JPH06327072A - Digital network synchronization system - Google Patents

Abstract

PURPOSE:To take countermeasures by specifying the generation part of abnormality when abnormal phase change is generated. CONSTITUTION:A host station 1 is provided with a reference oscillator 3 for generating reference clocks, a transmitter 4 for transmitting signals to a subordinate station 2 in synchronism with the reference oscillator 3 and receiving the signals of the subordinate station, a phase comparing part 5 for detecting a phase difference between the clock components of the signals from the subordinate station and the reference clocks and an information transfer means 6 for transferring phase difference information to the subordinate station. The subordinate station is provided with a phase synchronizing oscillator 7 synchronized with the clock components of the signals of the host station, the transmitter 8 for performing transmission in synchronism with the phase synchronizing oscillator 7 and also performing reception from the host station, the phase comparing part 9 for detecting the phase difference between the clock components of the signals from the host station and phase synchronizing oscillator output and a control part 10 for receiving the phase difference information of the host station and the phase difference information from the phase comparing part of the subordinate station.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital network synchronization system for synchronizing devices installed geographically apart in a digital communication network.

[0002]

2. Description of the Related Art In a digital communication network, a digital synchronization system in which all devices in the network operate at a common frequency in order to efficiently and economically perform processing such as time division multiplexing, branching, and insertion of information signals. Is the mainstream. In this digital synchronization system, a master-slave network synchronization system in which a reference clock signal output from an oscillator serving as a reference in a network is distributed to each device and each device operates in synchronization with the distributed clock is often adopted. There is.

FIG. 3 shows an example of a system configuration of a master-slave synchronization system which has been conventionally used. This is a method of distributing a reference clock among geographically distant devices to realize network synchronization. In the figure, upper station 101 and lower station 1
02 is geographically separated, and the transmission device 103,
A digital transmission path 105 is connected via 104.

The transmitter of the upper station is the reference oscillator 1 of the upper station.
It is operating in synchronization with 06. Lower station transmission device 104
Then, the clock component (frequency component) 107 of the signal transmitted from the transmission device 103 of the higher station is extracted. If the phase-locked oscillator 111 is synchronized with the extracted frequency component,
Since the frequency of the reference oscillator of the upper station and the frequency of the phase locked oscillator of the lower station are synchronized, network synchronization can be realized.

[0005]

[Problems to be Solved by the Invention] CCITT Recommendation (G.
824) specifies the phase fluctuation of the signal output from the node in the digital network. In order to satisfy this requirement, it is necessary to suppress the phase fluctuation amount of the reference clock in the digital network to 10 μs or less. However, in the conventional network synchronization method, as shown in FIG. 3, since the frequency component extracted by the lower station is used as the only reference for synchronization, it follows an abnormal phase fluctuation generated in the transmission line or the transmission device. There is an unfavorable problem that the phase of the phase locked oscillator of the lower station fluctuates.

That is, in the phase-locked oscillator, as shown in FIG. 3, the phase comparison unit 108 detects the phase difference between the frequency component 107 extracted by the transmission device and the clock 112 generated by the voltage-controlled oscillator, and the phase difference is detected. The frequency of the voltage controlled oscillator is controlled so that it becomes zero or a constant value. In current digital networks, highly stable crystal oscillators and rubidium atomic oscillators are used as voltage controlled oscillators.

Since the frequency stability of these voltage-controlled oscillators is very good, the phase-locked oscillator currently used for digital network synchronization uses a control loop having a long time constant of about 1000 seconds. Therefore, when a sudden phase change occurs due to a failure in the transmission path or a failure in the transmission device, the voltage-controlled oscillator does not immediately follow the phase change,
A phase change is detected in the phase comparison section of the phase locked oscillator.

However, the phase change in the phase comparison section is caused by an abnormality in the voltage controlled oscillator or a circuit failure in the phase locked oscillator. Therefore, in the conventional method, it is impossible to determine whether the phase change in the phase comparison unit is the phase change generated in the transmission line or the phase change in the phase locked loop. Therefore, in the conventional method, there is no choice but to follow the method in which the phase-locked oscillator follows the frequency component extracted from the transmitted signal. There was a problem that the accuracy of synchronization deteriorates.

The present invention has been made in order to solve such a conventional problem, and it is possible to identify an abnormal portion when an abnormal phase change occurs, and based on this, it is possible to identify an appropriate portion. It is an object of the present invention to provide a synchronization method capable of maintaining highly accurate phase synchronization by taking countermeasures.

[0010]

According to the present invention, the above-mentioned problems are solved by the means described in the claims.

That is, the present invention is a digital network synchronization system for transmitting a reference clock generated by a reference oscillator installed in an upper station to one or more lower stations to synchronize the lower stations with the reference clock. , A reference oscillator that generates a reference clock to the upper station, a transmission device that operates in synchronization with the reference clock, transmits a signal to the lower station, and receives a signal from the lower station, and a signal transmitted from the lower station And a information transfer means for transferring the phase difference information detected by the phase comparison unit to the lower station.

Further, a phase-locked oscillator that oscillates in phase synchronization with the clock component of the signal transmitted from the upper station to the lower station, and operates in synchronization with the phase-locked oscillator to transmit a signal to the upper station and , A phase comparator for detecting the phase difference between the clock component of the signal transmitted from the upper station and the output of the phase-locked oscillator, and the information transfer means for transferring from the phase comparator of the higher station. The phase difference information from the phase comparison unit of the lower station, the control unit receives the phase difference information from the phase comparison unit of the higher station when an abnormality occurs in the transmission line. This is a digital network synchronization system that has a function of identifying the location of an abnormality based on the phase difference information and the phase difference information from the phase comparison unit of the lower station.

[0013]

In the present invention, since the above-mentioned configuration is adopted, the control unit of the lower station is based on the phase difference information from the phase comparison unit of the upper station and the phase comparison unit of the lower station, and To judge. When a sudden phase fluctuation occurs in the phase difference, if the phase fluctuation is detected by the phase comparison unit of the lower station and not detected by the phase comparison unit of the higher station, the transmission path from the higher station to the lower station is Judge that an abnormality has occurred. If it is detected by the phase comparison unit of the upper station and not detected by the phase comparison unit of the lower station, it is determined that an abnormality has occurred in the transmission path from the lower station to the higher station. When the phase comparison unit of the upper station and the phase comparison unit of the lower station detect phase fluctuations with the same sign and equal absolute values,
It is determined that an abnormality has occurred in the phase locked oscillator of the lower station.

Based on such a judgment, the control unit of the subordinate station can identify the location where the abnormality has occurred. Therefore, by taking measures according to the location where the abnormality has occurred, it is possible to prevent the accuracy deterioration of the phase synchronization. You can

[0015]

1 is a block diagram showing the configuration of an embodiment of the present invention. Here, the case where there is one upper station and one lower station will be described. However, when a plurality of lower stations are directly connected to one upper station, or when several upper stations are connected in cascade, Even when the lower station is connected, the two stations directly connected can be regarded as the upper station and the lower station, and the present invention can be implemented with the same configuration.

As shown in FIG. 1, the higher station 1 is provided with a transmission device 4, a reference oscillator 3, and a phase comparison unit 5. A transmission device 8, a phase-locked oscillator 7, a phase comparison unit 9, and a control unit 10 are installed in the lower station 2. The transmission devices 4 and 8 of the upper station 1 and the lower station 2 respectively include clock receivers 4b and 8b that receive a clock from an oscillator, and a clock transmitter 4 that extracts the clock of the received transmission path signal and sends it to the outside.
a, 8a. Existing FTM type multiple repeater and M
The -20 type synchronous terminal device is already provided with a clock receiving unit and a clock transmitting unit, and can be used as it is as a transmission device of the present invention.

The transmission path signal output from the transmission device is synchronized with the clock received by the clock receiving unit. The transmission device of the upper station and the transmission device of the lower station are connected by an opposite transmission path. A transmission path transmitted from the upper station to the lower station is called a forward transmission path 14, and a transmission path transmitted from the lower station to the upper station is called a return transmission path 15. An information transfer transmission line 16 for transferring the phase difference information is installed from the upper station to the lower station. The phase difference information 13 detected by the phase comparison unit 5 of the upper station is the information transfer device 6 of the upper station, the information transfer transmission line 16,
Control unit 10 of the lower station via information transfer device 17 of the lower station
Transferred to.

In the present embodiment, the example in which the phase difference information detected by the upper station is transferred through the unique information transfer devices 6 and 17 has been shown. However, the phase difference information is multiplexed with the main signal, The present invention can be realized by transmitting data via the transmission devices 4 and 8 and the outward transmission line 14.

The transmission device 4 of the higher station operates in synchronization with the reference clock 11 output from the reference oscillator 3. Therefore, the transmission path signal output from the transmission device and transmitted to the lower station via the outward transmission path is synchronized with the reference oscillator 3. The transmission device of the lower station extracts the clock component 18 of the transmission path signal of the forward transmission path. The phase-locked oscillator 7 of the lower station oscillates in synchronization with the extracted clock component 18. The phase comparison unit 9 of the lower station detects the phase fluctuation of the output clock 20 of the phase-locked oscillator based on the phase of the extracted clock component 18. The phase difference information 19 detected by the phase comparison unit 9 is sent to the control unit 10.

The transmission device 8 of the lower station operates in synchronization with the output clock 20 of the phase locked oscillator. A transmission path signal is output from the transmission device of the lower station and transmitted to the upper station via the return transmission path. The transmission device of the upper station extracts the clock component 12 of the transmission path signal of the return transmission path. The phase comparison unit 5 of the upper station measures the phase fluctuation of the clock component 12 extracted with the phase of the reference clock 11 as a reference. The phase difference information 13 as a result of the phase comparison is transferred to the control unit 10 of the lower station via the information transfer device 6 of the upper station, the information transfer transmission line 16, and the information transfer device 17 of the lower station.

The operation of the embodiment for each state and the phase relationship of each signal will be described below. Normal operation First, let us consider a situation in which all devices are operating normally and there is no failure in the transmission path. In this state, the phase fluctuation applied to the transmission line is only the fluctuation of the transmission line delay time due to environmental changes such as temperature. At this time, the clock component 18 extracted by the lower station is synchronized with the phase of the reference clock of the upper station plus the delay variation of the forward transmission path. Since the delay fluctuation of the forward transmission path is a phase fluctuation having a longer period component than the time constant of the control loop of the phase locked oscillator, the output 20 of the phase control oscillator accurately follows the delay fluctuation of the forward transmission path. It will be. Therefore, no fluctuation appears in the phase difference between the extracted clock component 18 and the output 20 of the phase-locked oscillator, which is detected by the phase comparator 9 of the lower station.

On the other hand, since the transmission device of the lower station operates in synchronization with the phase control oscillator, the clock component 12 extracted from the transmission line signal of the return transmission line in the upper station is transmitted in the forward transmission as compared with the reference oscillator. The phase varies by the sum of the delay variation of the path and the delay variation of the return transmission path. Therefore, the phase comparison unit 5 of the upper station detects the phase fluctuation of the sum of the delay fluctuation of the forward transmission path and the delay fluctuation of the return transmission path.

It is known that the delay variation of the transmission line due to the environmental change is mainly due to the temperature variation around the transmission line, and the magnitude of the delay variation is proportional to the transmission line length and the temperature variation amount. The transmission path length is a known quantity, and the temperature fluctuation can be known from the season, which is several tens of degrees Celsius at the maximum. Therefore, the phase fluctuation amount detected by the phase comparison unit 5 of the upper station is a proper value calculated from the transmission path length and the temperature fluctuation amount, and the phase fluctuation is not detected by the phase comparison unit 9 of the lower station. If so, it can be determined that the clock distribution operation is normal and the phase-locked oscillator of the lower station is accurately synchronized with the reference oscillator of the upper station.

Operation when Abnormality Occurs in Outgoing Clock Path Next, let us consider a case where an abnormality occurs in the transmission path or the transmission device, causing a sudden phase fluctuation that cannot be followed by the phase-locked oscillator of the lower station. Here, the phase variation that is much larger than the delay variation due to the temperature change of the transmission line described in the normal operation is targeted.

Here, the clock receiving unit 4b of the transmission device of the upper station, the outward transmission line 14, and the clock transmission unit 8a of the transmission device of the lower station are collectively referred to as an outward clock path. Similarly, the clock receiving unit 8b of the transmission device of the lower station, the return transmission line 15, and the clock transmission unit 4a of the transmission device of the higher station are collectively referred to as a return clock path.

Now, let us consider the phase fluctuation detected at the moment when a sharp and large phase fluctuation occurs only in the forward clock path. Since the phase-locked oscillator 7 of the lower station does not immediately follow a sudden phase change, the phase comparator 9 of the lower station
The phase fluctuation of the forward clock path is detected as it is. On the other hand, the phase variation of the upper station is not detected by the phase comparison unit 5. If this state is left as it is, the phase-locked oscillator 7 follows an abnormal phase fluctuation in the forward clock path, and the clock synchronization accuracy deteriorates. Therefore, the control unit 10 determines that an abnormal phase fluctuation in the forward clock path is detected, and the control loop of the phase-locked oscillator is cut off to bring the phase-locked oscillator into a free-running state. By this operation, even if an abnormal phase variation occurs in the forward clock path, the deterioration of the synchronization accuracy can be minimized.

Operation when Abnormality Occurs in Return Path Clock Path Also, consider a case where a steep and large phase variation occurs only in the return path clock path. At this time, since the phase comparison unit 5 of the higher station detects the phase fluctuation of the return clock path and the phase comparison unit 9 of the lower station does not detect the phase fluctuation, it can be clearly distinguished from the phase fluctuation of the forward clock path. In this case, the forward clock path is normal, and the phase-locked oscillator 7 of the lower station is also normally synchronized with the reference oscillator 3 of the upper station, so that the control unit of the lower station does not need to operate in particular.

Operation When Abnormality Occurs in the Phase-Locked Oscillator of the Lower Station Further, consider the case where an abnormality occurs in the phase-locked oscillator 7 of the lower station. It is assumed that this abnormality causes a sudden phase change in the output phase of the phase locked oscillator. At this time, since the forward clock path is normal, the phase comparator 9 of the lower station can detect an abnormal phase fluctuation caused in the phase locked oscillator. Further, the phase fluctuation of the phase locked oscillator is transmitted to the upper station via the return clock path.

Therefore, the phase comparison section 5 of the upper station can detect the phase fluctuation of the phase locked oscillator. This state can be clearly distinguished from the case where the phase change occurs in the forward path clock path and the case where the phase change occurs in the forward path clock path. When the control unit of the subordinate station detects an abnormality in the phase-locked oscillator, it takes measures such as switching to a spare phase-locked oscillator.

Description of the phase relationship of each signal FIG. 2 shows the phases of the reference clock 11 of the upper station, the output clock 20 of the phase locked oscillator of the lower station, and the transmission path clocks 12 and 18 extracted by the upper station and the lower station. , The horizontal axis is time. In the figure, t ₁ , t ₂ ,
t ₃ and t ₄ are respectively the phase of the reference clock 11 of the upper station, the phase of the transmission path clock 18 extracted by the lower station, the phase of the phase-locked oscillator output 20 of the lower station, and the transmission path clock extracted by the upper station. 12 phases.

At this time, the delay time t of the forward clock path
_out is a t _out = t ₂ -t _1, delay time t _in the backward clock path is a t _in = t ₄ -t _3. Further, the phase difference φ _M detected by the phase comparison unit of the higher station is φ _M = t ₄ −t ₁ , and the phase difference φ detected by the phase comparison unit of the lower station is φ _M.
S is φ _S = t ₃ −t ₂ .

That is, the phase comparison unit of the upper station measures the phase of the transmission path clock extracted by the upper station with reference to the phase of the reference clock of the upper station. Further, the phase comparison unit of the lower station measures the phase of the phase locked oscillator of the lower station with reference to the phase of the transmission path clock extracted by the lower station.

Here, when a steep phase variation occurs in the forward clock path and the delay time increases by Δt _out ,
Since t ₃ does not follow immediately, φ _S becomes smaller by Δt _out , and φ _M does not change.

When a steep phase change occurs in the return clock path and the delay time increases by Δt _in, only t ₄ changes, so φ _S does not change and φ _M does not change Δt in.
_in only increases.

[0035] In addition, the abnormality occurs in the phase-locked oscillator of lower station, when the output phase of the phase-locked oscillator is delayed by Δt _p, if there is no change in the t _out and t _in, φ _S, φ _M
Both increase by Δt _p . That is, when an abnormality occurs in the phase locked oscillator of the lower station, both φ _S and φ _M undergo the same amount of phase change of the same sign.

Now, let us consider a case where the delay time of the forward clock path and the delay time of the backward clock path both suddenly increase by Δt _c . Such a thing occurs when both the forward transmission path and the return transmission path are switched to different transmission paths due to a failure of the transmission path. At this time, t ₂ and t ₄ are both Δt
_{Since c is} delayed and t ₃ does not follow immediately, φ _s is Δ
It decreases by t _c and φ _M increases by Δt _c . That is, in this case, since φ _S and φ _M change the phase by the same amount of absolute values of opposite signs, it can be clearly distinguished from the case where an abnormality occurs in the phase locked oscillator of the lower station.

When the delay time of the forward clock path and the delay time of the backward clock path have phase changes of opposite signs and the same absolute value, it is indistinguishable from the abnormality of the phase locked oscillator of the lower station. However, the probability that such a change (phase change of opposite sign and equal absolute value between the opposite transmission lines) will occur in the clock path is very low, so φ _s and φ _M are the same. When the phase change of the same sign is detected, it may be considered as an abnormality of the phase locked oscillator of the lower station.

Judgment and Operation of Control Unit Table 1 summarizes what judgment is made by the control unit when phase changes occur in the phase comparison units 5 and 9 of the upper station and the lower station. . The control unit 10 of the subordinate station has a determination function as shown in Table 1, and can compare the phase difference information from the two phase comparison units 5 and 9 to determine where an abnormality has occurred in the clock distribution system. it can. In addition, the control unit 1
With 0, it is possible to take an optimum protection operation such as free-running or switching of the phase-locked oscillator with respect to the determined abnormality occurrence part of the clock distribution system.

[0039]

[Table 1]

[0040]

As described above, according to the digital network synchronization system of the present invention, an abnormal phase variation occurs due to the phase difference being detected by the phase comparators installed in the upper station and the lower station. At this time, the cause can be identified, and appropriate measures can be taken. As a result, there is an advantage that deterioration of the phase synchronization accuracy can be suppressed to a small level even when a failure occurs in the transmission line or the phase locked oscillator.

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention.

FIG. 2 is a diagram illustrating a phase relationship of each signal in the present invention. Is.

FIG. 3 is a diagram showing an example of a system configuration of a conventional master-slave synchronization system.

[Explanation of symbols]

 1, 101 Upper station 2, 102 Lower station 4, 8, 103, 104 Transmission device 105 Digital transmission line 3, 106 Reference oscillator 12, 18, 107 Clock component of transmission line signal 108 Phase comparison unit in phase-locked oscillator 109 Filter 110 voltage controlled oscillator 7,111 phase-locked oscillator 20,112 output clock of phase-locked oscillator 5,9 phase comparator 10 controller 4b, 8b clock receiver 4a, 8a clock transmitter 14 forward transmission path 15 return transmission path 16 information Transfer transmission line 11 Reference clock 13,19 Phase difference information 6,17 Information transfer device

Claims (1)

[Claims] 1. A digital network synchronization system for transmitting a reference clock generated by a reference oscillator installed in an upper station to at least one lower station to synchronize the lower station with the reference clock, wherein the upper station (1) is A reference oscillator (3) that generates a reference clock, a transmission device (4) that operates in synchronization with the reference clock, transmits a signal to a lower station, and receives a signal transmitted from the lower station; A phase comparison section (5) for detecting a phase difference between the clock component of the transmitted signal and the reference clock of the higher station, and an information transfer means for transferring the phase difference information detected by the phase comparison section to the lower station, The lower station (2) includes a phase-locked oscillator (7) that oscillates in phase with the clock component of the signal transmitted from the upper station,
Transmission device (8) which operates in synchronization with a phase-locked oscillator and transmits a signal to a higher station and receives a signal from the higher station
And a phase comparison unit (9) for detecting the phase difference between the clock component of the signal transmitted from the host station and the output of the phase-locked oscillator.
And a control unit (10) for receiving the phase difference information transferred from the phase comparison unit (5) of the higher station by the information transfer means and the phase difference information from the phase comparison unit (9) of the lower station, The control unit (10), when an abnormality occurs in the transmission line,
A digital feature having a function of identifying an abnormal place based on the phase difference information from the phase comparing unit (5) of the upper station and the phase difference information from the phase comparing unit (9) of the lower station. Network synchronization method.