JP2896780B2 - Synchronization method and synchronization device for asynchronous data signal - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a method and apparatus for synchronizing an asynchronous data signal to generate a synchronous data signal.

Prior art From an input data signal that is asynchronous to the local clock frequency,
It is known to use stuffing techniques to generate data signals synchronized to a local clock frequency. Synchronous data signals are exchanged or multiplexed with other similarly synchronized data signals and transmitted.

The most frequently used stuffing technique is positive stuffing, where the frequency of the synchronous data signal is greater than or equal to the highest possible frequency of the asynchronous data signal, and the frequency difference is created by the insertion of stuff bits. For example, an asynchronous DS1 data signal is 1,544 Mb / s
It has a frequency of +/- 200 b / s and is converted by positive stuffing into a synchronous data signal having a frequency of at least 1.5442 Mb / s. Generally, higher frequencies are
Used for the synchronization data signal, it results from the stuffing process and allows the latency jitter typically having a frequency component equal to the stuffing frequency to be subsequently filtered from the synchronization data signal.

SUMMARY OF THE INVENTION Recently, synchronous communication networks, such as in the use of the so-called SONET format, have become increasingly important in the communication of data signals. In such networks, many proposals have been made to synchronize asynchronous data signals, especially asynchronous DS1 data signals. In one such proposal, the input asynchronous data signal has a frequency that is either lower or higher than the synchronous data signal frequency, and uses positive or negative stuffing, respectively, to generate a synchronous data signal from the asynchronous data signal. A synchronizing device to perform this is required. Positive stuffing provides stuff bits in the synchronous data signal to compensate for the relatively low asynchronous data signal frequency, while negative stuffing provides data transmission to compensate for the relatively high asynchronous data signal frequency. Use the "spare" bit of the synchronous data signal of

Due to this positive / negative stuffing, the synchronization data signal has a latency jitter component at a frequency equal to the positive or negative stuffing rate. The closer the asynchronous data signal frequency is to the synchronous data signal frequency, the lower the stuffing rate, here the jitter component frequency, and the more difficult it is to filter jitter from the synchronous data signal. This problem was addressed by Canadian Patent Application No. 510, filed May 29, 1986 by Mac Echan et al.
No. 260, "Synchronization of Asynchronous Data Signals". In the invention of that application,
The threshold for comparing the phase difference between writing to elastic memory and reading from elastic memory and generating a stuffing request is modified so that additional positive and negative stuffing occurs, thereby increasing the stuffing frequency. As a result, the jitter is filtered using a phase locked loop.

In another SONET proposal described in detail below, the incoming asynchronous data signal undergoes only positive stuffing. In the prior art, an appropriate stuffing ratio for positive stuffing was selected to achieve acceptable low levels of jitter by known methods and to be free of jitter. However, prior art procedures for SONET proposals have shown that jitter does not go to the expected low level, but rather to an unacceptably high jitter level. This difference is believed to result from the probability distribution that stuffing occurs at special stuffing opportunities. Although the prior art essentially assumes equal stuffing probabilities for all stuffing opportunities (although it does not seem to be discussed in the prior art), such an equal probability distribution has been proposed. Not applicable to SONET format stuffing.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an improved synchronization method and apparatus in which this problem is substantially avoided. According to a first aspect of the invention, the invention comprises a synchronization signal comprising a time division multiplexed frame comprising a plurality of frames, each of the time division multiplexed frames comprising a data bit and at least one stuffing opportunity bit. And at least one
A time-division multiplexed frame comprising the plurality of frames comprises a superframe, wherein at least two of the frames include a different number of data and / or overhead bits; A method for synchronizing the asynchronous signal by stuffing the asynchronous signal to generate a synchronous signal in accordance with the stuff request signal generated by comparing the phases of the synchronous signal and the synchronous signal. By comparing and generating a comparison signal, supplying a fixed threshold and a threshold correction waveform synchronized with the superframe, and correcting the fixed threshold with the threshold correction waveform, a threshold correction value is generated, and the comparison signal and the threshold correction value are generated. Generates a modified staff request signal from the Configured to distribute the stuffing between frames.

Preferably, the threshold correction waveform synchronized with the superframe is generated by a waveform having a period equal to the period of the superframe. The waveform is also a function of the difference between the number of data bits in a plurality of superframes and the number of frames in each superframe.

Also preferably, there is a difference of 1 between the number of data bits in one frame of the superframe and the number of data bits in each of the other frames of the superframe, and the waveform has one bit between successive frames. Is a sawtooth-shaped waveform having a different amplitude by a value obtained by dividing the phase difference by the number of frames in each superframe. Further preferably, the waveform is
It is a step-like sawtooth waveform. Also, preferably, each superframe includes four frames.

Further, preferably, generating the modified stuff request signal modifies either the threshold level or the phase difference.

According to a second aspect of the present invention, the present invention provides a method for generating a synchronous data signal including a time division multiplexed frame in response to a stuff request signal for stuffing an asynchronous data signal, wherein each time division multiplexed signal is generated. The frame includes data bits, at least one stuffing opportunity bit, and at least one overhead bit, wherein the plurality of frames constitute a superframe, wherein at least two frames have a different number of data and / or bits. Or means including overhead bits, means for comparing the relative phases of the asynchronous and synchronous data signals and generating a comparison signal, means for providing a fixed threshold and a threshold correction waveform synchronized with the superframe, Means for generating a threshold correction value by correcting the threshold with a threshold correction waveform; To generate a modified stuff request signal and a threshold modification value, configured to include a correction means thereby distributing the stuffing between frames in a superframe.

Preferably, the threshold correction waveform synchronized with the superframe is generated by a waveform having a period equal to the period of the superframe. The waveform is also a function of the difference between the number of data bits in a plurality of superframes and the number of frames in each superframe.

Also preferably, there is a difference of 1 between the number of data bits in one frame of the superframe and the number of data bits in each of the other frames of the superframe, and the waveform has one bit between successive frames. Is a sawtooth-shaped waveform having a different amplitude by a value obtained by dividing the phase difference by the number of frames in each superframe. Further preferably, the waveform is
It is a step-like sawtooth waveform. Further, preferably, each superframe includes four frames.

Further, preferably, the means for generating a modified staff request signal modifies either the threshold level or the comparison signal.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood from the following description with reference to the accompanying drawings, in which: FIG. Referring to FIG. 1, a proposed SONET format for mapping an asynchronous DS1 data signal to a synchronous dependent data signal is shown. Such dependent data are numbered 0 through 25 in FIG.
US patent application Ser.
As described in U.S. Pat. No. 494,466 "How to multiplex digital signals", it is multiplexed with other dependent data by a synchronized word interleaving method. More specifically, a plurality of such dependent data are multiplexed together in a SONET format to form one frame of a so-called STS-1 signal. Therefore, for consistency,
The content of 26 words represented in FIG. 1 is called one frame of dependent data.

The dependent frame contains the pointer overhead information shown in word 0 and the other OH overhead information of bits 1 and 2 of word 1, which will not be discussed further as the present invention is not relevant. The third I bit of word 1 is an information bit that identifies the dependent frame. For example, the I bit is binary 1 for three consecutive dependent frames and binary 0 for the next, ie, fourth, dependent frame, and the pattern is periodic for consecutive groups of dependent frames. , Thereby defining a superframe of four frames. In the following description, this 4-frame stuffing superframe format is used, but other formats can be used as well.

Indicated in the first bit of word 21 in each dependent frame
D / X indicates that when bit I is binary 0 and the data in each of the fourth subordinate frame is discarded or used for other purposes in the other three frames of each stuffing superframe, Used as a bit. The SO indicated in the second bit of word 21 is a stuffing opportunity bit, which is used as a data bit or stuffed data bit to provide the desired positive stuffing. SO in each dependent frame
The bits are represented by the first bit of words 6,11,16 of the frame and are provided redundantly for reliability and DS-3
It is called a stuff indicator or C bit, corresponding to the standard name used in the signal. All C bits have the same binary value (there is a reliable majority after transmission with one bit error), indicating that the SO bit is a data bit or a stuff bit.

FIG. 1 shows the number of bits remaining in each word 1, 6, 11, 16 and 21, which together with each 8 bits of the other 20 words make up a total of 192 bits. If the SO bit is used as a data bit in each frame, 3
Each of the frames is 193 data bits, and there are 194 data bits for every four frames, averaging 193.25 data bits per frame. If the SO bit is used as a stuff data bit in each frame, each of the three frames is 192 data bits,
There are 193 data bits for every four frames, averaging 192.25 data bits per frame. In fact, nominally 193 bits per frame of an asynchronous DS1 data signal are mapped to dependent frames in any orientation, one SO bit per four dependent frames is used as a stuff bit, and four dependent The other three per frame
The SO bits are used as data bits and can achieve an average of 193 data bits per frame and a nominal stuff ratio of 0.25.

FIG. 2 shows a synchronizing device of the present invention, which converts an asynchronous DS1 data signal input to an input line 10 into synchronous dependent data output to an output line 12. Multiplexer 14 performs positive stuffing under the control of a stuff control signal provided by timing and control circuit 16, and
The other overhead information shown in FIG. 1 is multiplexed into dependent data on output line 12.

Based on the recovered clock signal generated from the asynchronous data bit stream on input line 10 by clock recovery circuit 22, the asynchronous data is stored in a circular or elastic memory (hereinafter referred to as elastic memory) provided by write address generator 20. Written to 18 addresses. Meanwhile, the circuit
Under the control of the read address generator 24 based on the gap clock signal from 16, the data written to the elastic memory 18 is read to the multiplexer 14. To determine when stuffing is needed, phase comparator 26 compares the relative phases of writing to and reading from elastic memory 18 when enabled by circuit 16 via line 28, The comparison signal is supplied to the stuff control decision circuit 30. Since the relative write and read phases can vary significantly in each frame due to the presence of overhead information on the dependent data, as shown in FIG.
Are enabled by circuit 16 and at the same point in successive frames, for example, only one in each frame.
Times, phase comparison is performed.

The stuff control decision circuit 30 generates a stuff request based on the phase comparison for the threshold level. Traditionally, this threshold level was a fixed threshold level, as described below. However, in the present invention, this threshold level is cyclically changed or modified by a waveform having the same period as the stuffing superframe, as described more fully below. Therefore, in the present invention, a threshold correction waveform generator 32 is newly provided, and the threshold correction waveform generator 32 operates in synchronization with the circuit 16 to generate a waveform having the same period as the above stuffing superframe. This waveform is added (here corrected) to a fixed threshold in an adder 34, and the corrected threshold is supplied to a stuff control decision circuit 30 for comparison with an output signal from a phase comparator.

FIG. 3 illustrates the problem of the present invention. In FIG. 3, a fixed constant amplitude threshold level in the conventional synchronization device is indicated by a broken line. On the other hand, the solid line 36 shows a relative phase signal corresponding to the phase difference detected by the phase comparator 26 as a function of time. Solid line 36 does not represent the actual output signal of the phase comparator in terms of the cyclic relative phase change that occurs within each frame and the resulting periodic enable of the comparator, while solid line 36 does not Expresses the relative phase difference between writing to elastic memory 18 and reading from elastic memory 18 from one frame to the next, and is referred to as the "net relative phase."

At the bottom of FIG. 3, the frame numbers in the stuffing superframe are shown, thereby indicating the time scale, and the frames in which stuffing occurs are identified by the letter S on the "stuffing" line. Frame 4 in each stuffing superframe sequence is where the first bit of word 21 contains a data bit, and thus, as shown by line 36 in FIG.
At the last point of each frame number 4 (point P), a one-bit positive phase change occurs. Also, as shown by the solid line 36, in response to each stuff bit, a one-bit negative phase change occurs at the end (point Q) of the frame in which stuff occurs. The solid line 36 has a positive slope corresponding to an asynchronous DS1 data signal that has been exaggerated and has a real frequency slightly less than the nominal frequency of 1,544 Mb / S overall, and consequently In contrast, the average staff ratio averaged over a longer period is slightly greater than the nominal staff ratio of 0.25.

Each time the solid line 36 exceeds the threshold level 38, the stuff control decision circuit generates a stuff request to generate a stuff bit at the next stuffing opportunity, i.e., in the next frame, and the solid line 36 Also be below. Other than at time t, the slope of the solid line 36 causes an intersection at P above the threshold level 38 due to the additional data bits occurring every four frames, so that stuffing occurs at frame number 1. This situation changes only in the frame after time t, and the stuff occurs at frame number 3 in this example.

Thus, FIG. 3 shows that the long-term average stuff ratio of a nominal value of 0.25 is incorrect, and in fact, the nominal stuff ratio is approximately 1 for each frame number 4 and for each other frame. Indicates that it is 0. The slope of the solid line 36 causes an intersection at time t, and at low frequencies additional stuff (or is omitted for incoming asynchronous signals having frequencies higher than the nominal frequency), and such low frequency Causes an unacceptable jitter component.

FIG. 4 shows how the invention avoids this problem in a manner similar to FIG. In this case, the threshold level is modified with a step-like waveform having a period equal to the period of the stuffing superframe, and the steps are performed at frame numbers 1, 2, 3, and 4 respectively at 3/4 of the relative phase difference. Corresponds to the quarter, quarter, and zero bits. As shown in FIG. 3, there is a positive and negative 1-bit phase change at P and Q, respectively. At times P and Q, these changes cancel each other, so that the solid line 36 is not stepped at this point. In contrast to FIG. 3, in FIG.
The stepped nature of the threshold level 38 means that the solid line 36 is not always above the threshold level 38 at each time P, so that stuffing does not always occur at frame number 1. Conversely, stuffing occurs at the same average rate in many frames of the stuffing superframe. The redistribution of stuffing during stuffing opportunities more accurately corresponds to the conventional tolerance for the chosen nominal step ratio of 0.25.

More specifically, in FIG.
The periodic stuffing at causes relatively high frequency jitter components, which are easily removed by filtering in the phase locked loop, and the overall slope of the solid line 36 produces relatively low frequency jitter components, which are eliminated. It is very difficult. In contrast, FIG. 4 produces large jitter components only at relatively high frequencies, which are easily removed, resulting in an acceptable signal without jitter.

The present invention is described in further detail below. The stuffing opportunity SO occurs for each frame. If there is a staff request, stuffing is performed, and if there is no staff request, no stuffing is performed. The stuffing depends on whether the signal from the phase comparator 26 exceeds a threshold level supplied to the stuff control decision circuit 30.

In the prior art, the threshold level was a fixed threshold level. In the conventional device, the threshold correction waveform generator 32 and the adder 34 of FIG. 2 are not present. In the operation of the conventional circuit, a broken line 38 indicates a fixed threshold level, as shown in FIG.

The D / X bits in each frame are used as data bits in the fourth frame of each superframe,
It is not used as data bits for three frames. This means that "at least two frames contain different numbers of data and / or overhead bits". In the present invention, unlike the first to third frames, an additional bit is included in the fourth frame, so that a positive one-bit relative phase change occurs in the fourth frame of each superframe. At the timing of the end of each frame, detected by the phase comparator 26, the net relative phase indicated by the solid line 36 in FIG. 3 produces a one bit positive change at the end of each frame 4. This changing point is point P in FIG.

The stuff control decision circuit 30 generates a stuff request in response to the net relative phase 36 exceeding the threshold level 38, and stuffing is performed at the next stuffing opportunity SO, thereby generating a one-bit negative phase change. I do. In the prior art shown in FIG. 3, this is done after each P point, so the next stuffing opportunity SO in frame 1 is used for stuffing.
As a result, as shown at point Q in FIG. 3, a negative phase change occurs by one bit at the end of each frame. Further, when the phase line 36 exceeds the fixed threshold line 38 at the time t in FIG. 3, a stuff request is generated and a stuff is generated at the next stuffing opportunity SO (frame 3 in FIG. 3),
A negative phase change occurs at the Q point at the end of the frame including the stuff.

A problem to be solved by the present invention is to remove a jitter component. Generally, it is very difficult to remove low frequency jitter. As shown by the stuffing opportunity S in FIG. 3, in the prior art, the jitter has the following two different components.

(I) Jitter due to periodic regular stuff S in each frame 1. This component has a relatively high frequency so that it can be easily filtered.

(2) Jitter due to stuff that crosses the threshold at time t shown in FIG. This component has a very low frequency,
For this reason, it is difficult to filter.

Therefore, removing low frequency jitter has been a major problem in the prior art.

The apparatus of FIG. 2 of the present invention includes, in particular, a threshold correction waveform generator 32 and an adder 34 which are added to the prior art. Therefore, as shown in FIG. 4, the threshold level 38 is not fixed but changes stepwise as shown in the embodiment of the present invention.

This point P indicates a positive phase change of 1 bit at the end of each frame when the D / X bits in each frame 4 are data bits. The operation of the stuff control determination circuit 30 is the same as described above. That is, at each time, the threshold level 38 crosses the phase line 36, the stuff S occurs at the next stuffing opportunity (next frame), and at the end of the stuff frame, a negative phase change of 1 as shown by point Q. Only bits occur. At points P and Q in FIG.
A one-bit positive phase change due to the / X bit and a one-bit negative phase change due to the stuff in the frame occur. FIG.
This shows that the stuff S is distributed among the frames 1 to 4 of the superframe.

A feature of the present invention is that the stuffing point Q of FIG. 4 produces a large jitter component only at relatively high frequencies, so that this jitter is easily filtered and an acceptable signal without jitter is obtained. In particular, in the present invention, the time at which the stuff request signal is generated by the stuff control decision circuit 30 is modified. As a result of correcting the fixed threshold level with the waveform from the threshold correction waveform generator 32, the frame number 1 in FIG.
As shown in FIG. 4, the stuff S does not occur periodically as shown in FIG.
Staff S are distributed in the frame.

From the above description, the waveform generator 32
The step-like waveform shown in FIG. 4 is generated in synchronization with the super frame. The waveform generator 32 is simply configured in the form of a counter, and is realized using an adder 34, a phase comparator 26, and a stuff control decision circuit 30. If these are implemented using analog components, an analog-to-digital converter is required. However, on the other hand, the generated waveform is a sawtooth waveform or another waveform that matches the waveform for changing the frame, where stuffing is performed in the manner described above. Further, the amplitude of the waveform is different from the above. In particular, it is appropriate to use different waveforms for different synchronization data formats, where the waveform period corresponds to the period of the stuffing superframe.

In the present invention, although described above with respect to positive stuffing of asynchronous DS1 data signals, also negative stuffing and other bit rates and formats, in particular,
Applicable for signal synchronization with DSIC, DS2, and European 2,048 Mb / s data signals.

In addition, the principles of the present invention can be applied alone or in combination, especially in the case of + /-/ 0 stuffing, with the principles described in the already-disclosed patent application of McEchan et al. When this principle is combined, the threshold level for comparing the monitored phase difference to generate a stuff request adds the two modified waveforms. That is, the above-described waveform having a period equal to the period of the stuffing superframe is added to the waveform described in the application of McEchan et al. Which typically has a different frequency.

Also, as described above, the threshold level is modified by the waveform described above, while the output of the phase comparator is modified in a complementary manner to achieve the same result. Numerous other modifications, variations, and adaptations may be made in the embodiments described without departing from the scope of the invention as set forth in the claims.

[Brief description of the drawings]

FIG. 1 illustrates a SONET format proposed for accommodating asynchronous DS1 data signals in synchronous subordination. FIG. 2 is a block diagram illustrating a synchronization device according to an embodiment of the present invention. 3 and 4 are diagrams showing the phase relationship and the stuffing. 10 ... input line 12 ... output line 14 ... multiplexer 16 ... timing and control circuit 18 ... elastic storage circuit 20 ... write address generator 22 ... clock recovery circuit 24 ... read address generator 26 ... Phase comparator 30 stuff control decision circuit 32 threshold correction waveform generator 34 adder 38 threshold

 ────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Thomas Edward Moore K2L1P6 Ontario Kanata Axbridge Crescent, Canada 53 (56) References JP-A-57-212842 (JP, A) JP-A-61-224740 (JP, A) JP-B-7-112183 (JP, B2)

Claims (14)

(57) [Claims] The synchronization signal includes a time division multiplexed frame comprising a plurality of frames, each of the time division multiplexed frames comprising a data bit, at least one stuffing opportunity bit, and at least one overhead bit. The time-division multiplexed frame comprising a plurality of frames comprises a superframe, wherein at least two frames include a different number of data and / or overhead bits, and a phase of the asynchronous and synchronous signals. A method for synchronizing an asynchronous signal to generate a synchronous signal by stuffing an asynchronous signal in accordance with a stuff request signal generated by comparing the relative phases of asynchronous and synchronous data signals. Generate signal, fixed threshold and superframe Providing a synchronized threshold correction waveform, generating a threshold correction value by correcting the fixed threshold with the threshold correction waveform, generating a correction stuff request signal from the comparison signal and the threshold correction value, A method for synchronizing asynchronous data signals, wherein stuffing is distributed between frames in a superframe. 2. The method for synchronizing an asynchronous data signal according to claim 1, wherein the threshold correction waveform synchronized with the superframe is generated by a waveform having a cycle equal to the cycle of the superframe. . 3. The synchronization method according to claim 2, wherein the waveform is a function of a difference between the number of data bits in a plurality of superframes and the number of frames in each superframe. How to synchronize asynchronous data signals. 4. The synchronization method according to claim 2, wherein there is a difference of 1 between the number of data bits in one frame of the superframe and the number of data bits in each of the other frames of the superframe. Is a saw-tooth waveform in which amplitudes differ by a value obtained by dividing a 1-bit phase difference by the number of frames in each superframe between consecutive frames. 5. A method according to claim 4, wherein said waveform is a step-like sawtooth waveform. 6. The method of synchronizing an asynchronous data signal according to claim 1, wherein each superframe includes four frames. 7. The method of synchronizing an asynchronous data signal according to claim 1, wherein the step of generating the correction stuff request signal corrects one of a threshold level and a phase difference. . 8. A synchronous data signal including a time division multiplexed frame in response to a stuff request signal for stuffing an asynchronous data signal, wherein each time division multiplexed frame includes a data bit and at least one data bit. Means for including a stuffing opportunity bit and at least one overhead bit, wherein the plurality of frames constitute a superframe, wherein at least two frames in the superframe include different numbers of data and / or overhead bits; Means for comparing the relative phases of asynchronous and synchronous data signals to generate a comparison signal; means for supplying a fixed threshold and a threshold correction waveform synchronized with a superframe; and correcting the fixed threshold with the threshold correction waveform Means for generating a threshold correction value, the comparison signal and the threshold correction value And a correcting means for generating a corrected stuff request signal from the frames, thereby distributing the stuffing among the frames in the superframe. 9. The synchronization device according to claim 8, wherein the threshold correction waveform synchronized with the superframe is generated by a waveform having a period equal to the period of the superframe. . 10. The synchronizing device according to claim 9, wherein the waveform is a function of a difference between the number of data bits in a plurality of superframes and the number of frames in each superframe. Synchronizer for asynchronous data signals. 11. The synchronization apparatus according to claim 9, wherein there is a difference of 1 between the number of data bits in one frame of the superframe and the number of data bits in each of the other frames of the superframe. Is a saw-tooth waveform having amplitudes different from each other by a value obtained by dividing a 1-bit phase difference by the number of frames in each superframe between consecutive frames. 12. The synchronizing device according to claim 11, wherein the waveform is a step-like sawtooth waveform. 13. The synchronization apparatus according to claim 8, wherein each superframe includes four frames. 14. The synchronization device according to claim 8, wherein said means for generating the correction stuff request signal corrects either a threshold level or a phase difference. .