JP4711345B2 - Clock regeneration method and apparatus - Google Patents

Description

  In the present invention, when the data transmission device cannot use the clock source used by the data transmission device because the data transmission device and the data reception device are connected via an asynchronous network, the data transmission device The present invention relates to an adaptive clock method in which a data receiving apparatus regenerates a clock having the same frequency as a clock source used by a data transmitting apparatus based on the number of received packets or the amount of received data of a fixed bit rate signal to be transmitted. More specifically, the present invention relates to a technique for suppressing the influence of fluctuation of a packet received via an asynchronous network on a clock frequency to be reproduced based on an adaptive clock method.

  When the data receiving device cannot use the clock source of the data transmitting device, such as a communication system in which the data receiving device and the data transmitting device are connected via an asynchronous network such as an IP network, the data receiving device uses the clock of the data transmitting device. An adaptive clock method is widely known as a method of reproducing and synchronizing. The principle of the adaptive clock method is described in Non-Patent Document 1, for example.

  According to Non-Patent Document 1, in the adaptive clock method, when a data transmission device transmits data at a constant bit rate, even if the data reception device and the data transmission device are connected via an asynchronous network, the data reception device The principle is that the average value of the received data amount per unit time observed on the side of the data stochastically matches the transmission bit rate in the data transmission apparatus. That is, the data receiving apparatus can estimate the transmission bit rate in the data transmitting apparatus by dividing the amount of data received in a certain period by the period. Normally, the clock is regenerated by maintaining an equilibrium state of the amount of data stored in the buffer rather than dividing the amount of data received in a certain period by that period. In other words, the data receiving apparatus accumulates the received data in the buffer and extracts the data from the buffer at a bit rate proportional to the recovered clock, but the clock frequency is set so that the accumulated data amount in the buffer matches a certain target value. Control. Ideally, the amount of stored data always matches the target value, and stable clock recovery can be performed.

  Here, the relationship between the difference between the accumulated data amount and the target value and the reproduction clock frequency will be described. When the reproduction clock frequency is faster than the clock used by the data transmission device, the data extraction speed from the buffer is faster than the transmission bit rate of the data transmission device, and the amount of stored data is less than the target value. Conversely, when the recovered clock frequency is slower than the clock used by the data transmission device, the amount of stored data is greater than the target value. Therefore, the control of the reproduction clock frequency based on the accumulated data amount is to increase the reproduction clock frequency when the accumulated data amount is larger than the target value, and to decrease the reproduction clock frequency when it is small.

  When the transmission target is a signal having a fixed bit rate, the data transmission device generates a packet at a fixed cycle. If the transmission delay time of the packet is constant in all the packets, the data reception device Receive the packet at. Therefore, the amount of data stored in the buffer of the data receiving device increases rapidly each time a packet is received, and gradually decreases at a rate proportional to the recovered clock frequency. FIG. 1 is a diagram illustrating changes in the amount of accumulated data when packets are received at a fixed period.

  In FIG. 1, the data receiving apparatus receives a packet every fixed period T at times t1, t2, t3, t4, and t5, and the amount of stored data increases every time the packet is received. If the recovered clock frequency is changed in accordance with the change in the amount of stored data based on the principle of the adaptive clock method, the recovered clock frequency changes rapidly every time a packet is received. In order to suppress this fluctuation, it is conceivable to apply the moving average to the adaptive clock method. FIG. 1 also shows an average value of the accumulated data amount obtained by using the packet reception cycle T as a moving average calculation section. From FIG. 1, it can be seen that the moving average value of the accumulated data amount does not change at all. Therefore, if the clock is regenerated by the adaptive clock method based on the moving average of the accumulated data amount, fluctuations in the clock frequency can be suppressed.

  However, in an actual network, the packet transmission delay time is not constant for all packets, and fluctuations occur. FIG. 2 shows this state, and FIG. 3 shows the accumulated data amount and the moving average change of the accumulated data amount when the packet transmission delay fluctuates and the packet reception interval is not constant. In FIG. 3, the delay of the packet to be received at time t3 is large, and the packet is actually received at time t3 ′. For this reason, the moving average value of the accumulated data amount fluctuates and affects the reproduction clock frequency.

  In order to realize high-accuracy clock synchronization, it is essential to eliminate fluctuations in the clock frequency caused by fluctuations. Therefore, in order to perform the clock frequency control based on the “moving average of accumulated data amount”, removal of fluctuations in the value of “moving average of accumulated data amount” as shown in FIG. 3 becomes an issue.

  Patent Document 1 describes a method of using an average value of accumulated data amount for controlling a reproduction clock frequency. However, in the method described in Patent Document 1, as described above, the average value of the accumulated data amount also fluctuates due to fluctuations in packet transmission delay, and thus the recovered clock frequency also fluctuates.

  Patent Document 2 describes a method in which a data receiving apparatus has an independent clock and a packet transmission delay is predicted using the independent clock. For this method, the clock included in the data receiving apparatus needs to be highly accurate and stable, which increases costs.

  Patent Document 3 describes a method of changing a coefficient in a calculation formula for calculating a control value for a clock oscillator to a smaller one after a certain time has elapsed. This method increases resistance to fluctuations in packet transmission delay. However, in a network in which a packet whose transmission delay time is extremely larger than the average value is occasionally generated, a certain amount of fluctuation is caused in the recovered clock due to the packet.

Japanese Patent No. 3169335 Japanese Patent No. 3555833 Japanese Patent No. 3660517 ITU-T I.I. 363.1, “B-ISDN ATM Adaptation Layer Specification: Type 1 AAL”

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a clock recovery method and apparatus for removing a moving average variation in stored data amount and recovering a stable frequency clock even when packet transmission delay fluctuations occur. .

According to the clock recovery device of the present invention,
Data extraction means for accumulating data contained in the received packet in the buffer, and movement for calculating a moving average of the amount of accumulated data accumulated in the buffer using a period that is an integral multiple of the period in which the packet is transmitted as a calculation interval Average calculating means, means for calculating the average value of the moving average of the accumulated data amount, clock frequency control means for controlling the clock frequency output from the clock oscillator based on the average value, and clock output from the clock oscillator When the data reading means for reading data from the buffer at a speed proportional to the frequency and the moving average calculation means calculate the moving average of the new accumulated data amount, a value indicating the fluctuation of the moving average of the new accumulated data amount is calculated. Fluctuation determination that determines whether or not to use the moving average of the new accumulated data amount to calculate the average value based on the value indicating the fluctuation Characterized in that it comprises a stage.

According to another embodiment of the clock recovery device of the present invention,
The value indicating the variation is a difference between the moving average of the new accumulated data amount and the average value, and the variation determining means determines the new accumulated data amount when the difference is equal to or larger than the threshold value or larger than the threshold value. It is also preferable to determine that the moving average is not used for calculating the average value.

According to another embodiment of the clock recovery device of the present invention,
Based on the moving average of the accumulated data amount used for the calculation of the average value, it has means for calculating a value indicating the degree of dispersion of the moving average of the accumulated data amount, and the threshold is determined from the value indicating the degree of dispersion. It is also preferable.

According to the clock recovery method of the present invention,
A method of regenerating a clock in a data receiving device based on a stored data amount of a buffer in which data included in a received packet is accumulated and data is read at a rate proportional to a clock frequency output from a clock oscillator, A first step of calculating a moving average of the amount of accumulated data stored in the buffer, with a period that is an integral multiple of the period at which is transmitted as a calculation interval, and a value indicating a variation in the moving average of the new accumulated data amount A second step of calculating and a third step of determining whether or not to use the moving average of the new accumulated data amount for calculating the average value of the moving average of the accumulated data amount based on the value indicating the fluctuation Step, and, if used, a fourth step of updating the average value with a moving average of the new accumulated data amount, and clock oscillation based on the average value. There and having a fifth step of controlling the output to the clock frequency.

  By obtaining an average value of “moving average of stored data amount” and using it for clock recovery, even if a small fluctuation occurs in “moving average of stored data amount”, the fluctuation is eliminated and stable. Realize clock recovery at frequency.

  However, when a large change occurs in the new “moving average of accumulated data amount”, the average value also changes. Accordingly, the fluctuation determination means determines whether or not the fluctuation is a large fluctuation based on the index indicating the fluctuation amount of the “moving average of accumulated data amount”. If it is determined that the fluctuation is large, the "moving average of the accumulated data" is controlled so that it is not used for calculating the average value. it can. As a fluctuation amount index, a new “moving average of accumulated data amount” and a difference between average values are used, and it is determined whether or not there is a large fluctuation based on a threshold value. The threshold is determined from a value indicating the degree of dispersion, such as a standard deviation, or a predetermined number of times that indicates the degree of dispersion of the “moving average of accumulated data amount”.

  The best mode for carrying out the present invention will be described in detail below with reference to the drawings. FIG. 5 is a block diagram of a clock recovery apparatus according to the present invention. According to FIG. 5, the clock recovery device includes a data extraction unit 1, a buffer 2, a moving average calculation unit 3, a fluctuation removal unit 4, a clock frequency control unit 5, a clock oscillator 6, and a data reading unit 7. It has. Further, the fluctuation removal unit 4 includes a standard deviation calculation unit 41, an average value calculation unit 42, and a fluctuation determination unit 43.

  The data extraction unit 1 extracts the data contained in the received packet and stores it in the buffer 2. The data reading unit 7 stores the data stored in the buffer 2 at a speed proportional to the reproduction clock frequency output from the clock oscillator 6. Is read.

  The moving average calculation unit 3 obtains a moving average of the amount of accumulated data stored in the buffer 2 in each calculation section, with a period at which the packet is transmitted or a period that is an integral multiple of the period being transmitted.

  The average value calculation unit 42 calculates the average value of “moving average of accumulated data amount”, and the standard deviation calculation unit 41 calculates the standard deviation of “moving average of accumulated data amount”. However, when a new “moving average of accumulated data amount” is input from the moving average calculation unit 3, the fluctuation determination unit 43 sets the new “moving average of accumulated data amount” to “moving average of accumulated data amount”. When the average value and the standard deviation are determined not to be used for calculating the average value and the standard deviation, the average value calculating unit 42 and the standard deviation calculating unit 41 replace the new “moving average of the accumulated data amount” with the “moving average of the accumulated data amount”. It is not used to calculate the average value and standard deviation of "".

  The determination of whether or not the new “moving average of accumulated data amount” is used for calculation of the average value and the standard deviation by the fluctuation determining unit 43 is performed based on the new fluctuation amount of “moving average of accumulated data amount”. Specifically, when the fluctuation amount is less than a predetermined threshold value, it is determined that the average value and the standard deviation are updated with a new “moving average of accumulated data amount”. The new “moving average of accumulated data amount” is determined not to update the average value and the standard deviation. In addition, you may determine not updating, when updating below a threshold value and larger than a threshold value.

  Here, the fluctuation determination unit 43 calculates the average value as the fluctuation amount of the “moving average of accumulated data amount” and the new input value from the moving average calculation unit 3 and the time point before the new input value is input. Using the difference from the average value of the “moving average of accumulated data amount” output from the calculation unit 42 as a threshold, the standard deviation of the “moving average of accumulated data amount” obtained by the standard deviation calculation unit 41 is used as a threshold. Use the required value. More specifically, the threshold is a value determined by calculation from the standard deviation, such as a predetermined number of times the standard deviation, for example, twice, and if the variation is equal to or greater than the threshold, the average of the “moving average of accumulated data” Suppresses updating of values and standard deviations. FIG. 4 is a schematic diagram for explaining processing of the fluctuation removing unit 4 for packet reception shown in FIG.

  The clock oscillator 6 is an oscillator that changes the clock frequency based on an external control signal, such as a voltage-controlled crystal oscillator, and the clock frequency control unit 5 is based on the average value of “moving average of accumulated data amount”. 6 outputs a control signal for controlling the reproduction clock frequency output from 6.

  As described above, in the clock recovery apparatus according to the present invention, the average value calculation unit 42 obtains the average value of “moving average of accumulated data amount” and uses it for clock recovery, thereby obtaining “moving average of accumulated data amount”. Even if fine fluctuations occur, the fluctuations are removed and clock reproduction at a stable frequency is realized. However, when a large fluctuation occurs in the “moving average of the accumulated data amount”, the average value of the “moving average of the accumulated data amount” also fluctuates. When the “average” fluctuates greatly, control is performed so that the value is not used for updating the average value of “moving average of accumulated data amount”. As a result, clock regeneration at a stable frequency can be realized. A standard deviation of “moving average of accumulated data amount” indicating the degree of data dispersion is used as a reference for determination by the variation determination unit 43.

FIG. 6 is a flowchart of the moving average fluctuation removal processing of the accumulated data amount in the clock recovery method according to the present invention.
(S61, S62) The average value and the standard deviation are calculated from the past “moving average of accumulated data amount”.
(S63) It is determined whether or not the difference between the new “moving average of stored data amount” and the current “moving average of stored data amount” is less than a predetermined number of times the current standard deviation.
(S64) If the standard deviation is less than a predetermined number of times, the average value and standard deviation of the “moving average of accumulated data amount” are updated based on the new “moving average of accumulated data amount” value.

  FIG. 7 is a simulation result of the clock recovery method according to the present invention. In the simulation model, the data transmission device transmits a packet at a period of 8 unit time, and adds a random transmission delay time for each packet. The transmission is extremely large compared to other packets at a rate of about 1%. It was assumed that a delay time occurred. In the data receiving apparatus, each time a packet is received, the accumulated data amount is increased by one unit, and 0.125 unit data is subtracted from the accumulated data amount in one unit time period. “Moving average of accumulated data”.

  Since the amount of accumulated data continues to decrease until a new packet is received, the amount of accumulated data becomes extremely small every time a packet with a large transmission delay occurs. In the graph of FIG. 7, “moving average of accumulated data amount” protrudes downward in several places. In this simulation, the average value and standard deviation of 100 “moving averages of accumulated data” are calculated at 64 unit time intervals, and the threshold value, that is, the average value and the standard deviation is updated twice the standard deviation. Used to determine whether or not. FIG. 7 also shows a simple average of “moving average of accumulated data amount” for comparison.

  As is apparent from FIG. 7, the “moving average of accumulated data amount” fluctuates greatly, and if this is used as it is for clock frequency control, the reproduction clock frequency also fluctuates drastically, and a clock having a stable frequency cannot be reproduced. In addition, the fluctuation of the simple average is considerably suppressed compared to the “moving average of accumulated data amount”, but the value of the simple average also fluctuates when the “moving average of accumulated data amount” fluctuates greatly. The clock frequency also varies accordingly. In contrast, in the clock recovery according to the present invention, the values used for the clock frequency control hardly change, and it can be seen that a clock with an extremely stable frequency can be generated.

  In the above description, the average value and standard deviation of the “moving average of accumulated data amount” are used. However, instead of the average value, the amount indicating the average value of the data such as the mode and the median value is represented by the standard deviation. Instead of this, it is also possible to use an amount indicating the degree of data dispersion such as variance or quartile.

It is a figure which shows the change of the moving average of accumulation | storage data amount and accumulation | storage data amount when a packet reception interval is constant. It is a figure which shows the fluctuation of a packet. It is a figure which shows the change of the moving average of accumulation | storage data amount and accumulation | storage data amount when a packet reception interval is not constant. It is a figure explaining the process of the fluctuation | variation removal part with respect to the packet reception shown in FIG. 1 is a block diagram of a clock recovery device according to the present invention. It is a flowchart of the fluctuation removal process of the moving average of the amount of stored data in the clock recovery method according to the present invention. It is a simulation result of the clock reproduction | regeneration method by this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Data extraction part 2 Buffer 3 Moving average calculation part 4 Fluctuation removal part 41 Standard deviation calculation part 42 Average value calculation part 43 Fluctuation judgment part 5 Clock frequency control part 6 Clock oscillator 7 Data reading part

Claims (4)

Data extraction means for storing data contained in the received packet in a buffer;
A moving average calculating means for calculating a moving average of the amount of accumulated data stored in the buffer, using a period that is an integral multiple of the period in which the packet is transmitted as a calculation interval;
Means for calculating an average value of the moving average of the accumulated data amount;
Clock frequency control means for controlling the clock frequency output from the clock oscillator based on the average value;
Data reading means for reading data from the buffer at a rate proportional to the clock frequency output by the clock oscillator;
When the moving average calculator calculates the moving average of the new accumulated data amount, it calculates a value indicating the fluctuation of the moving average of the new accumulated data amount, and calculates the average value of the moving average of the new accumulated data amount. Fluctuation determining means for determining whether or not to use based on a value indicating the fluctuation;
A clock recovery device comprising: The value indicating the fluctuation is a difference between the moving average of the new accumulated data amount and the average value,
2. The clock according to claim 1, wherein the variation determination unit determines that the moving average of the new accumulated data amount is not used for calculating an average value when the difference is equal to or larger than the threshold value or greater than the threshold value. Playback device. A means for calculating a value indicating the degree of dispersion of the moving average of the accumulated data based on the moving average of the accumulated data used for calculating the average value;
The clock recovery device according to claim 2, wherein the threshold is determined from a value indicating the degree of dispersion. A method of regenerating a clock in a data receiving device based on an accumulated data amount of a buffer in which data included in a received packet is accumulated and data is read at a rate proportional to a clock frequency output from a clock oscillator,
A first step of calculating a moving average of the amount of accumulated data stored in the buffer, with a period that is an integral multiple of the period in which the packet is transmitted as a calculation interval;
A second step of calculating a value indicating a variation of the moving average of the new accumulated data amount;
A third step of determining whether or not to use the moving average of the new accumulated data amount for calculating the average value of the moving average of the accumulated data amount based on the value indicating the variation;
When using, a fourth step of updating the average value with a moving average of the new accumulated data amount;
A fifth step of controlling the clock frequency output by the clock oscillator based on the average value;
A clock recovery method comprising:

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