In ITU-Recommendation G.811 (Timing Characteristics of Primary Reference Clocks) a network clock stability of 1 x 10 E-11 for the Primary Reference Clock (RPC) is recommended. This accuracy is derived from 1 acceptable slip in a 2 MBit/s transmission in an observation time of 72 days. 1 (frame) = 125 us 125 us / 72 d x 24 h/d x 3600 s/h = 2 x 10 E –11 or+-1 x 10 E-11 This clock stability can be generated by atomic clocks only! NA = Not Applicable NS = Not Specified NE = Network Element CPE = Customer Premises Equipment
Synchronization and timing loop presentation -mapyourtech
G.781 –Synchronization layer functions
G.783 –Characteristics of synchronous digital hierarchy (SDH) equipment functional
G.810 –Definition and Terminology of Synchronous Networks
G.811 –Timing Characteristics of Primary Reference Clocks
G.812 –Timing requirements of slave clocks suitable for use as node clocks in
G.813 –Timing characteristics of SDH equipment slave clocks (SEC)
G.822 –Controlled Slip Rate Objectives on an international digital connection
G.823 –The control of jitter and wander within digital networks which are based on
the 2048 kbit/s hierarchy
G.824 –The control of jitter and wander within digital networks which are based on
the 1544 kbit/s hierarchy
G.825 –The control of jitter and wander within digital networks which are based on
the synchronous digital hierarchy (SDH)
The introduction of digital 64 kb/s circuit switches (end office and tandem switching systems)
and digital cross-connect systems in the late 1970s and early 1980s drove the need for network
Synchronization in telecommunications networks is the process of aligning the time scales of
transmission and switching equipment so equipment operations occur at the correct time and in
the correct order. Synchronization requires the receiver clock to acquire and track the periodic
timing information in a transmitted signal.
The transmitted signal (Fig. 1a) consists of data that is clocked out at a rate determined by the
transmitter clock. Signal transitions between zero and peak values contain the clocking
information and detecting these transitions allows the clock to be recovered at the receiver (Fig.
1b). The recovered clock is used to write the received data into a buffer, also called elastic store
or circular shift register, to reduce jitter (jitter is discussed later in this section). The data is then
read out of the buffer onto a digital bus for further multiplexing or switching (Fig. 1c).
Fig. 1a – Transmitter and Receiver Clocks
The received signal is processed by the clock recovery circuit, and the clock is then used to recover the
data. The transmitter and receiver circuits for both directions are identical. The receiver on the right is
shown in slightly more detail.Fig.1a is as below.
The synchronization network is a network that shall be able to provide all types of telecommunication traffic
networks with reference timing signals of required quality. The objective for the traffic networks, for example
switching, transport, signaling, mobile, is to not lose information. Loss of information is often caused by poor
synchronization. This can be avoided by properly connecting the traffic network to an adequate
synchronization network (how to connect to a synchronization network is normally called network
synchronization).In the best case, poor synchronization causes only limited inconvenience to the traffic
network. In the worst case, it can make the entire telecommunication network stop passing traffic.
Poor synchronization causes loss of information in varying degrees. Examples of
results of poor synchronization are:
• degraded traffic throughput;
• inhibition of set-up of calls (#7 signaling) due to re-transmission;
• re-sending of files;
• corrupt fax messages;
• degraded speech quality;
• freeze-frames on video;
• disconnection of calls during hand-over in mobile networks;
• partial or complete traffic stoppage.
The results for network operators providing poor synchronization to their networks are: reduced short and
long term income, decreased customer satisfaction, low network availability and low traffic throughput.
Synchronization Status Message (SSM)
It is defined in EN 300 417-6-1 . This is a signal that is passed over a synchronization interface to indicate the
Quality-Level of the clock sourcing the synchronization signal. This signal was originally defined for use over
STM-N interfaces in the S1 byte. It has since been proposed for use over 2 Mbit/s interfaces as well.
The SSM code transmitted reflects the quality of the clock that the interface is ultimately traceable to; i.e. the
grade-of clock to which it is synchronized directly or indirectly via a chain of network element clock's (the
synchronization trail),or how long this chain of clocks is. For example, the clock-source quality-level may be a
Primary Reference Clock (PRC) complying with EN 300 462-6-1 , or it may be a Slave Clock in holdover-mode,
complying with ETSI EN 300 462-4-1 , or an EN 300 462-5-1  Clock in holdover or free-run.
The clock-source quality-level is essentially, therefore, an indication only of the long-term accuracy of the
network element clock.
In a fully synchronized network all sources should be ultimately traceable to a PRC and this should be indicated
using a '0010' code. The "Do Not Use for Synchronization" code is used to prevent timing loops and is
transmitted in the opposite direction on interfaces used to synchronize an equipment's clock. The "Quality
Unknown" code was originally proposed for connection to equipment that did not use SSM codes. ETSI has
proposed that this coding is not used as it cannot be sensibly used in a quality based selection algorithm.
Synchronization Supply Unit (SSU):This unit is a high quality slave clock deployed in the
The SSU gives two key benefits: it filters out short term phase noise (jitter) and short term wander and
provides a highly accurate clock if there is a failure of synchronization supply from the PRC. There are a number
of different SSU implementations, these are usually differentiated on their Frequency Accuracy in holdover
mode. They vary from more expensive Rubidium based oscillators to cheaper Quartz oscillators. There are also
a number of Higher Quality Quartz Oscillators which use improved techniques to reduce the temperature and
ageing effects of Quartz.
Stand Alone Synchronization Equipment (SASE): this is a piece of synchronization equipment
that contains an SSU. This term is used to differentiate from the SSU clock function itself which can be located
within another piece of equipment for instance an SDH DXC or 64 kbit/s switch.
Timing Loops: timing loops are created when a clock is directly or indirectly synchronized to itself.
Timing loops must be prevented because all clocks in a timing loop are isolated from a Primary Reference Clock
and are subject to unpredictable frequency instabilities. There is no simple way of detecting Timing loops as no
alarms are generated on their creation. They can go undiscovered until service is effected by poor slip or error
performance leading to an investigation which will eventually locate the timing loop.
Primary Reference Clock (PRC): these are the highest quality Clocks in a network and are usually
based on a freerunning Caesium Beam oscillator giving a very accurate clock performance.
Global Positioning System (GPS): system using a number of Satellites orbiting the earth, these
satellites are primarily intended to give positioning information for navigation but can also be used to derive a
highly accurate timing source of PRC Quality. To use GPS an antenna and a post processing unit are required.
The deletion and repetition of a data block is called a slip. If the block consists of only one bit, it is called a bit slip
or clock slip. If the block consists of an entire data frame (for example, the 192 payload bits in a DS1 frame), it is
called a frame slip. If the slip occurs at the frame boundary and the framing bit is not lost, it is called a controlled
slip. On the other hand, an uncontrolled slip means that identification of the frame boundary was lost and there
was a Change Of Frame Alignment (COFA). An uncontrolled slip is more serious because meaningful transmission
is interrupted (the receiver is unable to accurately read bit values) until framing is re-established.
The overall object of network synchronization is to minimize controlled slips and eliminate uncontrolled slips.
This can be achieved only by synchronizing all equipment clocks in all nodes to the same master clock or to a
number of master clocks that operate plesiochronously (very closely matched).Frame slips have different effects
on different traffic types. Table 1 shows the effects of a single frame slip on an individual 64 kb/s (DS-0) channel.
The slip rate between plesiochronous networks can be calculated from Eq. 1.
SR = Δf FR 86,400⋅ ⋅ Eq. 1
SR = Slip rate (frame slips/day)
Δf = Frequency accuracy difference
FR = Frame rate (frames/second) [Note: All telecommunications systems operate at 8,000 frames/second]
86,400 = Number of seconds/day
As an example, consider two networks that are synchronized by different clocks. One clock has
an accuracy of +1x10–11 and the other has an accuracy of –1x10–11. The accuracy difference in
this case is 2x10–11. Therefore, the slip rate is 2x10–11 x 8,000 frames/second x 86,400
seconds/day = 0.0138 frame slips/day, or 72.3 days/frame slip.
Since decided by the opposite end, the external source signal speed needs to be converted into the speed of
the local switching equipment before entering digital switching network. This is called Retiming, and is realized
through the buffer memory. The clock extracted from the source signals acts as the writing clock, which will
write bit flow into the buffer memory at the speed extracted from the source clock, then the clock of the
receiving equipment (local clock), acting as the reading clock, will read the bit flow out from the buffer. Thus
the source signal speed is converted into the local clock speed, as shown in Figure below.
When the write clock frequency disaccords with the read clock frequency, the buffer memory will overflow or
underflow, and a group of bits will be reread or unread, which we call it Slip. Since the buffer memory capacity
is generally greater than the data quantity of one frame (the typical value is of two frames' length), a frame of
data will be unread or reread when slip occurs. Such kind of slip which only rereads or leaves out a complete
frame, but does not disturb the frame structure, is called Controlled Slip or Frame Slip.
Effects of Single Frame Slip
Reference input signals:
T1, derived from:
STM-N (ITU-T Recommendation G.707 )
34 368 kbit/s with 125μs frame structure
139 264 kbit/s with 125μs frame structure
T2, derived from:
2 048 kbit/s (EN 300 166 )
T3, derived from:
2 048 kHz (EN 300 166 )
2 048 kbit/s (EN 300 166 ) with SSM according to (ITU-T Recommendation G.704 ).
T4: External reference signal,
2 048 kHz (ITU-T Recommendation G.703 ) (after physical interface)
2 048 kbit/s (EN 300 166 ) with SSM according to (ITU-T Recommendation G.704 ) (after
NOTE: The main application of 2 048 kbit/s signals with SSM is the exchange of synchronization status
information between an SSU and an SDH network element within a node.
T0: Timing signals for equipment-internal signal processing and for generating outgoing SDH traffic signals:
Frequencies are implementation-specific.
Basic properties for T0:
Frequency Accuracy: ± 4,6 × 10-6
Holdover: 5 × 10-8 (initial frequency offset)
2 × 10-6 (temperature)
1 × 10-8/day (aging)
What are some timing/sync rules of thumb
1. A node can only receive the synchronization reference signal from another node
that contains a clock of equivalent or superior quality (Stratum level).
2. The facilities with the greatest availability (absence of outages) should be selected
for synchronization facilities.
3. Where possible, all primary and secondary synchronization facilities should be
diverse, and synchronization facilities within the same cable should be minimized.
4. The total number of nodes in series from the stratum 1 source should be
minimized. For example, the primary synchronization network would ideally look like a
star configuration with the stratum 1 source at the center. The nodes connected to
the star would branch out in decreasing stratum level from the center
5. No timing loops may be formed in any combination of primary
Working Mode of the Clock Monitoring Module
The clock monitoring module can be configured to work in one of the following two modes:
In this mode, the clock source is configured manually. The clock monitoring module does not switch the clock
source automatically, but just tracks the primary reference source. If the primary reference source is lost, the
clock monitoring module enters a holdover state.
In auto mode, the clock source is selected by the system automatically. When the primary clock source is lost
or not available, the clock monitoring module selects another clock source based on the following rules:
If SSM level is not activated, the clock source is decided by reference source priority. If two
reference sources have the same priority, the one with the smaller reference source number (1 to 18) is
selected. When the reference source with the highest priority is lost, the next available reference source with
the highest priority is selected. When the former clock source becomes available, the system switches to that
If SSM is activated, the clock source is decided by the SSM level. If two reference sources have the
same SSM level, the reference source priority takes effect, in the way described above.
The following clock sources are excluded in clock selection (when SSM is activated):
Clock sources whose signals are lost;
Clock sources whose priority is 255.
Clock sources whose SSM level is DNU (DoNotUse).
Free Running Mode
An operating condition of a clock, the output signal of which is strongly influenced by
the oscillating element and not controlled by servo phase-locking techniques. In this
mode the clock has never had a network reference input, or the clock has lost external
reference and has no access to stored data, that could be acquired from a previously
connected external reference. Free-run begins when the clock output no longer
reflects the influence of a connected external reference, or transition from it. Free run
terminates when the clock output has achieved lock to an external reference.
An operating condition of a clock (a slave clock or a primary reference clock), where
the output signal’s frequency is not controlled with servo phase-locking to an external
reference and is, therefore, determined exclusively by an internal oscillator. In this
mode the clock has never had an external reference or, in the case of a slave clock,
the clock has lost the external reference and has no access to stored data acquired
from a previously connected external reference.
An operating condition of a slave clock in which the output signal is controlled by an
external input reference such that the clock’s output signal has the same long
term average frequency as the input reference, and the time error function
between output and input is bounded. Locked mode is the expected mode of
operation of a slave clock.
An operating condition of a clock which has lost its controlling reference input and is
using stored data, acquired while in locked operation, to control its output. The
stored data are used to control phase and frequency variations, allowing the
locked condition to be reproduced within specifications. Holdover begins when
the clock output no longer reflects the influence of a connected external
reference, or transition from it. Holdover terminates when the output of the clock
reverts to locked mode condition.
An operating condition of a slave clock which has lost its controlling input and is
using stored data, acquired while in locked operation, to control its output. The
stored data is used to control frequency, attempting to reproduce locked mode
performance. Holdover begins when the clock output no longer reflects the
current behaviour of the external reference, or transition from it.
The system changes to the holdover mode when it loses all its external reference sources. Holdover performance controls the
maximum wander at the output in holdover mode.
The maximum fractional frequency offset Y(S) for the output signal is
Where x(S) is the output phase drift over the period of S seconds. The output Y(S) for Stratum 1 and Stratum 2 in the holdover mode
must meet the requirements in following table.
For Stratum 3 holdover as below in table
Time Interval (s) Average Frequency Deviation
0 < S 5000
The output signal phase in the first 5000 seconds meets the
requirements specified in 3.4.11 Phase Transient.
5000 < S 86400 1.5E-11
86400 < S 259200 2.5E-11
259200< S 604800 3E-11
Holdover Interval Average Frequency Deviation
1 day 1E-10
3 days 5E-10
Why is Holdover Important?
Holdover is critical to ensuring service continuity as well as keeping the operator's OPEX to a minimum (e.g. elimination
of emergency truck rolls to the BTS site). In certain geographical areas where GPS signals are received only
intermittently, holdover is crucial for the operation of base stations. Holdover technologies are also necessary to
maintain sync during GPS outages caused by external events, such as in 2007 in San Diego when the US Air Force's
wide-scale denial of GPS caused a major GPS outage. Criminals can also jam GPS signals locally, given the commercial
availability of GPS jammers, and environmental factors such as sun spots also contribute to GPS disruptions. Out-of-
sync base stations dramatically increase the operator's OPEX. Further, if telecommunications regulations such as E911
requirements are violated, the operator may incur additional expenses of reporting outages and carrying out audits.
Holdover Requirements and Technologies
Holdover is achieved by equipping base stations transceivers with oscillators that temporarily "hold over" sync
signals. Holdover capability can range from several hours to several days depending on the quality of the BTS's
oscillator. Holdover requirements are not standard; they vary depending on the type, complexity, and operator'
requirements. LTE TDD networks have stringent timing requirement, + 1.5 microseconds.
For the reliable operation of synchronization network, there are usually two or more
reference signals used as active and standby signals for the clocks in hierarchical
master-slave synchronization network. When the active reference signal fails or
transmission link is disconnected, the clock reference signal will be switched to
standby signal. When timing signals are transferred via active/standby reference
transmission link, a timing loop may form, that is, a clock may receive the timing signal
sent from itself via a specific link.
When a timing loop is formed, clocks in the loop form a self-feedback loop, and after a
period of time, the frequencies of respective clocks will gradually deviate from the
reference clock frequency, resulting in the deterioration of signal quality.
So special consideration is required to prevent the forming and existing of timing loop
in synchronization network design and planning.
A timing loop is a situation where a portion of the normal timing distribution structure
inadvertently feed timing to itself. In a worst case scenario, a timing loop can cause a traffic
outage. Networks must be designed to avoid timing loops at all costs.
When designing a network, the designer must first work through the potential traffic paths of
all the timing signals and identify possible potential loop points. Once the location of these
critical points is known, a few different methods of preventing these loops can be used
. Two approaches are described here:
Create a table for each NE listing every potential timing source. NEs that may
potentially send looping timing signals must not be included. Timing signals from NE
sources not included in the table are ignored.
The timing source table includes a quality/priority ranking for each timing source. Timing
signals from better-quality/higher-priority sources are preferred to timing signals from
The highest quality timing source is given first priority. Secondary priority is given to the
alternative timing sources. These alternative sources are automatically used as backup
sources if the primary timing source goes down. Up to four levels of timing sources can
be defined for each NE. The NE automatically reverts to the highest quality timing
source currently available. This means that as higher-quality timing sources are
restored, the NE automatically gives preference to the highest-quality timing signal
Each NE can automatically send a Do Not Use (DNU) message back to every other
direct source NE from which it receives a timing signal. This ensures that the source
NEs do not receive back their own timing signals from the current NE.
Simple timing loop and prevention of timing loops through use of DNU messages
Example of timing loop generation
Here each of the two external timing nodes (node with external timing) can be synchronized, either by is
external clock or via any of the 2 lines. In case of failure of the active external clock in timing node 1, this node
selects its fourth priority, which carries the timing generated by itself. This is a timing loop situation.
(within S1 byte)
Highest 1 G.811 (PRC) 0010
2 G.812T (SSU transit) 0100
3 G.812L (SSU local) 1000
4 G.813 (SEC) 1011
Lowest 5 Do not use1)
Quality unknown 0000
This signal should not be used for synchronization. It is automatically
transmitted in the backward direction of the port, which is selected as clock
reference. This is done to prevent synchronization loops.
Jitter and Wander already have been described as any kind of phase noise or fluctuations of the recovered timing
signal along the time axis. The phase noise can be generated by the transmission system, in the clock recovery
circuits of an interface, and at the outputs of synchronization supply units (SSU).
Jitter and wander is described in time units such as seconds (seconds, microseconds, nanoseconds , picoseconds) ,
phase angle (radians) or unit interval (UI). The most common unit is the Unit Interval, which is equal to the pulse
period (reciprocal of the interface bit rate). For example, 1 UI at the DS-1 rate corresponds to (1 ÷ 1,544,000 b/s)
648 ns; therefore, a peak-to peak jitter of 0.1 UI at a DS-1 interface is the same as 64.8 ns.
Jitter tolerance requirements that apply to the input of Stratum 2, 3E, 3 and 4 clocks are given in ANSI T1.102. A
DS-1 timing output interface shall not generate more than 0.05 UI peak-to-peak jitter (per ANSI T1.101 Annex G).
The effect of transmission cables on wander is in the order of
• ~ 80 ps/km/°C for fiber optic cables
• ~ 725 ps/km/°C for metallic twisted pair cables
Time Interval Error (TIE) is the variation in time delay of a timing signal over a particular observation period
relative to an ideal timing signal (or a very high quality reference signal). If the timing signal has a fixed frequency
offset, the TIE will increase linearly over time with no upper bound – the longer the observation period the higher
the TIE. If the timing signal phase delay varies during the observation period it will have some maximum value, or
Maximum TIE (MTIE), during that period. MTIE is specified to ensure that slip performance is properly bounded
particularly during loss and rearrangement of reference timing sources
Time Deviation (TDEV) is a measure of the expected time variation of a timing signal as a function of integration
time. It provides information about the spectral content of the timing signal’s phase noise. It is calculated from a
sequence of time error samples and is expressed in time units (usually nanoseconds). A constant frequency offset
does not affect TDEV. In the case of linear frequency drift (linear increase in frequency offset), TDEV is proportional
to the square of the observation period. TDEV gives more information on timing signal noise than MTIE.
MTIE and TDEV are specified as a function of measurement (observation) period because clocks have different
observed behavior depending on the observation period. As previously mentioned, the output circuits of all
telecommunications clocks use high quality quartz crystal oscillators to provide short-term stability. Crystal
oscillators age and drift over time and are sensitive to temperature variations so they are steered by atomic clock
generators (cesium or rubidium), which have very good long-term stability but relatively poor short-term stability.
The combination of a high quality crystal oscillator and atomic clock generator provides the necessary stability over
all observation periods.
Troubleshooting Synchronization Issues
The following tips may be helpful when diagnosing synchronization issues:
Be careful with loopbacks. A local loopback placed on a source in the hierarchy can
cause a timing loop. In addition, a remote loopback towards a source on the hierarchy
of another NE can cause a loopback.
Synchronization alarms as defined in the Network Time Protocols (NTPs) can be
used to find synchronization-related problems. Other alarms that often accompany
synchronization problems include:
PDH port input and output buffer alarms
Loss of pointer alarms
Loss of frame alarms
AU and TU pointer justification events can be measured through performance
The Network Elements (NE) within SDH networks have to be synchronized to a central clock.
A hierarchical structure of clock generators with different qualities is used to distribute the clock reference
signal through the entire network:
The high precision Primary Reference Clock (PRC), according to ITU-T recommendation G.811, builds up the
highest level of the synchronization hierarchy and provides clock reference signals for the NEs within the
It should be redundant in the synchronization network.
The second level is represented by the Synchronization Supply Unit (SSU), according to ITU-T recommendation
G.812. It filters the incoming clock reference signal and provides the network with a high quality clock
reference signal in case of a loss of the PRC reference.
The SSU can be a separate equipment or part of a SDH NE.
Max. 10 SSU are allowed in a chain.
The third and last level of synchronization is realized by the SDH Equipment Clock (SEC) according to ITU-T
It is normally part of the NE and provides the NE with an holdover capability for at least 24 hours.
Max. 20 SEC are allowed one after the other and max 60 SEC in a chain between two SSU or after a PRC.
General principles Hierarchy of the synchronization clocks
Primary reference clock (PRC)
Frequency accuracy greater than 1 part in 10-11
Compliant to ITU-T Recommendation G.811
Synchronization supply unit (SSU)
Holdover mode : frequency drift lower than 2 x 10-10
Compliant to ITU-T Recommendation G.812
SDH equipment slave clocks (SEC)
Holdover mode : frequency drift lower than 7 x 10-12
Frequency accuracy not greater than 4.6 ppm under free-running conditions
Compliant to ITU-T Recommendation G.813
International Synchronization Standards
Holdover initial slip-rate
Pull-in range Application
Stratum 1, PRS, PRC G.811 1.0 x 10-11 ≤1 Slip in 72 days NA Network Master
Stratum 2 G.812 Type II 1.6 x 10-8 1 slip per 14 days +/- 1.6 x 10-8 Stratum 2 BITS
G.812 Type I --- 1 slip in first 24 hours +/- 1.0 x 10-8 Type I SSU
Transit node clock (TNC) G.812 Type V NS 1 slip in first 24 hours NS TNC BITS, TNC SSU
Stratum 3E G.812 Type III 4.6 x 10-6 8 slips in first 24 hours +/- 4.6 x 10-6 Stratum 3E BITS
Local node clock (LNC) G.812 Type I NS 17 slips in first 24 hours NS LNC SSU
Stratum 3 G.812 Type VI 4.6 x 10-6 255 slips in first 24 hours +/- 4.6 x 10-6
Stratum 3 BITS,
Stratum 3 NE
SDH equipment clock (SEC) G.813 Option 1 4.6 x 10-6 1643 slips in first 24 hours +/- 4.6 x 10-6
minimum clock (SMC)
G.813 Option 2 20 x 10-6 3520 slips in first 24 hours +/- 20 x 10-6
Stratum 4 NA 32 x 10-6
22118 slips in
first 24 hours
+/- 32 x 10-6 Stratum 4 NE, CPE