Transcript CXR Larus - dB Levels.com

Basic Timing & Synchronization
GPS, NTP and PTP/IEEE 1588
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GPS - Global Positioning System, is a satellite navigation system consisting of 24 satellites have atomic clocks that are accurate to within
a billionth of a second – 1ns.
UTC or Coordinated Universal Time - A high precision atomic time standard that is used as a time reference for many Internet and WWW
applications. Specified in ITU-R TF.460-4.
Accuracy - A measure of how closely the frequency generated by the standard corresponds to its assigned value (e.g., the atomic
transition frequency for an atomic standard). A measurement of a 100-Hz frequency that is accurate to the sixth decimal place is said to
be accurate to 1 part in 108, 0.1 parts per billion, or to have 10−8 accuracy.
Precision - A measure of the repeatability of a frequency measurement. It is generally expressed in terms of a standard deviation of the
measurement.
Stability - A measure of the maximum deviation of the standard’s frequency when operating over a specified parameter range.
Holdover - The mode that a clock enters into when it loses connectivity with an input reference. While in holdover, the clock uses stored
data to control its output and its stability depends on the stability of its internal oscillator.
Jitter - deviation of a time signal from its ideal point in time.
Wander - Wander is a phase variation at slow frequency of DC to 10Hz. It requires wider measurement range than Jitter. (The required
range is at least 1 x 109 ns according to ITU-T Rec. O.172.).
BITS – Building Integrated Timing System – A standard for distributing a precision clock among telecommunications equipment .
TIE – Time Interval Error - The variation in time delay of a given timing signal with respect to an ideal timing signal over a particular
time period.
TDEV - a measure of how much the phase (in time units) of a clock could change over an interval of duration T assuming that any
systemmatic (i.e. constant) frequency offset has been removed.
MTIE – Maximum Time Interval Error – A measure of the worst case phase variation of a signal with respect to a perfect signal over a
given period of time.
PDV – Packet Delay Variation - The variation in the amount of Latency among Packets being received, has an impact on jitter and wander
for Pseudowire implementations.
ACR – Adaptive Clock Recovery – method of recovering frequency from the arrival rate of packets, not recommended in heavily loaded or
best effort networks.
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Confidential
Reference
Isochronous - same frequency, out of phase
A
Asynchronous – out of frequency, out of phase
B
Synchronous – same frequency, same phase
C
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A reference timing source
provides a precise clock
that is used for framing
and timeslot inference in
network elements
Received
Signal
Timeslots
RX
F4
Data
F3
Data
F2
Data
F1
Data
TX
Imperfect timing can
cause buffer underflow
and overflow conditions
leading to frame slips
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VCXO
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Relationship between frequency and phase:
ω=dФ/dt
Frequency is the slope in the phase plot
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TDEV(t) is the rms of filtered TIE, where the
bandpass filter (BPF) is centred on a
frequency of 0.42/t.
TIE
BPF
H(f)
RMS
TDEV
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Both MTIE and TDEV are measures of wander
over ranges of values from very short-term
wander to long-term wander
MTIE is a peak detector: shows largest phase
swings for various observation time windows
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TDEV is a highly averaged, “rms” type of
calculation showing values over a range of
integration times
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Stratum 1
S1
S2
S3
S4
S1
S2
S3
S2
S3
S4
S4
Stratum 2
Receive sync signals from multiple sources,
good holdover capability
Frequency accuracy: ± 16 ppb
Used in Central Offices
Stratum 3
Receive sync signals from multiple sources,
reasonable holdover capability
Frequency accuracy: ± 4.6 ppm
Used in local offices
Stratum 4
Receive sync signals from multiple sources,
Tolerable holdover capability for CPE applications
Frequency accuracy: ± 32 ppm
Used in CPEs, Set-top boxes, etc.
S2
S3
S3
S4
Most accurate clock sources in the network
Frequency accuracy: ±0.01 ppb to UTC
Used in Network Gateways
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PRC
G.812
Type I
G.813
Number of
G.813 clocks  20
Number of
G.812 type I clocks  10
G.813
G.812
Type I
G.813
Total number of G.813
clocks in a synchronization
trail should not exceed 60
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SDH
SONET
PRC
PRS
Primacy Reference
Clock
Primacy Reference
Source
SSU
BITS
Synchronization
Supply Unit
Building Integrated
Timing Source
SEC
SMC
SDH Equipment
Clock
SONET Minimum
Clock
Decreasing Accuracy
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MAN
SEC
Local Exchange
Ring STM-1/4
MAN
MAN
 Digital switching equipment
must be synchronized to
avoid slips
SEC
MAN
 Slips have a major impact
on circuit-switched services
SSU
ADM
 SONET and SDH
technologies of the 1990s
put stringent requirements
on network synchronization
ADM
SSU
ADM
Transit Ring
STM-4/16
SSU
ADM
PRC
Backbone Ring
STM-16
PRC
f
 Network synchronization plays an
important role in next generation
packet switched networks too
T1/E1
IWF
Remote Terminal
TDM to Packet
IP Network
IWF
T1/E1
Central Office
Packet to TDM
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 Predominantly IP-based
 Access networks owned by different
service providers
 Networks provide transit as well as
access
 Timing is required at points where
legacy networks meet IP networks
and for QoS assurance across the
entire network
Wireless Service Provider’s
IP Network
Synchronization required
IP Transit Network
Broadband Wireline
Service Provider’s
IP Network
PSTN Service Provider’s
IP Network
Local
Exchange
Reproduced from ATIS NGN Framework
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WEB Services
Parlay/OSA
Service
Profiles
Content
Distribution
SCP
Session Control
Messaging
Dynamic Service Data
(Presence, Location)
WiMAX
Existing PSTN
DSL
Access CPE
Management
AAA
SIP Apps
Cable
Core IP Centric Network
Existing Mobile
UMTS
Edge
Existing Internet
GigE
Access
NGN Core v1.1, Reproduced from ATIS
Network
Resources
Broadband
Interconnect
Existing Internet
Time synchronization required
Frequency synchronization required
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Source: Cisco
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Distributed database transaction journaling and logging (Time-of-day)
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Stock market buy and sell orders (Time-of-day)
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Secure document timestamps (with cryptographic certification) (Time-of-day)
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Aviation traffic control and position reporting (Time-of-day)
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Radio and TV programming launch and monitoring (Time-of-day, frequency)
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Intruder detection, location and reporting (Time-of-day)
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Multimedia synchronization for real-time teleconferencing (Time-of-day, frequency)
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Network monitoring, measurement and control time (Frequency)
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Early detection of failing network infrastructure devices (Time-of-day, frequency)
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Differentiated services traffic engineering (Time-of-day, frequency)
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Distributed network gaming and training (Time-of-day)
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Real-Time Applications
Mobile Sync Requirements
Wireless System
Frequency
Synchronization
Phase (Time)
Synchronization
APPLICATION
SYNCHRONIZATION
REQUIREMENT
UMTS
+/- 50 ppb
Not required
Voice
< 32 ppm (part per million)
CDMA2000 (US,
Asia, 3GPP2)
+/- 50 ppb
+/- 3 µs
(+/- 10 µs worst case)
Ethernet Best Effort
< 100 ppm (part per million)
WCDMA (3GPP,
Europe, Asia)
and GSM
+/- 50 ppb
+/- 1.25 µs between
Reference and BTS; +/- 2.5
µs between basestations
Two-way Video
< 50 ppb (part per billion)
One-way Video MPEG
< 500 ppb (part per billion)
Pico RBS
(WCDMA and
GSM)
+/- 100 ppb
+/- 3µs
One-way Video HDTV
< 100 ppb (part per billion)
Femtocells
+/- 100 ppb
+/- 3 µs
(+/- 10 µs worst case)
One-way Video IPTV
< 100 ppb (part per billion)
Mobile WiMAX
+/- 50 ppb
+/- 2.5 µs down to +/- 1.0
µs for some WiMAX
profiles
Source: Vodafone & others
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Attribute
PTP
NTP
Accuracy
Sub-microsecond accuracy.
Nanosecond accuracy with good
oscillator
Millisecond accuracy. Brilliant achieves
sub-microsecond accuracy using
hardware implementation
Network
topology
Version-1 suitable for LANs only.
Version-2 is under development for
WANs
Has been designed for use in public
networks and can be used across
WANs
Synchroniza
tion
mechanism
Single Grandmaster “pushes” time to
one or more slaves in a multicast
mode
NTP client regularly polls one or more
NTP servers
Redundanc
y
Version 2 supports multiple clock
sources running a best-master
selection algorithm
Built-in redundancy through multiple
clock sources (NTP servers)
Security
Hash codes and improved clock
selection mechanism in v2 prevents
security risks
Cryptographic security mechanism
Applications
Military and aerospace, industrial
automation (synchronization of CNC
systems, sensors, actuators, etc.),
telecommunications (synchronization
of base stations), home networking
(standard for Audio-video-bridging)
Enterprise IT applications,
synchronization of computers in the
home network, IPTV related
applications (DRM), generic timestamping applications in a variety of
industries
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ITSF - International Telecom Sync Forum
ITU-T - International Telecommunication Union
ANSI - American National Standards Institute
ATIS - Alliance for Telecommunications Industry Solutions
IEEE - Institute of Electrical and Electronics Engineers
Bellcore/Telcordia - http://www.telcordia.com/
NIST - National Institute of Standards and Technology
IETF - Internet Engineering Task Force
TicToc BOF – timing and frequency distribution over IP BOF
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ITU-T G.811: Timing Characteristics of Primary Reference Clocks
ITU-T G.812: Timing requirements of slave clocks suitable for use as node clocks in
synchronization networks
ITU-T G.813: Timing characteristics of SDH equipment slave clocks (SEC)
ITU-T G.823: The control of jitter and wander within digital networks which are based on the 2048
kbit/s hierarchy (i.e. E1)
ITU-T G.824: The control of jitter and wander within digital networks which are based on the 1544
kbit/s hierarchy (i.e. T1)
Draft ITU-T Recommendation G.8261/Y.1361 - Timing and synchronization aspects in packet
networks – formally G.pactiming
GR-1244-CORE, Clocks for the Synchronized Network: Common Generic Criteria Generic
Requirements
GR-378-CORE, Building Integrated Timing Systems
GR-378-CORE, Timing Signal Generator Generic Requirements (supersedes above)
GR-436-core: Digital Network Synchronization Plan
GR-499-core: Transport Systems Generic Requirements (TSGR): Common Requirements
GR-253-CORE, SONET Transport Systems: Common Generic Requirements
GR-2830-CORE: Primary Reference Sources: Generic Criteria
ANSI T1.101-1999: Synchronization Interface Standard
DTI: DOCSIS Timing Interface Specification
PTPv1 – 2002: uSec accurate timestamps and distribution
PTPv2 – 2008???: sub nSec accurate, correction (offsets) for asymmetric topologies, redundancy, etc.
NTP - Network Time Protocol: (NTPv3, RFC 1305, Obsoletes: RFC-1119, RFC-1059, RFC-958),
(SNPTv4, RFC 2030, Obsoletes RFC 1769)
ITU-R TF.460-4: STANDARD-FREQUENCY AND TIME-SIGNAL EMISSIONS
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ITU-T Recommendation G.803 (2000), Architecture of transport networks based on the
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ITU-T Recommendation G.810 (1996), Definitions and terminology for synchronization
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ITU-T Recommendation G.811 (1997), Timing characteristics of primary reference
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ITU-T Recommendation G.812 (1998), Timing requirements of slave clocks suitable for
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ITU-T Recommendation G.813 (1996), Timing characteristics of SDH equipment slave
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ITU-T Recommendation G.823 (2000), The control of jitter and wander within digital
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ITU-T Recommendation G.824 (2000), The control of jitter and wander within digital
synchronous digital hierarchy (SDH).
networks.
clocks.
use as node clocks in synchronization networks.
clocks (SEC).
networks which are based on the 2048 kbit/s hierarchy.
networks which are based on the 1544 kbit/s hierarchy.
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ANSI T1-101: Synchronization Interface Standard
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Bellcore GR-253-core: Synchronous Optical Network (SONET)
Transport Systems: Common Generic Criteria
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Bellcore GR-1244-core: Clocks for the Synchronized Network:
Common Generic Criteria
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Bellcore GR-436-core: Digital Network Synchronization Plan
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Bellcore GR-378-core: Generic Requirements for Timing Signal
Generators
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Bellcore GR-499-core: Transport Systems Generic Requirements
(TSGR): Common Requirements
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1985
1988
1989
1992
2006
NTPv0
RFC958
NTPv1
RFC1059
NTPv2
RFC1119
NTPv3
RFC1305
NTPv4
Work In
progress
Evolution from Time
Protocol and ICMP
Timestamp message
Specification of protocol,
algorithms state variables
and operational modes
Management of clients,
authentication based on
64-bit DES
Sanity checks for lost or
corrupted packets, clock
algorithm improved, new
peering algorithm
2002
2007
1588v1
1588v2
Initial release for
Industrial Automation,
T&M
Enhanced for telecom
applications,
nanosecond accuracy
IETF
IEEE
Feb 2008
Network modeling
G.
pacmod
Improved algorithm,
security enhancements
Feb 2008
Synchronous
Ethernet
G.8262
Apr 2004
ITU-T
Question 13
Oct 2003
Study Group 15
G.
pactiming
Timing and
Synchronization
aspects of Packet
Networks
G.
2008
Paclock. Profile for telecom
bis
G.
paclock
Sep 2004
G.8261
Y.1361
Network reference
model for timing
over IP networks
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Network Synchronous Operation: network-synchronous operation by using a PRS/PRC traceable network derived clock or a
local PRS/PRC as the service clock. In effect, the TDM signal is “retimed”. The clock accuracy of ingress TDM clock (clk1) must
be PRS/PRC traceable, otherwise the use of a network clock reference in the egress IWF (i.e. clk3) will cause jitter buffer
overflow/underflow events in the egress IWF.
Differential Clock Recovery: The principle of operation of any differential method is based on the availability of “equal” clock
references at the ingress and egress IWFs. The difference between the service clock and the reference clock is encoded and
transmitted across the packet network.The service clock is recovered on the far end of the packet network making use of the
“equal” reference clock. Synchronous Residual Time Stamp (SRTS) is an example of this family of methods. Differential
methods can support the plesiochronous circuit timing (also known as asynchronous circuit timing) mode whereby the TDM
service clock can have an offset from PRS/PRC provided it is within defined limits. Correct timing in the output TDM signal
implies that the clocks generating the TDM signal (clk1) and retiming (clk4) the TDM signal must have the same long term
frequency (or within the PRS/PRC limits) otherwise jitter buffer overflow/underflow events will be generated in the egress IWF
and the destination TDM NE may experience slips. It is easy to show that wander (and frequency inaccuracy) in the egress
TDM signal (clk4) is directly related to the relative wander between the reference clocks clk2 and clk3. Figure 5 shows that the
references come from two distinct PRS/PRC units though obviously they could be the same. If the synchronization trail
between clk2 and the PRS/PRC and that of clk3 and the PRS/PRC has a “common” node, that node could be in holdover
without adversely impacting the differential mode of operation.
Adaptive Clock Recovery: In Adaptive Clock Recovery (ACR) methods, timing is recovered based on the inte-rarrival time of
the packets or on the fill level of the jitter buffer. Adaptive methods can support the plesiochronous circuit timing mode
whereby the TDM service clock can have an offset from PRS/PRC provided it is within defined limits. If the transit time across
the packet network of the packets varies, also known as packet delay variation (PDV) or time-delay variation (TDV), the clock
recovery process is affected. In particular, PDV, on a short-term basis, is indistinguishable from a change in the
phase/frequency of the service clock and/or the local oscillator. Consequently ACR implementations require high quality
oscillators and apply filtering corresponding to bandwidths of the order of milli-hertz (mHz) (time constants of the order of
1,000s). However, if a network clock reference is not available, then ACR is the only available method for service clock
recovery. There are several causes of delay variation including the following that are covered in G.8261:
◦ • Random delay variation (e.g. queuing delays)
◦ • Low frequency delay variations (e.g. day/night traffic patterns)
◦ • Systematic delay variation (e.g. transmission window)
◦ • Routing changes (e.g. network re-configuration)
◦ • Congestion effects (e.g. network overload)
Since the performance of adaptive clock recovery is very dependent upon PDV, it is recommended for use only when the PDV
can be tightly controlled.
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Point-to-point distribution of timing signals in Ethernet
environments
Synchronize the Ethernet physical layer as currently done in
SONET/SDH
Packetize Synchronization Status Messaging protocol (SSMoETH)
Bring carrier-grade telecom-quality clocks to Ethernet switches
Maintain SONET/SDH network synchronization principles &
guidelines
Implementation conformant with IEEE 802.3 specification
High-level definition part of ITU-T G.8261 clause 8.1.1
Specification to be established within ITU-T G.pacmod &
G.paclock
Point to point – i.e. all network elements must support in order to
be effective frequency distribution mechanism
Frequency only, no sense of phase or Time of Day (ToD)
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Figure 15/G.8261 - IWF synchronization functions (Packet to TDM direction)
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Frequency transfer
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Network Synchronous
Differential Clock Recovery
Adaptive Clock Recovery
Synchronous Ethernet
Time transfer
 NTP – Network Time Protocol v4
 IEEE 1588v2 (also known as Precision Time Protocol, PTP)
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Figure 6/G.8261 – Example of Network Synchronous Operation
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PRC Traceable network clock is used as a service clock
Implies that PRC traceable clock is available at both ends
Reference signals at IWF must comply with G.823 and G.824
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f
T1/E1
IWF
Remote Terminal
TDM to Packet
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•
IP Network
IWF
MSC
Packet to TDM
T1/E1
IWF: Interworking Function
Synchronization path is broken
• Traditional synchronization techniques are not available
IP Networks introduce complexities to data traffic flow
• Different upstream and downstream paths
• Time varying delays
• Asymmetric delays
•
Synchronization must include both frequency and phase:
• Frequency only
• Synchronous Ethernet
• Adaptive Clock Recovery (ACR)
• Frequency and Phase
• GPS-based timing that provides T1 retiming capabilities
• Differential clock recovery – NTP & IEEE 1588 (PTP)
NextGen timing and sync distribution methods
must include time and phase in addition to frequency
Figure 8/G.8261 – Example of Adaptive Method
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Timing is recovered based on the inter-arrival time of the packets or on the
fill level of the jitter buffer
Service clock is preserved
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Proprietary – non standard, requires “bookend” approach
Frequency only – no sense of phase
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Unpredictable wander, can’t be filtered out
No measurement of differential delay, handoffs a challenge
Recovery from network perturbations (e.g. fiber cut, re-route, traffic loading) also a
challenge
No way to prevent instantaneous phase jumps
Jitter buffer adds to latency – lowers user QOE
Buffer overruns and underruns cause packet and data loss
Point to point – multiplexing/aggregation a challenge
No Global reference for time/sync – no visibility of timing loops and islands
Additional PWE on network degrades sync performance
Even with PWE as highest priority traffic, it is self-interfering
PWE packets tend to “clump”
Better local oscillators actually exacerbate the problem – the tighter the freq control,
the more the wander:

Amp <= Ts * BWpwe / BWlink
Figure 7/G.8261 – Example of Differential Method
•
•
•
•
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•
•
Distributes Global time. phase and frequency based on Primary Clock Reference
Preserves service clock – frequency and phase difference from Global reference
Hierarchical distribution from global reference prevents timing loops
Synchronization Status Messaging (SSM) – manageability and traceability
Scalable, supports point-to-point, point-to-multipoint, multipoint and broadcast services.
Signals at IWF comply with ITU G.823 and G.824 sync requirements
Standards-based – IEEE1588v2, NTP
Differential Clock Recovery distributes
phase and absolute time, not just frequency
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Global clock reference – GPS based
◦ Understanding of time, freq and phase
◦ No tendency of timing packets to “clump” – can be staggered to
ensure no impact to wander characteristics
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Timing updates are negligible BW, not traffic
dependent
Standards compliant – IEEE 1588, NTP
◦ Internal algorithms are proprietary, but multiple vendors’
servers and clients are interoperable, unlike ACR and PWE
equipment
◦ Brilliant servers are more accurate than competitors, leading to
better timing and sync at the edge
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Better time, sync and phase mean better QOE
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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 cesium oscillator or GPS receiver
Current implementations are primarily software-based. Non-deterministic
delays in networking stacks contribute to significant timing inaccuracy

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 cesium oscillator or GPS receiver
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Current implementations are primarily software-based. Non-deterministic delays
in networking stacks contribute to significant timing inaccuracy
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Stratum 1
S1
S1
Stratum 2
S2
S2
S2
S2
Stratum 3
S3
S3
S3
S3
S3
Stratum 4
S4
S4
S4
S4
 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 ITU-T Stratum levels!
Next Generation Network Services require ITU-T Stratum level synchronization
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Client
Client sends
T1
request at
T1 = 10:15:00
Server
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Server receives
T2 request at
T2 = 10:15:12
Clock offset:
◦ [(T2 – T1) + (T4 – T3)] / 2

Round-trip delay:
◦ (T4 – T1) – (T3 – T2)
Server sends
Client receives
response at T4
T2 = 10:15:30
T3 response at
T2 = 10:15:15
» 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
Client
Client sends
T1
request at
T1 = 10:15:00
Server
Clock offset:
Server receives
T2 request at
T2 = 10:15:12
Server sends
Client receives
response at T4
T2 = 10:15:19
T3 response at
T2 = 10:15:15
◦ [(T2 – T1) + (T4 – T3)] / 2
◦ (2 + 4) / 2 = 3 seconds
Round-trip delay:
◦ (T4 – T1) – (T3 – T2)
◦ 19 – 3 = 16 seconds
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
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IEEE1588 Overview

IEEE 1588 (commonly known as Precision Time Protocol, PTP)
was ratified as a standard in September 2002

Provides timing for the control of distributed applications

Version 1 of the protocol used for applications in
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Industrial automation
Test and measurement
Electric power
Military
Residential (Audio-Video Bridging)
Version 2 developed for telecom applications
◦ Early adopters include Vodafone, T-Mobile, etc.
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•
IEEE 1588v2 meets accuracy requirements for Telecom applications
• High refresh rates up to 64 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
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Slave Clock Time
Master Clock Time
Data at
Slave Clock
t1
Sync Message
t2m
t2
t2
Followup Message
containing value of t1

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:
t1 t2
t3m
t3
Delay Request
Message
t1 t2 t3
t2 –t1 = o + d
t4 - t3 = -o + d
t4
o = (t2 + t3 – t1 – t4) / 2
Delay Response Message
containing value of t4
t1 t2 t3 t4
Time
d = (t2 – t1 + t4 – t3) / 2
Master
Sent at 1001 s
Received at 1015 s
PTP
1001
SYNC
PTP
UDP
1001
SYNC
UDP
IP
1001
SYNC
IP
MAC
1001
SYNC
MAC
MII
MII
PHY
PHY
Tm = 1000s
Ts = 1010s
1003
Master sends ‘SYNC’ message at 1001
seconds. Timestamp in packet shows 1001,
but the packet is actually sent out at 1003
seconds. Slave clock is not synchronized.
Slave receives the packet at 1015 seconds
local time
Slave
1015
1001: SYNC
1015: SYNC
46
Master
Sent at 1004 s
Received at 1018 s
Slave
PTP
1003
FOLLOW UP
PTP
UDP
1003
FOLLOW UP
UDP
IP
1003
FOLLOW UP
IP
MAC
1003
FOLLOW UP
MAC
MII
MII
PHY
PHY
Master sends ‘SYNC’ message at 1001 seconds.
Timestamp in packet shows 1001, but the packet is
actually sent out at 1003 seconds. Slave clock is not
synchronized. Slave receives the packet at 1015
seconds local time
Master sends ‘FOLLOW UP’ message with actual time
of transmission of the SYNC message, i.e. 1003
seconds
1018
1001: SYNC
1015: SYNC
1003: FOLLOW UP
1018: FOLLOW UP
Offset = 1015 – 1003 – Unknown line delay
= 12 – Unknown line delay
Adjusted slave time = 1018 – 12 – Unknown line delay
= 1006 – Unknown line delay
47
Master
Received at 1013 s
Sent at 1009 s
Slave
PTP
1009
DELAY REQ
PTP
UDP
1009
DELAY REQ
UDP
IP
1009
DELAY REQ
IP
MAC
1009
DELAY REQ
Master sends ‘FOLLOW UP’ message with actual time
of transmission of the SYNC message, i.e. 1003
seconds
Slave tries to determine unknown line delay by sending
a ‘DELAY REQ’ message to the master
MAC
MII
MII
PHY
PHY
1012
Master sends ‘SYNC’ message at 1001 seconds.
Timestamp in packet shows 1001, but the packet is
actually sent out at 1003 seconds. Slave clock is not
synchronized. Slave receives the packet at 1015
seconds local time
1010
1001: SYNC
1015: SYNC
1003: FOLLOW UP
1018: FOLLOW UP
Offset = 1015 – 1003 – Unknown line delay
= 12 – Unknown line delay
Adjusted slave time = 1018 – 12 – Unknown line delay
= 1006 – Unknown line delay
1010: DELAY REQ
1012: DELAY REQ
48
Master
Sent at 1014 s
Received at 1018 s
Slave
PTP
1012 DELAY RESP
PTP
UDP
1012
UDP
IP
MAC
DELAY RESP
1012 DELAY RESP
1012 DELAY RESP
IP
MAC
MII
MII
PHY
PHY
Master sends ‘SYNC’ message at 1001 seconds.
Timestamp in packet shows 1001, but the packet is
actually sent out at 1003 seconds. Slave clock is not
synchronized. Slave receives the packet at 1015
seconds local time
Master sends ‘FOLLOW UP’ message with actual time
of transmission of the SYNC message, i.e. 1003
seconds
Slave tries to determine unknown line delay by sending
a ‘DELAY REQ’ message to the master
Master responds with ‘DELAY RESP’ message
containing timestamp when ‘DELAY REQ’ was
received
Slave calculates average delay by assuming symmetric
path and dividing total delay by 2
1001: SYNC
1015: SYNC
1003: FOLLOW UP
1018: FOLLOW UP
Offset = 1015 – 1003 – Unknown line delay
= 12 – Unknown line delay
Adjusted slave time = 1018 – 12 – Unknown line delay
= 1006 – Unknown line delay
1009: DELAY REQ
Line delay = ((1012 – 1009) + (1006 – 1003)) / 2
= 3 seconds
1013: DELAY REQ
1012: DELAY RESP
1015: DELAY RESP
Slave time = 1015 – 3 = 1012 seconds
49
Boundary Clock
S
M
M
S
Grandmaster
Grandmaster
S
M
IP Network
Slave
Boundary Clock
Boundary Clock
Slave
A Boundary Clock extends
synchronization across an
intermediate network element
Slave
 A boundary clock contains more
than one PTP port:
Master
PTP
PTP
PTP
PTP
UDP
UDP
UDP
UDP
IP
IP
IP
IP
MAC
MAC
MAC
MAC
MII
MII
MII
MII
PHY
PHY
PHY
PHY
 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
50
Transparent Clock
S
M
IP Network
Grandmaster
Grandmaster
Slave
Trasparent Clock
Transparent Clock
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
Slave
 A Transparent Clock contains
no PTP ports.
PTP
PTP
UDP
UDP
IP
IP
MAC
MAC
MAC
MAC
MII
MII
MII
MII
PHY
PHY
PHY
PHY
 Timestamp in incoming
message is modified
before sending the
message out
 Creates security issues,
since original crypto
checksum is not valid
anymore
51
Boundary
Clock
M
S
M
S
M
S
Endpoint
M
Time Offset
Grandmaster
Boundary
Clock
• Point-to-point
synchronization
• Cascading of error
offsets
Hops
M
Transparent
Clock
Transparent
Clock
S
Time Offset
Grandmaster
• End-to-end
synchronization
• Corrects only
residence time
• Causes less jitter
in a highly
cascaded network
Hops
52
Attribute
PTP
NTP
Accuracy
Sub-microsecond accuracy. Nanosecond
accuracy with good oscillator
Millisecond accuracy. Brilliant achieves submicrosecond accuracy using hardware
implementation
Network topology
Version-1 suitable for LANs only. Version-2 is
under development for WANs
Has been designed for use in public networks
and can be used across WANs
Synchronization
mechanism
Single Grandmaster “pushes” time to one or
more slaves in a multicast mode
NTP client regularly polls one or more NTP
servers
Redundancy
Version 2 supports multiple clock sources
running a best-master selection algorithm
Built-in redundancy through multiple clock
sources (NTP servers)
Security
Hash codes and improved clock selection
mechanism in v2 prevents security risks
Cryptographic security mechanism
Applications
Military and aerospace, industrial automation
(synchronization of CNC systems, sensors,
actuators, etc.), telecommunications
(synchronization of base stations), home
networking (standard for Audio-videobridging)
Enterprise IT applications, synchronization of
computers in the home network, IPTV related
applications (DRM), generic time-stamping
applications in a variety of industries
53
GPS receiver at every node
 Deliver frequency and time (up to 50ns accuracy claimed)
 Not always viable (indoor cells)
 Expensive oscillators required ($$$) for periods of unavailability (not 99.999% solution)
Network synchronous
Sync Ethernet
 Use the PHY clock from bit stream (similar to SDH/PDH), each node recovers clock
 Only deliver frequency and not phase
 Independent from network load
 Represent an excellent SDH/PDH replacement option -> viable ‘interim’ solution
Packet –based
In-band synchronization
(adaptive clock recovery)
 The clock is reconstructed using the packet inter-arrival rate (i.e. leaky bucket algorithm)
 Inexpensive solution
 Subjected to network load conditions, not ‘always-on’ and deliver frequency (not phase)
 Could represent a viable ‘interim’ solution only in certain scenarios
Packet –based
Out-of-band
synchronization
 Clock information is transmitted via dedicated timing packets (master <-> slave)
 ‘Always-on’ solution (even without traffic data)
 Ubiquitous solution (works over any transport technology)
 Can deliver frequency and phase (FDD and TDD systems)
 Major protocols: IEEE 1588v2, IETF NTP version 4
IEEE 1588v2 represents the most promising ‘long-term’ solution
(in conjunction with Sync Eth)
54
Enterprise VAP
Micro and Pico cells
IP<->TDM
IWF
2G
TDM
3G R99
BTS
ATM
TDM/ATM<->IP
IWF
PSN Backhaul
Node B
MGWs
Servers
BSC
IP/Ethernet over fibre, MW,
leased lines, etc.
3G R5+,B3G
Internet
STM-1
TDM
CPN
STM-1
ATM
E-NodeB
RNC
IP<->ATM IWF
GRX
(IP/MPLS)
SGSNs
Ethernet (IP)
GGSNs
DSLAM
3G R5+, B3G
E-NodeB
Residential VAP
S-GW/MME
ASN GW
DSL
TDM/ATM<->IP
IWF
IP link
IEEE 1588 sync packets
2G
BTS
3G R99
IEEE 1588v2
Grand Master
Need
Primary reference clock
PRC
IEEE 1588v2 Grandmaster
Node B
Typical Network
IEEE 1588v2 Client
IEEE 1588 slave board
IEEE 1588 slave chip
IEEE 1588v2 Differential Clock Recovery (DCR)
drives Typical NGN network
VPN
Corporate
dB Levels, Inc.
Dallas, TX, USA
www.dblevels.com
Telecom Measurement Consultants
*Some material in this tutorial courtesy CXR Larus, Inc.
www.cxrlarus.com
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