate frequency reference to all
Network Elements in an effort to elevate the internal
Oscillators of the network elements to that of the frequency
source (Stratum 1)

ANSI Clock Standards
Stratum-1
Stratum-2
Stratum-3E
Stratum-3
1 x 10-11
1.6 x 10-8
4.6 x 10-6
Not Defined
< 255 DS1 slips,
1st 24 hrs.
1 x 10-10 per
day, 1st 24 hrs.
4.6 x 10-6 1 x 10-8 per
day, 1st 24 hrs.
1.6 x 10-8
Not Defined
4.6 x 10-6
4.6 x 10-6
SMC 2.0 x 10-5
Stratum-4 Not Defined3.2 x 10-5 3.2 x 10-5
4.6 x 10-6 2.0 x 10-5
The maximum MTIE during a reference rearrangement for
SONET interfaces is 1 ms or 20 ns in any 14 ms.

The Objective of Synchronization Is …
Df = The average rate of phase accumulation (Dt/t)
1 picosec/sec  1x10-12 (1 ppt) (part per trillion)
10 picosec/sec  1x10-11
100 picosec/sec  1x10-10
1 nanosec/sec  1x10-9 (1 ppb) (part per billion)
10 nanosec /sec  1x10-8
100 nanosec /sec  1x10-7
1 msec /sec  1x10-6 (1 ppm) (part per million)
Df = fractional frequency offset

8
Frame Slips Due to Clock Offsets
Clock Extraction
Buffer
(Typically two frames or 250 usec)
System Clock
Write Clock
Read
Clock
To
Switching
Matrix
Incoming
Signal
Frequency offsets between traffic terminating elements are
reflected in the phase alignment of the read and write clocks
of Slip Buffers. Poorly synchronized networks cause these
buffers to slip, resulting in frame slips.
Frequency offsets between traffic terminating elements are
reflected in the phase alignment of the read and write clocks
of Slip Buffers. Poorly synchronized networks cause these
buffers to slip, resulting in frame slips.

Technology
Optical Cesium
Hydrogen Maser
Cesium Standards
GPS Receivers
Rubidium Oscillators
HQQ +
Quartz Crystal Oscillators
Tuned Circuits
Stratum Levels
“Stratum 0”
Stratum-1
Stratum-2E
Stratum-2
Stratum-3E
Stratum-3
Stratum-4
1 X 10-15
1 X 10-5
Frequency Standards
9

PRS - Cesium
In 1967 the esium atom was recognized as the basis
for the international standard (SI) unit of time
Atomic resonant frequency is exactly 9,192,631,770 Hz
All PRS equipment will be traced back to a Cesium standard.
Telecom Cesium devices have, typically, rT/T of 1 picosecond/second
(or a 1x10-12 frequency offset)
One Day rT = 86,400 picoseconds
One Year rT = 31,536 nanoseconds
12 Years rT = 378 microseconds
Less than 1/2 ms time error (with respect to UTC) for the life of the tube.
(“Turn it on, and forget about it”)
10

All GPS satellites contain multiple Stratum 1 Clocks (Cesium and/or Rubidium standards).
The Clocks in the satellites keep accurate time to within three nanoseconds. A GPS
Primary Reference Receiver will derive the accuracy of the standards and provide a very
stable Stratum 1 clock source. The GPS satellites are in half synchronous orbit so they
circle the earth twice a day.
PRS - Global Positioning System
11

GPS Space Segment
•The space segment consists of low
earth orbit (LEO) satellites, 24 in all: 21
navigational SVs and 3 active spares
orbit at 11,000 nautical miles above the
Earth. There are six orbital planes (with
4 SVs in each), equally spaced (60
degrees apart), and inclined about 55
degrees with respect to the equatorial
plane.
12

GPS Control Segment
• The GPS control segment consists of a system
of monitor stations located around the world
(Hawaii and Kwajalein in the Pacific Ocean;
Diego Garcia in the India Ocean; Ascension
Island in the Atlantic Ocean; and Colorado
Springs, Colorado) a master ground station at
Falcon Air Force Base in Colorado Springs,
Colorado; and four large ground antenna
stations that broadcast signals to the satellites.
13

14
 Highly reliable PRS
Roof access required for antenna
Long-term capitalization factor is high
Time-of-day available
GNSS
Town
 Self-contained, highly-reliable PRS
Antenna not required
Long-term capitalization factor is low
Time-of-day not availableCESIUM
City
Primary Reference Source Options

15
TSG Architecture

Building Integrated Timing Supply - BITS
DS3
TSG
MUX
Channel
Banks
DCS STP
Toll
Switch
CC/DS1
ATM
Switch
I
S
D
N
SONET
ADM
SONET
ADM
CC EC1
51 Mbit
MSS
PRS
Stratum 1DSL
DSLAM
DS1
DS1
DS1
DS1
DS1
DS1
CC
CC
CC
CC/DS1
DS3
DS0
DS0
DS0
DS0
DS1 OC192
OC192
OC48
OC48
OC3
OC3IP
ATM DS3
16

17
Inter-Office Timing Distribution - SONET

18
Timing Loops

19
Bi-Directional Switched Ring

Bi-Directional Switched Ring

21
SSM – Sync Status Messaging

SONET SSM Formats

SSM Rules for Implementation

24
SSM Bi-Directional Ring Configuration

25
SSM – Bi-Directional Ring Configuration

Packet Timing
Technologies
Chuck Perry
512-431-3103

27
NTP Overview
 Network Time Protocol (NTP) synchronizes clocks of hosts and routers
in the Internet
 The NTP architecture, protocol and algorithms have been evolved over
the last two decades. Currently NTP Version 4 is being developed
 Well-tested and widely-deployed protocol
 NIST estimates 10-20 million NTP servers and clients deployed in
the Internet and its tributaries all over the world. Every
Windows/XP has an NTP client
 NTP provides nominal accuracies of low tens of milliseconds on WANs,
submilliseconds on LANs, and submicroseconds using a precision time
source such as a GNSS receiver
 Traditional implementations are primarily software-based. Non-
deterministic delays in networking stacks contribute to significant
timing inaccuracy

NTP Protocol Overview
• Clock offset:
• [(T2 – T1) + (T4 – T3)] / 2
• Round-trip delay:
• (T4 – T1) – (T3 – T2)
Client Server
Client sends
request at
T1 =
10:15:00
T1
T2
T3
T4
Server
receives
request at
T2 =
10:15:12
Server sends
response at
T2 =
10:15:15
Client
receives
response at
T2 =
10:15:30
» Key Assumptions:
– Network delay is symmetric in both
directions
 One-way delay is half of round-trip delay
– Client and server clocks drift at the same
rate

29
NTP Stratum Levels
S1 S1
S2 S2 S2 S2
S3 S3 S3 S3 S3
S4 S4 S4 S4
Stratum 1
Stratum 2
Stratum 3
Stratum 4
 Hierarchical layering of clocks based on
number of hops from primary
reference source
 Stratum 1 servers are synchronized
with a GPS source
 Stratum 2 servers use client/server
mode to synchronize with up to six
Stratum 1 servers and symmetric
mode to synchronize with other
servers on the same stratum level
 Stratum 4 clocks work in client mode
to synchronize with servers in Stratum
3
NTP Stratum levels are not the same as ANSI/ITU-T Stratum levels!

IEEE1588 Protocol Overview
• The Slave collects the time
values t1, t2, t3, t4 during a
transaction and calculates
final offset (o) between
Master and Slave clocks
canceling out network delay
(d) as follows:
t2 –t1 = o + d
t4 - t3 = -o + d
o = (t2 + t3 – t1 – t4) / 2
d = (t2 – t1 + t4 – t3) / 2
Master Clock Time Slave Clock Time
Data at
Slave Clock
t1
t2
t2m t2
Sy nc Message
Followup Message
containing v alue of t1
t1 t2
t3t3m
Delay Request
Message
t1 t2 t3
t4
Delay Response Message
containing v alue of t4
t4
t1
t2 t3
Time

31
• IEEE 1588 (commonly known as Precision Time Protocol,
PTP) was ratified as a standard in September 2002
• LAN technology providing timing for the control of
distributed applications
• Slow transaction rates
• Version 1 of the protocol used for applications in
• Industrial automation
• Test and measurement
• Military
• Version 2 developed for telecom applications
• Early adopters include Vodafone, T-Mobile, etc.
IEEE-1588 Version 1

IEEE 1588v2 Enhancements
• IEEE 1588v2 meets accuracy requirements for Telecom applications
• High refresh rates up to 128 messages per second
• Correction field for asymmetric measurements
• Several modes supported
• Broad-cast, Multi-cast and Uni-cast are permitted
• Smaller message length to conserve bandwidth
• 72 octets (44 for 1588v2 payload)
• Multiple Master Clock selection methods
• Manual, Semi-automatic, Fully-automatic
• Transparent Clocks to reduce accumulation of timing errors across
network elements in cascaded topologies
• Enhanced security
• Configurable network in combination with Best Master Clock algorithm for GrandMaster
• HASH codes
32

Precision Time Protocol (PTP) Explained
Grandmaster
(Server)
L2/L3 device
External
Slave (client)
1588 Packet Flow
1588
1588
1588
• Protocol used to synchronize clocks
throughout a network.
• The Grandmaster (GM) “reference
clock” sends a series of time-
stamped messages to slaves
• Slaves receive the messages, and
eliminate the round-trip delay by
synchronizing to the Grandmaster.
• Frequency/Time-of-Day/Phase is
recovered from the accurate time of
day reference from the GM.
• Boundary Clocks (BC) can receive
PTP as a reference, while providing
GM functionality downstream to other
clients.
Boundary Clock (BC)
External
Slave (client) Embedded
Slave (client)
1588
1588
1588
(BC)
(BC)
(BC)

34
Transparent Clock
MAC
PHY
MII
MAC
PHY
MII
PTP
UDP
IP
MAC
PHY
MII
PTP
UDP
IP
MAC
PHY
MII
Grandmaster Trasparent Clock
Transparent Clock
Slave
Grandmaster Transparent Clock Slave
 A Transparent Clock
contains no PTP ports.
 Timestamp in
incoming message is
modified before
sending the message
out
 Creates security
issues, since original
crypto checksum is
not valid anymore
A Transparent Clock is
neither a master nor a
slave. It is merely a switch
that adjusts a PTP
message’s timestamp to
compensate for its own
queueing delays
IP Network
M S

35
Boundary Clock
PTP
UDP
IP
MAC
PHY
MII
PTP
UDP
IP
MAC
PHY
MII
PTP
UDP
IP
MAC
PHY
MII
PTP
UDP
IP
MAC
PHY
MII
Slave Master
IP Network
Grandmaster Boundary Clock
Boundary Clock
Slave
Grandmaster Boundary Clock Slave
 A boundary clock contains
more than one PTP port:
 a slave port that is
synchronized with a
remote master, and
 a master port that
synchronizes other
slaves downstream
 Synchronization
messages are
terminated at each port
and not forwarded
A Boundary Clock extends
synchronization across an
intermediate network
element
M S
M
MS
S

Mobile Backhaul / The Challenges
Application Radio Interface Backhaul
Frequency Phase Frequency Phase
CDMA 2000 ±50ppb ±3 to 10µs GPS GPS
GSM/WCDMA ±50ppb n/a ±16ppb n/a
LTE (FDD) ±50ppb n/a ±16ppb n/a
LTE (TDD) (large cell) ±50ppb ±5µs ±16ppb ±1.1µs
LTE (TDD) (small cell) ±50ppb ±1.5µs ±16ppb ±1.1µs
LTE-A MBSFN ±50ppb ±1 to 5µs ±16ppb ±1.1µs
LTE-A CoMP* ±50ppb
±500nsec to
5µs
±16ppb 500ns - ±1.1µs
LTE-A eICIC* ±50ppb ±1 to 5µs ±16ppb ±1.1µs
* The performance requirements of the LTE-A features are under study by 3GPP
New timing distribution architectures are Now Required!

Network Synchronization Migration (L2/L3)
1st mile 2nd mile Aggregation Core (IP/MPLS)
GPS PRS
SSU/
TSG
CENTRAL OFFICE
FREQUENCY IS STILL REQUIRED,
BUT MANY APPLICATIONS NOW
REQUIRE PRECISE PHASE & TIME
Ethernet
Ethernet

Distributed Architecture
• Can be added to any exiting NE’s
• Wide Temperature range
• Simple configuration
• Small footprint and Cost effective
• Highly integrated
• Very low power (1.2W)
T-GM
GNSS
Mid-PTP
Grandmaster
Packet-Based Backhaul Network
GNSS
T-SC
T-SC
T-SC
First Aggregation Node
GM closer to end application
T-SC
Mini-GM
Embedded or
Low Cost SFP
GNSS/GM/T-SC

GNSS only at first aggregation site
1588v2 with Full/Partial On Path Support to Cell Sites
APTS in case of GNSS failure
T-GM
GNSS
PTP
Grandmaster
PTP unaware or partly aware
G.8265.1 /G.8275.2
Remote
Base Station
T-SC
T-SC
T-SC First
Aggregation
Node
Boundary
Clock &
PTP fully aware or partly aware network
G.8275.1/G.8275.2
BC
GNSS
5420
GM
Mid Scale GM with APTS in Aggregation Node
– Core GM Protection

GNSS only at first aggregation site
1588v2 with Full/Partial On Path Support to Cell Sites
APTS in case of GNSS failure
T-GM
GNSS
PTP
Grandmaster
PTP unaware or partly aware
G.8265.1 /G.8275.2
Remote
Base Station
T-SC
T-SC
T-SC First
Aggregation
Node
Boundary
Clock &
PTP fully aware or partly aware network
G.8275.1/G.8275.2
BC
GNSS
Mid-Scale
GM
Mid Scale GM with APTS in Aggregation Node
– Core GM Protection

Performance Monitoring & Probing
• Software feature that
measures and reports
the status/state of the
sync network
• Embedded in Many
Synchronization products
• Analogous to Perf Mon
and Bit Error Rate in
traditional networks
• SLA Verification

PTP Network Probe Statistics and Results
• Packet Counters (arrived, lost)
• PD (Path Delay) – min, max, avg., forward only
• MPD (Mean Path Delay) – min, max, avg., both
directions
• RPDV (Residual Path Delay Variation) – min,
avg., both directions (based on observed Delay
Floor); Current Value and Histogram
• Network Usability (based on G.8261.1 FPP);
Current Value and Histogram
Network Score -FR
Network Score -REV
Lost /Received

Network Management
• Build, Monitor and Expand a Reliable Sync network
• Reduce on-site Operation with a Centralized Management process
• Performance monitoring, Fault and Event management
• Dedicated for sync network monitoring
• Provide access to
• Core, edge and access sync equipment

1588 for Power Utilities
Chuck Perry
512-431-3103

Why Is Timing Relevant?
There are many definitions for the smart grid, but
in short …
• We need to do more with less
• Utilities need to do things differently
• Distributed generation & energy flows create network
stability concerns
• New reliability and security regulation
Protection, Telecommunication, Metering and
Control (PTM&C) technologies will have the biggest
impact
Timing is inherent to PTM&C
GenerationTransmission
&Distribution
Power Flow
Power Flow

And There Is the Regulatory Need
Au3 blackout… the case for forensics
• Blackout in north-east USA focused attention on the electric power grid
• After investigating, NERC suggested GPS time stamping throughout the grid
• Power System Outage task force endorsed NERC’s timing recommendation
NERC standard PRC-018-1
• Utilities must maintain a time accuracy
better than 2 ms in disturbance recorders
• Time shall be in Coordinated Universal
Time (UTC) format
NERC … North American Electric Reliability
Corporation

Timing Dependent Applications
Traditional Applications
• TDM communication networks
• Grid frequency management
• Event correlation (network and substation level)
• relays record reason for operation
• analogue trace recorders
• Control centre computers & terminal units (RTU)
• Scheduled load shedding
• Quality of Supply metering
• Energy metering (time of use tariffs)
Advanced Applications
• Lightning strike monitoring
• Travelling wave fault location
• Synchrophasor measurement
• Merging Units/Sample Values

Smart Grid Timing Needs Today!
Application Measurement
Accurac
y
Time Interface Sync Source
TW Fault Locator 300 m (line span) 1 μs PTP, IRIG-B, PPO GPS, 1588 GMC
Phasor Measurements ± 0.1 degree 1 μs PTP, IRIG-B (1344) GPS, 1588 GMC
Lightning Strike Correlation Grid-wide events 1 ms IRIG-B GPS
Protection Relaying events < 1 cycle 1 ms
PTP, IRIG-B
IEC 61850
GPS, IRIG-B, 1588
GMC
Event/Disturbance Recorders < 1 cycle 1 ms PTP, IRIG-B, PPO GPS, 1588 GM
Network, Distribution &
Substation Control
Grid-wide events 1 ms PTP, IRIG-B
GPS, Control Centre,
1588 GMC
Quality of Supply Metering Freq, time error 0.5 sec PTP, IRIG-B, PPO GPS, 1588 GMC
Bulk Metering Energy registers 0.5 sec Proprietary, PPO Proprietary
Customer Premises Metering Energy registers 1 sec NTP, Proprietary Proprietary, NTP
SCADA/EMS/PAS Grid-wide status 1 ms NTP, ASCII GPS
Frequency Measurement Frequency 1 ms N/A GPS
Sampled Values Volt/Current 1 μs PTP 1588 GM
Telecommunication SDH/PDH G.812/813
PTP G.8265
2.048 Mbps/MHz
GPS, 1588 GMC

IEEE 1588 Overview
IEEE 1588-2008 …
• Also referred to as Precision Time Protocol (PTP)
• Grandmaster “reference clock” sends time-stamped messages to IED’s
• Communication is over the Ethernet with traffic
• IED’s (slaves) eliminate round-trip delay & synchronize to the GMC
Accuracy is enhanced by:
• Hardware time-stamping (eliminate software processing delays)
• 1588 capable switches (transparent/boundary clocks)
• Best Master Clock schemes
IED
IEEE 1588
Aware Switch
Grandmaster
(GMC)
61850 LAN
1588 15881588 1588

IEEE 1588 Power Profile
• IEEE PSRC co-ordinates with IEC TC57 WG10 (IEC
61850)
• Tasked with developing IEEE Standard PC37.238 –
“Profile for Use of IEEE 1588 Precision Time Protocol
in Power System Applications”
• Profile characteristics:
• LAN (Layer 2 Ethernet mapping)
• Multicast addressing only
• Peer-to-peer delay measurement
• Switches must be transparent clocks
• Holdover time defined
• http://www.pes-psrc.org/h/ Power Profile
Defined by IEEE PSRC (C37.238)
Substation LAN applications

What Has Changed For Timing
From application perspective
• Accuracy increasing from 1ms to 1μs
From the regulatory perspective
• Timing of recorders is mandatory
Within the substation
• IRIG-B ports won’t be in future IED’s
• Timing is needed in the switch yard
• Sampled values make synchronization a necessity - no
longer an option
• Must know clock quality and status (unmanaged GPS
clocks not sustainable)
• Substations LANs change the way timing is distributed
- IEEE 1588

IEEE 1588 on the IEDs
• IEDs with IEEE 1588 power profile
LAN 1
LAN 2
LAN 3

Interoperability Between Vendors
• Implementation of C37.238 (power profile) grandmasters, switches and slaves in
IED’s
• Inter-operability of C37.238 aware components
• IEEE PSRC working group
standard for conformance &
performance testing
• IEEE 1588 plugfests
IEEE Plugfest, ISPCS Conference
New Hampshire, USA

55
Phasor Measurement Unit
A phasor measurement unit (PMU) is a device which
measures the electrical waves on an electricity grid.

56
What is a Synchrophasor
Time synchronization allows synchronized real-time measurements of multiple
remote measurement points on the grid. The resulting measurement is known
as a synchrophasor.
• A mailbox-sized, monitoring device that can
measure the instantaneous voltage, current
and frequency at specific locations on the grid.
• They give power grid operators a near-real-
time picture of what is happening on the
system.
Gives power grid operators a near-real-time
picture of what is happening on the system.

Synchophasor Applications

Is GPS Safe Enough?
N.J. man fined
$32K for illegal
GPS device that
disrupted Newark
airport system

59
Is GPS Safe Enough?
GPS car
jammers
GPS jammer
5 band
jammer L1,
L2, L3, L4, L5
+ RF lowjack

60
Jammer Hunting with UAV
A fully autonomous,
unmanned aerial
vehicle (UAV)-based
system for locating
GPS jammers, currently
under development,
seeks to localize a
jammer to within 30
meters in less than 15
minutes in an area
comparable to that of
an airport

Protecting The GNSS
Aging/Day Temperature
stability
Operational
temperature
Quartz ± 5e-10 ± 5e-9 -40 to 65C
Quartz
HQ++
± 5e-11 ± 1e-11 -40 to 65C
Rubidium ± 5e-12 ± 2e-10 -40 to 45C
Aging can be estimated with
GNSS and eliminated
Enter
holdover
24 Hours
500
ns
Quartz HQ++ Holdover
• Tested in the oven with
temperature profile +/-20C
• Phase holdover over 24
hours below 500nsec !

Timing Delivery for Substations
… Today and Tomorrow
Event
Recorder
GNSS
Primary
IRIG B
Protection Schemes
ADM
Remote
Terminal
Unit (RTU)
IRIG-B Bus
DS1
G.703
9/13
1PPS
Protection
Relay
PMU
Alarm
Annunciator
Substation
Gateway
QOS
Meter
Substation
Switch
61850 LAN
 PTP
IEEE C37.118
 NTP
RELAY ROOM
PTP
Telecom Profile
G.8265.1
G.8275.1
Mid PTP GM
PTP
Secondary

Thank You
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alleged to have been caused by or in connection with using and/or relying on the information contained in this presentation.
Copyright © for the entire content of this presentation: ADVA Optical Networking.

Abbreviations
3ϕ Three Phase
61850 A standard for the design of an electrical substation (more detail)
AMI Advanced Metering Infrastructure
ADM Add-Drop Multiplexor (SDH/SONET terminal)
AMR Automatic Meter Reading
CES Circuit Emulation Service
CT Current Transformer
DFR Digital Fault Recorder
DNP Distributed Network Protocol
DR Demand Regulation, or
DR Disturbance Recorder
DSM Demand Side Management
EHV Extra High Voltage
EMC Electro-Magnetic Compatibility
EV Electric Vehicle
FERC Federal Energy Regulatory Commission
GMC Grandmaster Clock
GOOSE Global Object Oriented Substation Event
GUI Graphic User Interface
FTM Frequency & Time Deviation Monitor
HV High Voltage
I Current
IP Internet Protocol
IEC International Electrotechnical Commission
IED Intelligent Electronic Device
IEEE Institute of Electrical & Electronic Engineers
IRIG-B Inter-Range Instrumentation Group time-code B
NERC North American Electric Reliability Corporation
NTP Network Time Protocol
OC Ordinary Clock (PTP reference)
PAS Power Application Software
PDH Plesiochronous Digital Hierarchy
PMU Phasor Measurement Unit
PPO Programmable Pulse Output (e.g. 1PPS)
PQ Power Quality
PSN Packet Switched Network
PSRC Power System Relaying Committee
PT Potential Transformer (sometimes called a VT
PTM&C Protection, Telecommunication, Metering/Measurement and Control
PTP Precise Time Protocol
QOS Quality of Supply
RTU Remote Terminal Unit
SCADA Supervisory Control & Data Acquisition
SDH Synchronous Digital Hierarchy
SV’s Sampled Analog Values
TW Travelling Wave
UTC Universal Coordinated Time (world standard)
VT Voltage Transformer (sometimes called a PT)

Evolution of Network Synchronization Technologies

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