SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKSDigital networks – Design objectives for digital networks
Timing characteristics of SDH equipment slave clocks (sec)
CONTENTS
Page
1 Scope 4
2 References 4
3 Definitions 5
4 Abbreviations 5
5 Frequency accuracy 6
6 Pull-in, hold-in, and pull-out ranges 6
6.1 Pull-in range 6
6.2 Hold-in range 6
6.3 Pull-out range 7
7 Noise generation 7
7.1 Wander in locked mode 7
7.2 Non-locked Wander 11
7.3 Jitter 11
8 Noise Tolerance 12
8.1 Wander Tolerance 13
8.2 Jitter Tolerance 16
9 Noise Transfer 18
10 Transient response and holdover performance 19
10.1 Short-term phase transient response 19
10.2 Long-term Phase Transient Response (Holdover) 21
10.3 Phase response to input signal interruptions 23
10.4 Phase discontinuity 24
11 Interfaces 24
Appendix I – Guidance on the relationship between network limits and input noise tolerances 25
I.1 Option 1 network limits 25
I.2 Option 2 network limits 25
Page
Appendix II – Considerations on bandwidth requirements, noise accumulation, and payload wander accumulation 27
II.1 Introduction 27
II.2 Relevant network requirements and assumptions for option 1 27
II.2.1 G.825 STM-N jitter acceptance 27
II.2.2 Wander accumulation in a synchronization distribution chain 27
II.2.3 Phase transients due to automatic timing restoration 28
II.2.4 Conclusion 29
II.3 Relevant network requirements and assumptions for option 2 30
II.3.1 Clock bandwidth requirements 30
II.3.2 Network limit TDEV mask 30
II.3.3 Wander accumulation in a synchronization distribution chain 33
Recommendation G.813
TIMING CHARACTERISTICS OF SDH EQUIPMENT SLAVE CLOCKS (SEC)
(Geneva, 1996, revised in 2003)
1 Scope
This Recommendation outlines requirements for timing devices used in synchronizing network equipment that operate according to the principles governed by the Synchronous Digital Hierarchy (SDH). These requirements apply under the normal environmental conditions specified for the SDH equipment. In normal operation SDH equipment contains a slave clock traceable to a primary reference clock. In general the SDH equipment clock (SEC) will have multiple reference inputs. In the event that all links between the master and the slave clock fail, the equipment should be capable of maintaining operation (holdover) within prescribed performance limits.
The SEC is part of the SDH equipment, the functions of which are specified in Recommendation G.783 as the Synchronous Equipment Timing Source (SETS). Slave clocks used in SDH equipment must meet specific requirements in order to comply with network jitter requirements for plesiochronous tributaries.
This Recommendation contains two options for the SEC. The first option, referred to as "Option 1," applies to SDH networks optimised for the 2048 kbit/s hierarchy. These networks allow the worst-case synchronization reference chain as specified in Figure 8.5 /G.803. The second option, referred to as "Option 2," applies to SDH networks optimized for the particular 1544 kbit/s hierarchy that includes the rates 1544 kbit/s, 6312 kbit/s, and 44 736 kbit/s. The synchronization reference chain for these networks is defined in Appendix II of this Recommendation.
An SDH equipment slave clock should comply with all of the requirements specific to one option and should not mix requirements between options 1 and 2. In the sections where one requirement is specified, the requirements are common to both options. It is the intention that options 1 and 2 should be harmonised in the future.
Careful consideration should be taken when interworking between networks with SECs based on option 1 and networks with SECs based on option 2.
This Recommendation defines the minimum requirements for clocks in SDH NEs. However, some SDH NEs may have a higher quality clock. This Recommendation allows for proper network operation when a SEC (Option 1 or 2) is timed from another SEC (like option), or a higher quality clock. Hierarchical timing distribution is recommended for SDH networks. Timing should not be passed from a SEC in free-run/holdover mode to a higher quality clock since the higher quality clock should not follow the SEC signal during fault conditions.
2 References
The following Recommendations and other references contain provisions which, through reference in this text, constitute provisions of this Recommendation. At the time of publication, the editions indicated were valid. All Recommendations and other references are subject to revision: users of this Recommendation are therefore encouraged to investigate the possibility of applying the most recent edition of the Recommendations and other references listed below. A list of the currently valid ITU-T Recommendations is regularly published. The reference to a document within this Recommendation does not give it, as a stand-alone document, the status of a Recommendation.
[1] ITU-T Recommendation G.703 (2001), Physical/electrical characteristics of hierarchical digital interfaces.
[2] ITU-T Recommendation G.783 (2002), Characteristics of Synchronous Digital Hierarchy (SDH) equipment functional blocks.
[3] ITU-T Recommendation G.801 (1988), Digital transmission models
[4] ITU-T Recommendation G.803 (2000), Architecture of transport networks based on the Synchronous Digital Hierarchy (SDH)
[5] ITU-T Recommendation G.810 (1996), Definitions and terminology for synchronization networks.
[6] ITU-T Recommendation G.811 (1997), Timing requirements at the outputs of primary reference clocks suitable for plesiochronous operation of international digital links.
[7] ITU-T Recommendation G.812 (1998), Timing requirements at the outputs of slave clocks suitable for plesiochronous operation of international digital links.
[8] ITU-T Recommendation G.822 (1988), Controlled slip rate objectives on an international digital connection.
[9] ITU-T Recommendation G.823 (2000), The control of jitter and wander within digital networks which are based on the 2048 kbit/s hierarchy.
[10] ITU-T Recommendation G.824 (2000), The control of jitter and wander within digital networks which are based on the 1544 kbit/s hierarchy.
[11] ITU-T Recommendation G.825 (2000), The control of jitter and wander within digital networks which are based on the Synchronous Digital Hierarchy (SDH).
[12] ITU-T Recommendation Q551 (2002), Transmission characteristics of digital exchanges
3 Definitions
The terms and definitions used in this Recommendation are contained in Recommendation G.810.
4 Abbreviations
For the purposes of this Recommendation, the following abbreviations are used:
CMI Coded Mark Inversion
FPM Flicker Phase Modulation
MTIE Maximum Time Interval Error
NE Network Element
OAM Operation And Maintenance
PLL Phase Locked Loop
PRC Primary Reference Clock
SDH Synchronous Digital Hierarchy
SEC SDH Equipment Clock
SSMB Synchronization Status Message Byte
STM Synchronous Transport Module
TDEV Time Deviation
UI Unit Interval
UTC Coordinated Universal Time
WFM White Frequency Modulation
5 Frequency accuracy
a) Option 1
Under free-running conditions, the SEC output frequency accuracy should not be greater than 4.6 ppm with regard to a reference traceable to a G.811 clock.
NOTE – The time interval for this accuracy is for further study. Values of 1 month and 1 year have been proposed.
b) Option 2
Under free-running conditions, the SEC output frequency accuracy should not be greater than 20 ppm. Note that payload performance is not guaranteed for a fractional frequency deviation with a magnitude greater than 4.6 ppm. However, Operations and Maintenance (OAM) functionality shall be supported when a SEC is performing at the minimum accuracy.
6 Pull-in, hold-in, and pull-out ranges
6.1 Pull-in range
a) Option 1
The minimum pull-in range should be ±4.6 ppm, whatever the internal oscillator frequency offset may be.
b) Option 2
The minimum pull-in range shall be ±20 ppm, whatever the internal oscillator frequency offset may be.
6.2 Hold-in range
a) Option 1
The hold-in range for Option 1 is not required.
b) Option 2
The minimum hold-in range shall be ±20 ppm, whatever the internal oscillator frequency offset may be.
6.3 Pull-out range
a) Option 1
The pull-out range is for further study. A minimum value of ±4.6 ppm has been proposed.
b) Option 2
The pull-out range is not specified.
7 Noise generation
The noise generation of a SEC represents the amount of phase noise produced at the output when there is an ideal input reference signal or the clock is in holdover state. A suitable reference, for practical testing purposes, implies a performance level at least 10 times more stable than the output requirements. The ability of the clock to limit this noise is described by its frequency stability. The measures MTIE and Time Deviation (TDEV) are useful for characterisation of noise generation performance.
MTIE and TDEV are measured through an equivalent 10 Hz, first-order, low-pass measurement filter, at a maximum sampling time t0 of 1/30 seconds. The minimum measurement period for TDEV is twelve times the integration period (T = 12t).
