YD/T 2375-2019 PDF in English
YD/T 2375-2019 (YD/T2375-2019, YDT 2375-2019, YDT2375-2019)
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Technical requirements for high accuracy time synchronization
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YD/T 2375-2019: PDF in English (YDT 2375-2019) YD/T 2375-2019
COMMUNICATION INDUSTRY STANDARD
OF THE PEOPLE’S REPUBLIC OF CHINA
ICS 33.040.20
M33
Replacing YD/T 2375-2011
Technical requirements for high accuracy time
synchronization
ISSUED ON: NOVEMBER 11, 2019
IMPLEMENTED ON: JANUARY 01, 2020
Issued by: Ministry of Industry and Information Technology of PRC
Table of Contents
Foreword ... 4
1 Scope ... 6
2 Normative references ... 6
3 Abbreviations ... 7
4 Time synchronization requirements and precision grade division ... 8
4.1 Classification of time synchronization precision ... 8
4.2 Requirements for high-precision time synchronization ... 10
5 Requirements for high-precision time synchronization networking ... 10
5.1 Hierarchical structure of high-precision time synchronization network ... 10
5.2 Composition and basic functions of time synchronization device ... 11
5.3 Principles of time synchronization networking ... 11
6 Requirements for high-precision time synchronization performance indicators ... 15
6.1 Overview ... 15
6.2 Allocation of performance indicators of time synchronization network when satellite
timing is available ... 16
6.3 Allocation of performance indicators of time synchronization network when satellite
timing service is unavailable ... 17
7 High-precision time synchronization transmission technology ... 18
7.1 High-precision time synchronization technology ... 18
7.2 Inter-office transmission technology ... 19
7.3 Intra-office distribution technology ... 19
8 Requirements for high-precision time synchronization interface ... 19
8.1 Overview ... 20
8.2 PTP time interface requirements ... 20
8.3 1PPS+ToD time interface ... 22
9 Reliability requirements of high-precision time synchronization network ... 27
9.1 Reliability requirements for time sources ... 27
9.2 Requirements for time synchronization device and time link settings ... 27
9.3 Redundant configuration requirements of time synchronization device ... 28
9.4 Selection requirements for time reference source ... 28
10 Basic requirements for high-precision time synchronization related device ... 31
10.1 High-precision time synchronization device ... 31
10.2 Basic requirements for time synchronization of bearer device ... 33
10.3 Basic Requirements for time synchronization of synchronized devices ... 35
Appendix A (Informative) IRIG/DCLS time coding ... 37
Appendix B (Informative) Setting location and quantity of time synchronization device
... 41
Appendix C (Informative) Engineering feasibility requirements ... 42
Appendix D (Informative) Estimation of time distribution of time source part in case of
satellite timing failure ... 44
Appendix E (Informative) Key factors affecting inter-office transmission accuracy . 45
Appendix F (Informative) 1588V2 protocol and BMC algorithm ... 48
Technical requirements for high accuracy time
synchronization
1 Scope
This standard specifies the networking requirements for high-precision time
synchronization, the performance index requirements for high-precision time
synchronization, the reliability requirements for high-precision time synchronization
networks, the basic requirements for equipment related to high-precision time
synchronization.
This standard applies to high-precision time synchronization networks, based on ground
transmission using PTP technology.
2 Normative references
The following documents are essential to the application of this document. For the dated
documents, only the versions with the dates indicated are applicable to this document;
for the undated documents, only the latest version (including all the amendments) is
applicable to this standard.
YD/T 900-1997 Technical requirement for SDH equipment - Clock
YD/T 1012-1999 Node clock set of digital synchronization network and its timing
feature
YD/T 1479-2006 Technical requirements and testing methods of primary reference
clock equipment
YD/T 2879-2015 (OAM) technical requirements based on the synchronous network
operation management and maintenance of a packet network
ITU-T G.8262 Timing characteristics of synchronous Ethernet equipment slave
clock
ITU-T G.8275.1 Precision time protocol telecom profile for phase/time
synchronization with full timing support from the network
IEEE 1588-2008 Standard for a precision clock synchronization protocol for
networked measurement and control systems
3GPP TR 25.836 Node B synchronization of time division duplex (TDD)
Figure 1 -- Hierarchical structure of time synchronization network
5.2 Composition and basic functions of time synchronization device
The level-1 time synchronization device shall be composed of two cesium clocks and
two satellite timing receivers, etc., which can be traced to a higher-level domestic time-
keeping reference (for example, the National Time Service Center), through a dedicated
comparison method.
The level-2 time synchronization device shall be equipped with at least rubidium clocks.
The device consists of two rubidium clocks and two satellite timing receivers. It shall
support the traceability of time to the level-1 time synchronization device, through
ground means; support the punctuality function of ground frequency signals. It shall be
able to be reliably traced to China's frequency synchronization network.
The level-3 time synchronization device shall be equipped with at least a high-stable
crystal oscillator. The device consists of two high-stable crystal oscillators and at least
one satellite timing receiver. It shall support time traceability to the level-2 time
synchronization device or the level-1 time synchronization device, through ground
means; support the punctuality function of ground frequency signals. It shall be able to
be reliably traced to China's frequency synchronization network.
See Appendix B, for the location and quantity of time synchronization devices, at all
levels.
5.3 Principles of time synchronization networking
5.3.1 General principles of time synchronization networking
Under normal circumstances, the time synchronization device shall prefer the signal of
the satellite timing receiver. When the satellite signal is unavailable, the low-level time
synchronization device shall trace to the high-level time synchronization device,
through the ground time link. In the case where both the satellite signal and the ground
time link are unavailable, the low-level time synchronization device shall be able to use
the signal from the frequency synchronization network, to keep time. If conditions
permit, the time synchronization network shall be traced to the national time-frequency
standard system.
When building a time synchronization network, according to the actual network
conditions, flexible networking structures (for example, level-2 or level-3 networking)
and setting methods (for example, a level-3 time synchronization device with a
rubidium clock) can be used.
5.3.2 Relationship between time synchronization network and frequency
the best network element of frequency time keeping as the reference. See 8.3.4, for the
specific mapping mechanism of scheme 1 and scheme 2. See 9.4.2, for the source
selection processing mechanism.
5.3.3 Principles of reference source setting
5.3.3.1 The principle of setting the timing reference of the level-1 time
synchronization
The principle of setting the timing reference of level-1 time synchronization is as
follows:
a) Under normal circumstances, the level-1 time synchronization device shall track
the signal of the satellite timing receiver;
b) When the satellite timing receiver fails, the level-1 time synchronization device
shall be able to keep time, based on the cesium clock;
c) If the relevant technical conditions are available, the level-1 time synchronization
device can be traced to the national time-frequency system, as the fundamental
guarantee for the entire time synchronization network.
5.3.3.2 The principle of setting the timing reference of the level-2 time
synchronization
The principle of setting the level-2 time synchronization timing reference is as follows:
a) Under normal circumstances, the level-2 time synchronization device shall track
the signal of the satellite timing receiver;
b) The level-2 time synchronization device shall be equipped with at least two
ground time input reference signals, which are respectively connected to the
level-1 time synchronization device, from different physical routes. When the
satellite signal is unavailable, the level-2 time synchronization device shall be
able to track the ground time input reference signal;
c) The level-2 time synchronization device shall be equipped with at least one ground
frequency input reference signal, which shall be traceable to the frequency
synchronization network. In the event that satellite signals are not available, the
level-2 time synchronization device shall be able to track the ground frequency
input reference signal. In the event that both satellite signals and ground time
input reference signals are unavailable, the level-2 time synchronization device
shall be able to use the signal, which is from the frequency synchronous network,
to keep time.
5.3.3.3 The principle of setting the timing reference of the level-3 time
synchronization
The principle of setting the timing reference of the level-3 time synchronization are as
follows:
a) Under normal circumstances, the level-3 time synchronization device shall track
the signal of satellite timing receiver;
b) The level-3 time synchronization device shall be equipped with two ground time
input reference signals, which are respectively connected to the level-1 or level-
2 time synchronization device, from different physical routes. When the satellite
signal is unavailable, the level-3 time synchronization device shall be able to track
the ground time input reference signal;
c) The level-3 time synchronization device shall be equipped with a ground
frequency input reference signal, which shall be traceable to the frequency
synchronization network. When the satellite signal is unavailable, the level-3 time
synchronization device shall be able to track the ground frequency input reference
signal. When both the satellite signal and the ground time input reference signal
are unavailable, the level-3 time synchronization device shall be able to use the
signal, which is from the frequency synchronous network, to keep time.
5.3.4 Principles for organizing time synchronization path
In order to ensure the reliability of the time synchronization network and the precision
of time allocation, the organization of time synchronization paths shall follow the
following principles:
a) Ground time transmission shall follow the principle of one-way layer-by-layer
downward (applicable to scheme 1 only);
b) Ground time signal transmission is only allowed from high-level time
synchronization device to low-level time synchronization device; it is not allowed
for the low-level time synchronization device to transmit timing to high-level or
same-level time synchronization device (only applicable to scheme 1);
c) The time transfer between the ground offices, between the time synchronization
device and the synchronized device (such as base station), shall use PTP
technology; the time transfer, between the ground offices between time
synchronization devices, can use PTP technology or other technologies;
d) Level 2 and Level 3 time synchronization device shall be able to obtain time
synchronization, from at least two different physical routing ground timing links;
the ground timing physical links are preferably buried optical cables;
e) The number of time synchronization devices, which are connected in series to the
time synchronization path, shall not exceed 3, of which there are at most one
level-1 time synchronization device, one level-2 time synchronization device, one
level-3 time synchronization device;
f) The number of bearer device network elements, which are connected in series in
the time synchronization path, from the time synchronization device to the
synchronized device, should not exceed 30. The number of bearer device network
elements, which are connected in series between any two adjacent levels of time
synchronization devices (including the OA station, which supports the PTP
function) should not exceed 20. Refer to clause 10.2 for the time synchronization
function and performance requirements of the bearer device;
g) For bearer device, the frequency reference shall be taken from the frequency
synchronization network;
h) For the level-2/level-3 time synchronization device, when the satellite timing
receiver is unavailable, the frequency reference shall preferably come from the
intra-office frequency synchronization device, OR trace to the frequency
synchronization network, through the shortest path.
6 Requirements for high-precision time synchronization
performance indicators
6.1 Overview
The working state of the satellite timing receiver will cause changes in the time
transmission link of the time synchronization network. When the satellite signal is
available, the time synchronization network forms a single-level structure, that is, there
is only one time synchronization device, in the time path, as the time source, as shown
in Figure 3. When the satellite timing is unavailable, it may connect, in series, multiple
time synchronization devices, into the time synchronization network, to form a multi-
level structure, that is, a maximum of three time synchronization devices may be
connected in series in the time path, as the time source, as shown in Figure 4.
For the above two cases, the end-to-end time performance indicator consists of three
parts: The time source part, the bearer network part, the end distribution part.
Considering that the time deviation, which is introduced by the line asymmetry and
compensation error, that may exist in the engineering construction, will adversely affect
the end-to-end indicator allocation, the delay asymmetry shall be accurately
compensated, in the actual engineering construction. For specific requirements, see
Appendix C.
the PTP technology to track the time signal from the level-1 time
synchronization device. The time output performance Δt1' of the last-level
time synchronization device is composed of two parts: one is the fixed time
deviation introduced by the punctuality of the cesium clock; the other is the
time deviation introduced by the inter-office allocation, in the inner-province
backbone and inter-province backbone. In this case, the specific indicators of
the time allocation of the time source part are to be studied. For reference
estimates, see Appendix D.
2) For the case where the satellite timing receiver of the level-1 time
synchronization device is normal, whilst the satellite timing receiver of the
level-2/level-3 time synchronization device is faulty, the time output
performance of the last-level time synchronization device mainly includes the
time output performance of the level-1 time synchronization device, when
normally tracking the satellite timing receiver (better than ±150ns), AND the
time deviation introduced by the inter-office distribution of the time signal in
the inner-provincial backbone and the inter-provincial backbone; its time
output performance Δt1' shall be better than ±250ns (relative to UTC). The
transmission method of the time signal, from the level-1 time synchronization
device to the level-2/level-3 time synchronization device, is to be studied;
3) When all satellite timing receivers fail, if conditions permit, the time
synchronization network can be traced to the national time-frequency standard
system by other means; the time output performance Δt1' of the final time
synchronization device shall be better than ±250ns (relative to at UTC). The
specific traceability and transmission methods are to be studied.
b) Bearer network part: The time performance indicator Δt2 is the same as that in
clause 6.2 b).
c) End allocation part: The time performance index Δt3 is the same as that in clause
6.2 c).
7 High-precision time synchronization transmission
technology
7.1 High-precision time synchronization technology
The realization technology of high-precision time synchronization involves inter-office
transmission technology and intra-office distribution technology. Among them, the
inter-office transmission technology refers to the realization of time synchronization,
between different computer rooms, which can be realized by using PTP technology,
based on different transmission platforms; the intra-office distribution technology is
mainly to realize the time synchronization, between different devices in the computer
room, the most commonly used implementation technology includes 1PPS+ToD
interface technology and PTP interface technology. See Appendix E, for an introduction
to the key factors affecting the accuracy of inter-office transmissions.
7.2 Inter-office transmission technology
Interoffice transmission technology includes the following two categories.
a) The PTP mode is supported point by point: The bearer network element device,
which is involved in the time synchronization path, shall support the PTP function.
b) Transparent transmission mode: For the transparent transmission of high-
precision time synchronization based on TDM mode, the intermediate node does
not need to support the PTP function, whilst the end-to-end time synchronization
is realized, through the exchange of PTP protocol, between the master and slave
devices at both ends. The way of realizing high-precision time synchronization
and transparent transmission, based on the grouping method, needs to be studied.
See Appendix F, for the requirements of the PTP protocol and its BMC algorithm.
7.3 Intra-office distribution technology
Intra-office distribution techniques include the following two categories.
a) Technologies that support intra-office two-way message exchange to achieve
high-precision time transmission, mainly including PTP, general timing interface,
improved NTP, etc. Among them, PTP technology is relatively mature at present,
whilst other technologies need to be further studied. When using the PTP interface
or the general timing interface for intra-office distribution, the downstream
synchronized devices can simultaneously obtain time and frequency reference
signals, through this interface.
b) Technologies that support intra-office one-way signals to achieve high-precision
time transmission, mainly including 1PPS+ToD. The frequency synchronization,
which is required by the downstream synchronized devices, shall be obtained,
preferentially through the intra-office frequency synchronization node.
8 Requirements for high-precision time synchronization
9.3 Redundant configuration requirements of time synchronization
device
The redundant configuration requirements of the time synchronization device are as
follows:
a) The important card boards, such as the clock card and power supply card of the
time synchronization device, shall be redundantly configured;
b) The time synchronization device shall be equipped with dual-mode satellite
receivers (for example: GPS + Beidou) or two single-mode satellite receivers;
c) Support simultaneous reception of various satellite signals and ground signals;
realize automatic switching of various time reference source signals;
d) After all time reference sources fail, the ground frequency input signal or internal
clock can be used to keep time;
e) The mean time between failure (MTBF) of the device shall not be less than 20
years. The device card board shall be pluggable, during operation.
9.4 Selection requirements for time reference source
9.4.1 Overview
The requirements in this chapter are mainly applicable to the selection of the time
reference source in the BC mode; the OC mode may use it as a reference; the TC mode
is to be studied.
9.4.2 Switching conditions for time reference source
The high-precision time synchronization network shall support automatic failover, so
that when the synchronization link fails, it can automatically switch to the backup
reference source or link, to realize the self-healing of the synchronization link and
performance, thereby meeting the synchronization requirements of the service.
The automatic switching conditions for reference source include:
a) The clock class (clockClass) is lowered or the pulse-per-second state class of
1PPS+ToD interface is lowered;
b) The clock accuracy (clockAccuracy) is lowered;
c) The Priority 1 (Priority1) and priority (Priority2) are lowered;
d) Packet message timing failure PTSF;
e) PTP link LOS;
f) PTP link failure (linkdown);
g) 1PPS or ToD interface's input signal is lost.
9.4.3 Selection mechanism of time reference source
The selection mechanism of time reference source is as follows:
a) The PTP port's source selection algorithm shall follow the BMC algorithm, which
is defined in IEEE 1588-2008 (hereinafter referred to as 1588v2), which mainly
includes two parts: The data set comparison algorithm and the state decision
algorithm. Please refer to Appendix F.3, for the specific implementation process.
b) Principle of 1PPS+ToD source selection algorithm.
1) Scheme 1
- The source selection of the time source device shall follow the following
principles:
● When tracking GPS normally, the output code of pulse-per-second state is
0x00;
● After the satellite timing receiver fails, when the time synchronization
device uses the cesium atomic clock, to maintain or trace the frequency
synchronization signal to the PRC, to keep time, THEN, the output code of
pulse-per-second state is 0x01;
● After the satellite timing receiver fails, the time synchronization device uses
the tungsten atomic clock, to keep the output code of pulse-per-second state
as 0x05;
● After the satellite timing receiver fails, the time synchronization device uses
the crystal clock, to maintain the output code of pulse-per-second state as
0x03;
● When the time source device fails, the output code of pulse-per-second state
is 0x02;
● The output clockClass code and the pulse-per-second state code of the time
source device shall maintain a consistent corresponding relationship.
- The source selection principle of bearer device shall follow the following
principles:
......
Source: Above contents are excerpted from the PDF -- translated/reviewed by: www.chinesestandard.net / Wayne Zheng et al.
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