Sep. 01, 2025
Mechanical Parts
The MSC is a digital switching center which contains components like TRAU, BSCs and BTSs for the radio interface.
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The MSC is synchronized by local clock sources, e. g. PRCs, GPS receivers or clock outputs (T4) of SDH network elements.
Plesiochronous: (Greek - plesio, meaning near and chronos, meaning time) The meaning of Plesiochronous therefore is "nearly synchronous".
Plesiochronous Digital Hierarchy is used to multiplex PCM30 transmissions to data streams with higher data rates and for transmission over digital transport equipment such as fiber optic and microwave radio systems. Although this transmission clock accuracy is not highly exact, the PCM30 data flow is transferred from synchronized switching centers, regarding data and clock transparency.
Digital switching centers transfers synchronous PCM30 - data flow with this transmission technique to other switching centers, and the recovered clock can be used for its own synchronization.
Much of the transport infrastructure based on plesiochronous digital hierarchy (PDH) is being replaced with SONET or SDH based infrastructure.
The accuracy of PRC must be > 1 x 10-11.
If the receiving switching center has a lower frequency than the transmitting switching center, the buffer cannot read the data in the same rate as these is written by the transmitter. A buffer overflow occurs with the result of a frame slip.
If the receiving switching center has a higher frequency than the transmitting switching center, the buffer is faster read out, than it is filled. A buffer underflow occurs followed by a slip frame.
The slip rate is proportional to the frequency deviation between transmitter and receiver.
These inputs can be synchronized with 2,048 MHz or 2,048 MBit/s. The interfaces are either symmetrical or asymmetrical.
These output can deliver 2,048 MHz or 2,048 MBit/s. The interfaces are either symmetrical or asymmetrical.
The first time slot of the second PCM frame is the non frame alignment signal (NFAS) to transfer alarms and checksums (CRC4).
It is synchronized over the PCM30-Transmission from MSC and transmits the received clock accuracy over PCM30 transmissions to the base station control (BSC).
The TRAU is located at the BSC or directly at the MSC (mobile Switching center).
This technology is characterised by higher data rates and makes the transmission of the existing data formats from the earlier PDH technology possible. Contrary to the PDH technology SDH network elements are synchronized.
The payload data is stored in "containers" with additional information - the "Overhead" and the "Payload" is transmitted as synchronous transport module (e.g. STM-1). This transmission is normally accomplished over optical media.
STM-1 designates a data rate of 155 MBit/s - e.g. 63 PCM30 transmissions can be transported. These 155 MBit/s data stream transports also the clock accuracy to the next SDH network element and synchronizes its clock generator. Thus a continuous synchronization of the SDH network is possible.
The clock outputs (T4) of the SDH network elements can be used for the synchronization of further SDH network components or other connected mobile equipment.
SDH uses the following Synchronous Transport Modules (STM) and rates:
STM-1 (155 MBit/s)Time synchronization has recently emerged as an important requirement in telecom networks for several reasons. It ensures that devices and systems across networks operate cohesively, which is crucial for data integrity and maintaining the quality of the service. Additionally, accurate timekeeping is vital for security protocols, as many encryption methods rely on precise timestamps.
This blog post provides an overview of the concepts and terms used in telecom network time synchronization. The topic is complex and will be developed in the following few blogs to discuss details. It is the first blog in a series to elaborate on basic concepts such as frequency, phase, and time synchronization. Differences between the oscillator, generator, and clock will be explained, and at the end, we will touch on the topic of clock quality measurements.
Events that occur in time in a consistent manner are called synchronous events. They can occur spontaneously or be the result of deliberate actions. The concept of synchronicity is inextricably linked to the concept of time, the passage of which we can measure with high precision using dedicated devices – clocks. The basic problem is that the readings of different clocks can vary, which raises the question of which one indicates the „correct time”.
The word synchronization derives from the ancient Greek language and consists of two parts: syn- (gr. σύγ-), which means coexistence, and the word -chronos (gr. -χρονος), which is the name of the god of time from Greek mythology. In Greek beliefs, there were two gods responsible for time: the already mentioned Chronos (gr. Χρόνος), who was depicted as a sage with a beard and was responsible for the continuous time in which everything is immersed, and Kairos (gr. Καιρός) being the god of the moment. According to legends, Kairos was bald and had bangs, and whoever saw him had only a moment to grab his bangs and get luck. According to the analysis of the word, „synchronization” means coexistence in a common continuous time. Currently, there are three types of synchronization:
• frequency synchronization,
• phase synchronization,
• time synchronization.
Frequency synchronism is a state in which two or more periodic signals from different systems change at the same rate, i.e., have the same frequency (Fig. 1) . The process of achieving synchronism is called synchronization. Frequency synchronization is also called syntonization. The etymology indicates that the word syntonization consists of two parts: syn-, which means, as already mentioned, common, and -tone, i.e., a sound of a given frequency. The most famous example of syntonization is the tuning of musical instruments.
Phase synchronism is a state when two or more signals, clocks, objects or systems, are not only in a state of syntonism, but also their cycles or actions begin and end at exactly the same moment. The time shift between signals is called phase error. In the syntonistic state, the phase error is constant but not necessarily zero. Phase synchronization means zero phase error over time (Fig. 2.).
A time synchronism is a state when two or more clocks, objects, or systems, not only have a synchronized phase but also their cycles and actions follow an abstract time scale within the same inertial frame (Fig. 3). Quite often in English-language literature the name time synchronization is used interchangeably with phase synchronization, but this is not entirely correct and precise. Phase synchronization becomes time synchronization if you add additional information about when the event happened relative to the time scale [1].
Concepts such as oscillators, generators, and clocks are important in the work. These terms are often used interchangeably in technical documentation and English-language literature, but this is not entirely precise.
An oscillator is an electronic device that generates an oscillating signal [2]. An oscillation is an event that repeats at a constant frequency. The device in which the process takes place is called a resonator. A resonator needs an energy source to sustain the oscillations. Combined, the power source and the resonator form an oscillator. Although there are many simple types of oscillators, both mechanical and electronic, two types of oscillators are mainly used for time and frequency measurements: crystal oscillators and atomic oscillators.
Generators are electronic systems and devices that produce periodic electrical waveforms or electrical impulses at the expense of power supply energy. The most important parameters of the generator include the frequency or frequencies of the generated waveforms, frequency constancy, frequency tuning range and the shape of the output signal. Generators most often have their own built-in oscillator, but they can also generate output waveforms based on an external reference periodic signal.
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A device that generates periodic, precisely spaced signals for timekeeping applications is called a clock. A clock consists of at least three parts: an oscillator, a device that counts and converts oscillations into units of time interval, such as seconds, minutes, hours, and days, and a device that displays or records the results [3].
Clocks and generators used in telecommunications systems may be in the following states [4]:
• FREE RUNNING,
• PULL-IN,
• LOCKED,
• HOLDOVER.
The FREE RUNNING state is an operating condition in which the output signal depends on internal oscillating elements and is not controlled by any synchronization mechanisms. In this state, the device cannot access the reference signal or has lost an external reference and does not have access to the stored data that may have been obtained from the previous reference connection.
The LOCKED state is the operating conditions under which the output of the synchronized device is controlled by an external reference input in such a way that the output signal has the same long-term average frequency as the reference signal, and the time error function is limited. This mode is the expected operating state of the synchronized device.
The PULL-IN state occurs during the process of getting to the LOCKED state.
The HOLDOVER state is the operating conditions in which the synchronized device has lost its reference signal and uses the stored data obtained during operation in the LOCKED state for control. The stored data is used for frequency and phase control. This mode starts when the clock output does not reflect the influence of an external reference, and ends when the output starts to be controlled by a reference signal
Other terms that describe the quality of a synchronizing and synchronized device are accuracy and stability.
Accuracy is defined [5] as the closeness of the agreement between the measured value and the true value of the measured parameter, for example frequency or phase. This concept is not generally a quantitative measure, but it can be said that something is more accurate when it provides less error. It can also be understood as the compliance of many measurements with a measurable quantity.
Stability should be understood as immutability. The purpose of the stability assessment [6] is to characterize the phase and frequency fluctuations (variability) of a time source in the time domain and frequency domain. Stability analysis can refer to stochastic (noise) and deterministic properties of the device under test. It is assumed that stochastic properties are invariant for a given clock (the random process is stationary and ergodic). Environmental effects affecting the stability rating, such as temperature change, shock, or radiation, should be minimized in the test environment. A common problem in assessing stability in the time domain is to obtain the longest possible observation interval while minimizing test time and cost. The relationship between precision, accuracy and stability is shown in Figure 4 showing instantaneous frequency waveforms for different cases.
The results of the time error measurement are influenced by the configuration of the measurement system. If the measurement concerns two clocks, and the tested clock tracks the reference signal, such a configuration is called a synchronized clock configuration (Fig. 5) [4]. The time error measured in this configuration does not depend on frequency detuning and frequency drift of the master clock. The parameters calculated from the measured time values reflect only the phase noise of the clocks used in the measurement.
Over the years, many methods of comparing clock readings and methods of assessing the quality of their work, i.e. accuracy and uniformity of running, have been developed. Thanks to this, we know which of them is „worse” and which is „better”. In many cases, we can also force a clock with lower accuracy or greater unevenness of movement to improve both parameters. One way is to use a Digital Phase-Locked Loop (DPLL) (Figure 6). The method is so effective that it has practically displaced other methods, such as the injection synchronization method [7].
The input of the loop shown in Figure 6 is supplied with a reference signal to which the frequency and/or phase of the VCO (Voltage Controlled Oscillator) generator is synchronized. Another element of the system is a digital phase detector with a time-to-digital converter (Time to Digital Converter). It allows you to measure the time difference between the edges of two signals, the reference and the local. In this way, the phase error is determined and converted into a digital form. The Digital Loop Filter implements a control algorithm that converts the received information about the phase error into an appropriate control word, causing a change in the voltage on the Digital Analog Coverter. Changing the input voltage VCO changes the frequency of the output signal, the phase of which is again compared to that of the reference signal, and the loop duty cycle closes [7].
A reference signal, also known as a reference signal, can come from a variety of sources. It can be a Global Navigation Satellite System (GNSS), for example GPS (Global Positioning System). We then say that the system is disciplined to GPS. GNSS systems are often used as reference sources for the synchronisation process, as they can be used anywhere on Earth and free of charge. The reference signal can also be from any other source with higher quality than the VCO.
There are many types of generators that produce periodic signals. In telecommunications, highly stable quartz, rubidium and cesium generators were used as VCOs. By highly stable generators we mean generators whose daily frequency inconstancy is less than or equal to 10-8. This corresponds to the Stratum 3E network layer (or higher) operating in HOLDOVER mode.
The requirements for a system for frequency and phase synchronization of high-stability generators is quite simple in theory: The output signal must meet the telecommunications standards (PRC, PRTC B or PRTC A) on MTIE and TDEV.
Maximum Time Interval Error [4] (MTIE) is the maximum difference between the phase of the test clock and the phase of the reference clock, expressed in units of time, observed for all locations (positions) of the tau observation intervals (τ=nτ0, where τ0 is the sampling period) existing at the time of the measurement T. MTIE is non decreasing function of the observation interval τ and describe the accuracy of the clock
Time deviation [4] (TDEV or σx) measures the expected variation in time signal as a function of integration time tau. TDEV also provides information about the spectral content of the signal’s phase noise. TDEV is given in units of time and describes the stability of the clock in τ observation intervals.
The PRC (Primary Reference Clock) standard defines the requirements for the most important clocks in the telecommunications network, and the exact indications can be found in ITU-T Recommendation G811 [6]. According to the G811 standard, the recommended minimum measurement time, in order to obtain a reliable estimation of the TDEV parameter, for a given maximum integration period τ is equal to twelve integration periods (T=12 τ).
In conclusion, in this blog, we learned the meaning and etymology of the word synchronization. We know the states in which the clock can be in typical operating conditions. The definition of clock accuracy has been presented, and we know that its measurement in practice is MTIE. Similarly, the stability of the clock is presented, and we know that we can estimate it using TDEV. We learned about two basic measurement configurations: independent clock configuration and synchronized clock configuration. The construction of the phase synchronization loop was also presented. In this way, we finished the first blog in the series.
In the next blog, different telecommunications standards will be compared, and requirements for the Front-haul (FH) at O-RAN will be presented. Various technologies of synchronization and distribution of the reference signal, such as SyncE, PTP, and White Rabbit, will be analyzed. Different approaches to network synchronization will be discussed.
[1] Deep-Learning-Aided_Distributed_Clock_Synchronization
[2] NIST oscillator
[3] NIST clock
[4] ITU-T G810
[5] NIST accuracy
[6] ITU-T G811
[7] Tutorial on Digital Phase-Locked Loops
Many thanks to Marcin Dryjanski for his guidance during my work at Rimedo Labs and tips for this blog post.
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