dB Levels, Inc.
Basic Timing & Synchronization
GPS, NTP and PTP/IEEE 1588
1
IEEE 1588 Tutorial
2
Mini Glossary 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.
Confidential
Timing & Synchronization 101
4
Synchronization SchemesReference
A
C
Isochronous - same frequency, out of phase
Asynchronous – out of frequency, out of phase
Synchronous – same frequency, same phase
B
5
How is timing used in network equipment?
ReceivedSignal
Timeslots
A reference timing sourceprovides a precise clock that is used for framingand timeslot inference innetwork elements
F4 Data F3 Data F2 Data F1 DataRX TX
Imperfect timing can cause buffer underflow and overflow conditions leading to frame slips
7
Time Interval Error (TIE)
8
Frequency and PhaseRelationship between frequency and phase:
ω=dФ/dtFrequency is the slope in the phase plot
9
Analysis from phase: MTIE
10
Analysis from phase: TDEV
TDEV(t) is the rms of filtered TIE, where the bandpass filter (BPF) is centred on a frequency of 0.42/t.
BPFH(f)
RMSTIE TDEV
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Synchronization Analysis (MTIE)
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
12
Synchronization Analysis (TDEV)
TDEV is a highly averaged, “rms” type of calculation showing values over a range of integration times
13
Synchronization Hierarchy in North America
S1 S1
S2 S2 S2 S2
S3 S3 S3 S3 S3
S4 S4 S4 S4
Stratum 1
Stratum 2
Stratum 3
Stratum 4
Most accurate clock sources in the networkFrequency accuracy: ±0.01 ppb to UTC
Used in Network Gateways
Receive sync signals from multiple sources, good holdover capability
Frequency accuracy: ± 16 ppbUsed in Central Offices
Receive sync signals from multiple sources, reasonable holdover capability Frequency accuracy: ± 4.6 ppm
Used in local offices
Receive sync signals from multiple sources, Tolerable holdover capability for CPE applications
Frequency accuracy: ± 32 ppmUsed in CPEs, Set-top boxes, etc.
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ITU-T Sync Reference ChainPRC
G.813
Number ofG.813 clocks £ 20
Number ofG.812 type I clocks £ 10
Total number of G.813 clocks in a synchronization trail should not exceed 60
G.812Type I
G.813
G.813
G.812Type I
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Clock Types
PRCPRC PRSPRS
SSUSSU BITSBITS
SECSEC SMCSMC
Primacy Reference Clock
Primacy Reference Source
SynchronizationSupply Unit
SDH EquipmentClock
Building IntegratedTiming Source
SONET MinimumClock
SDH SONET
Decreasing Accuracy
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Timing is critical
PRC
SSU
SEC
SSU
SSU
SEC
Backbone RingSTM-16
Transit RingSTM-4/16
Local ExchangeRing STM-1/4
MANMAN
MAN
MAN
ADM
ADM
ADM
ADM
PRC
IWF
Remote TerminalTDM to Packet
IWF
Central OfficePacket to TDM
IP Network
f
T1/E1T1/E1
Digital switching equipment must be synchronized to avoid slips
Slips have a major impact on circuit-switched services
SONET and SDH technologies of the 1990s put stringent requirements on network synchronization
Network synchronization plays an important role in next generation packet switched networks too
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The Next Generation Network
Reproduced from ATIS NGN Framework
Wireless Service Provider’sIP Network
IP Transit Network
Broadband Wireline Service Provider’s
IP NetworkPSTN Service Provider’s
IP Network
Local Exchange
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
Synchronization required
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NGN applications rely on time and frequency synchronization
Core IP Centric Network
Existing PSTN
Existing Mobile
Existing Internet
Existing Internet
Dynamic Service Data(Presence, Location)
Dynamic Service Data(Presence, Location)
Content Distribution
Content Distribution
WEB ServicesWEB Services
MessagingAAA
ServiceProfiles
SIP AppsSIP Apps SCPSCP
WiMAXWiMAX
Parlay/OSAParlay/OSA
Session ControlSession Control
DSLDSL
CableCable
UMTSUMTS
EdgeEdge
GigEGigE
BroadbandInterconnect
BroadbandInterconnectNetwork
Resources
NetworkResources
Access
Acc
ess
CP
EA
cces
s C
PE
M a
n a
g e
m e
n t
NGN Core v1.1, Reproduced from ATIS
Time synchronization required
Frequency synchronization required
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Service-specific Synchronization Requirements
Source: Cisco
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Examples of applications that need precise time and frequency
Distributed database transaction journaling and logging (Time-of-day)
Stock market buy and sell orders (Time-of-day)
Secure document timestamps (with cryptographic certification) (Time-of-day)
Aviation traffic control and position reporting (Time-of-day)
Radio and TV programming launch and monitoring (Time-of-day, frequency)
Intruder detection, location and reporting (Time-of-day)
Multimedia synchronization for real-time teleconferencing (Time-of-day, frequency)
Network monitoring, measurement and control time (Frequency)
Early detection of failing network infrastructure devices (Time-of-day, frequency)
Differentiated services traffic engineering (Time-of-day, frequency)
Distributed network gaming and training (Time-of-day)
Next Generation Sync Requirements
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< 100 ppb (part per billion)One-way Video IPTV
< 100 ppb (part per billion)One-way Video HDTV
< 500 ppb (part per billion)One-way Video MPEG
< 50 ppb (part per billion)Two-way Video
< 100 ppm (part per million)Ethernet Best Effort
< 32 ppm (part per million)Voice
SYNCHRONIZATION REQUIREMENTAPPLICATION
Real-Time Applications
Wireless System Frequency Synchronization
Phase (Time) Synchronization
UMTS +/- 50 ppb Not required
CDMA2000 (US, Asia, 3GPP2)
+/- 50 ppb +/- 3 µs (+/- 10 µs worst case)
WCDMA (3GPP, Europe, Asia) and GSM
+/- 50 ppb +/- 1.25 µs between Reference and BTS; +/- 2.5 µs between basestations
Pico RBS (WCDMA and GSM)
+/- 100 ppb +/- 3µs
Femtocells +/- 100 ppb +/- 3 µs (+/- 10 µs worst case)
Mobile WiMAX +/- 50 ppb +/- 2.5 µs down to +/- 1.0 µs for some WiMAX profiles
Mobile Sync Requirements
Source: Vodafone & others
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How does PTP compare to NTP?
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
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-video-bridging)
Enterprise IT applications, synchronization of computers in the home network, IPTV related applications (DRM), generic time-stamping applications in a variety of industries
Standards Bodies 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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Applicable Standards◦ 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 Synchronization Standards
ITU-T Recommendation G.803 (2000), Architecture of transport networks based on the synchronous digital hierarchy (SDH).
ITU-T Recommendation G.810 (1996), Definitions and terminology for synchronization networks.
ITU-T Recommendation G.811 (1997), Timing characteristics of primary reference clocks.
ITU-T Recommendation G.812 (1998), Timing requirements of slave clocks suitable for use as node clocks in synchronization networks.
ITU-T Recommendation G.813 (1996), Timing characteristics of SDH equipment slave clocks (SEC).
ITU-T Recommendation G.823 (2000), The control of jitter and wander within digital networks which are based on the 2048 kbit/s hierarchy.
ITU-T Recommendation G.824 (2000), The control of jitter and wander within digital networks which are based on the 1544 kbit/s hierarchy.
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North American Synchronization Standards ANSI T1-101: Synchronization Interface Standard
Bellcore GR-253-core: Synchronous Optical Network (SONET) Transport Systems: Common Generic Criteria
Bellcore GR-1244-core: Clocks for the Synchronized Network: Common Generic Criteria
Bellcore GR-436-core: Digital Network Synchronization Plan
Bellcore GR-378-core: Generic Requirements for Timing Signal Generators
Bellcore GR-499-core: Transport Systems Generic Requirements (TSGR): Common Requirements
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NGN Timing StandardsIETF NTPv0
RFC958NTPv1
RFC1059NTPv2
RFC1119NTPv3
RFC1305
NTPv4Work In progress
IEEE 1588v1 1588v2
ITU-T
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
Improved algorithm, security enhancements
Initial release for Industrial Automation,
T&M
Enhanced for telecom applications,
nanosecond accuracy
G.pactiming
G.paclock
G.8261Y.1361
G.pacmod
G.Paclock.
bis
G.8262
Question 13Oct 2003
Apr 2004
Timing and Synchronization
aspects of Packet Networks Network reference
model for timing over IP networks
Sep 2004
Feb 2008Network modeling
2008Profile for telecom
Feb 2008Synchronous
Ethernet
1985 1988 1989 1992 2006
2002 2007
Study Group 15
Clock Recovery Methods over Packet
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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Synchronous Ethernet 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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Synchronization in the IWF
Figure 15/G.8261 - IWF synchronization functions (Packet to TDM direction)
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Synchronization Techniques
Frequency transfer 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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Network Synchronous Operation
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
Figure 6/G.8261 – Example of Network Synchronous Operation
Synchronization Challenges in the IP Backhaul Network
• 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)
IWF
Remote TerminalTDM to Packet
IWF
MSCPacket to TDM
IP Network
f
T1/E1T1/E1
IWF: Interworking Function
NextGen timing and sync distribution methodsmust include time and phase in addition to frequency
NextGen timing and sync distribution methodsmust include time and phase in addition to frequency
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Adaptive Clock Recovery
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
Figure 8/G.8261 – Example of Adaptive Method
Adaptive Clock Recovery (ACR) issues
Proprietary – non standard, requires “bookend” approach Frequency only – no sense of phase
◦ 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
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Differential Clock Recovery
• 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
Figure 7/G.8261 – Example of Differential Method
Differential Clock Recovery distributesphase and absolute time, not just frequency
Differential Clock Recovery distributesphase and absolute time, not just frequency
PTP/NTP Timing Recovery 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 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
Better time, sync and phase mean better QOE
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 cesium oscillator or GPS receiver
Current implementations are primarily software-based. Non-deterministic delays in networking stacks contribute to significant timing inaccuracy
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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 cesium oscillator or GPS receiver
Current implementations are primarily software-based. Non-deterministic delays in networking stacks contribute to significant timing inaccuracy
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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 ITU-T Stratum levels!Next Generation Network Services require ITU-T Stratum level synchronization
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
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NTP Protocol Overview
Clock offset:◦ [(T2 – T1) + (T4 – T3)] / 2◦ (2 + 4) / 2 = 3 seconds
Round-trip delay:◦ (T4 – T1) – (T3 – T2)◦ 19 – 3 = 16 seconds
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 = 10:15:19
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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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◦ 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.
IEEE1588 OverviewIEEE1588 Overview
Enhancements to IEEE 1588v2• 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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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 + dt4 - t3 = -o + d
o = (t2 + t3 – t1 – t4) / 2
d = (t2 – t1 + t4 – t3) / 2
Master Clock Time Slave Clock Time
Data atSlave Clock
t1
t2t2m t2
Sync Message
Followup Messagecontaining value of t 1
t1 t2
t3t3mDelay Request
Message
t1 t2 t3
t4
Delay Response Messagecontaining value of t 4
t4t1 t2 t3
Time
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1003
1001 SYNC
1001 SYNC
SYNC1001
1001 SYNC
PTP overview - SyncMaster 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
1001: SYNC
1015: SYNC
PTPPTP
UDPUDP
IPIP
MACMAC
PHYPHY
MIIMII
PTPPTP
UDPUDP
IPIP
MACMAC
PHYPHY
MIIMII
Master Slave
Tm = 1000s Ts = 1010s
Sent at 1001 s Received at 1015 s
1015
47
1003 FOLLOW UP
1003 FOLLOW UP
FOLLOW UP1003
1003 FOLLOW UP
PTP overview – Follow Up
PTPPTP
UDPUDP
IPIP
MACMAC
PHYPHY
MIIMII
PTPPTP
UDPUDP
IPIP
MACMAC
PHYPHY
MIIMII
1001: SYNC
1015: SYNC
1003: FOLLOW UP
1018: FOLLOW UP
Offset = 1015 – 1003 – Unknown line delay
= 12 – Unknown line delay
Master sends ‘FOLLOW UP’ message with actual time of transmission of the SYNC message, i.e. 1003 seconds
Adjusted slave time = 1018 – 12 – Unknown line delay
= 1006 – Unknown line delay
1018
Master SlaveSent at 1004 s Received at 1018 s 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
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1009 DELAY REQ
1009 DELAY REQ
DELAY REQ1009
1009 DELAY REQ
PTP overview – Delay Request
1012
1010: DELAY REQ
1012: DELAY REQ
PTPPTP
UDPUDP
IPIP
MACMAC
PHYPHY
MIIMII
PTPPTP
UDPUDP
IPIP
MACMAC
PHYPHY
MIIMII
Master sends ‘FOLLOW UP’ message with actual time of transmission of the SYNC message, i.e. 1003 seconds
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 tries to determine unknown line delay by sending a ‘DELAY REQ’ message to the master
1001: SYNC
1015: SYNC
1003: FOLLOW UP
1018: FOLLOW UP
1010
Master SlaveReceived at 1013 s Sent at 1009 s
Offset = 1015 – 1003 – Unknown line delay
= 12 – Unknown line delay
Adjusted slave time = 1018 – 12 – Unknown line delay
= 1006 – Unknown line delay
49
PTP overview – Delay Response
1012 DELAY RESP
1012 DELAY RESP
DELAY RESP1012
1012 DELAY RESP
PTPPTP
UDPUDP
IPIP
MACMAC
PHYPHY
MIIMII
PTPPTP
UDPUDP
IPIP
MACMAC
PHYPHY
MIIMII
1012: DELAY RESP
1015: DELAY RESP
Master SlaveSent at 1014 s Received at 1018 s
Master sends ‘FOLLOW UP’ message with actual time of transmission of the SYNC message, i.e. 1003 seconds
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 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
1009: DELAY REQ
1013: DELAY REQ
1001: SYNC
1003: FOLLOW UP
1018: FOLLOW UP
1015: SYNC Offset = 1015 – 1003 – Unknown line delay
= 12 – Unknown line delay
Adjusted slave time = 1018 – 12 – Unknown line delay
= 1006 – Unknown line delay
Line delay = ((1012 – 1009) + (1006 – 1003)) / 2
= 3 seconds
Slave time = 1015 – 3 = 1012 seconds
50
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
51
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 NetworkM S
52
Boundary clocks and transparent clocks: How do they compare?
M S M S M S M
Hops
Tim
e O
ffse
t
M S
Grand-master
Grand-master
TransparentClock
TransparentClock
Hops
Tim
e O
ffse
t
Boundary Clock
Boundary Clock
End-point
• Point-to-point synchronization
• Cascading of error offsets
• End-to-end synchronization
• Corrects only residence time
• Causes less jitter in a highly cascaded network
53
How does PTP compare to NTP?
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
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-video-bridging)
Enterprise IT applications, synchronization of computers in the home network, IPTV related applications (DRM), generic time-stamping applications in a variety of industries
Mobile Operator view of Sync Techniques
54
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)
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
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 –basedIn-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 –basedOut-of-band synchronization
Network synchronous Sync Ethernet
IEEE 1588v2 represents the most promising ‘long-term’ solution(in conjunction with Sync Eth)
NGN Sync Architecture
RNC
BSC
SGSNs
GGSNs
Internet
CPN (IP/MPLS)
MGWs Servers
GRX
VPNCorporate
STM-1ATM
IP<->ATM IWF
IP<->TDMIWF
STM-1TDM
TDM/ATM<->IPIWFBTS
Node B
2G
ATM
TDM3G R99
3G R5+,B3GEth (IP
)
E-NodeB
PSN Backhaul IP/Ethernet over fibre, MW,
leased lines, etc.
3G R5+, B3GEth
ernet (
IP)
E-NodeBS-GW/MME
ASN GW
Ethernet (IP)
DSLResidential VAP
DSLAM
BTS
2G
SHDSL (TDM)
3G R99
TDM/ATM<->IPIWF
Node B
SHDSL (A
TM)
Enterprise VAPMicro and Pico cells
Ethernet (IP)
PRC
IEEE 1588v2 Grand Master
IEEE 1588 slave board
IEEE 1588 sync packetsPrimary reference clockIP link
IEEE 1588v2 Grandmaster
IEEE 1588v2 Client
IEEE 1588 slave chip
Need
IEEE 1588v2 Differential Clock Recovery (DCR)drives Typical NGN network
Typical Network
Tutorial Provider
56
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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