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

6

Generic Block Diagram of a Clock

VCXO

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

11

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.

14

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

15

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

16

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

17

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

18

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

19

Service-specific Synchronization Requirements

Source: Cisco

20

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

21

< 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

22

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

24

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

25

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.

26

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 

27

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.

29

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)

30

Synchronization in the IWF

Figure 15/G.8261 - IWF synchronization functions (Packet to TDM direction)

31

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)

32

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

34

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

36

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

39

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

40

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

42

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

43

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

44

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

46

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

48

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