Note: Descriptions are shown in the official language in which they were submitted.
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SYNCHRONIZATION OF ASYNCHRONOUS DATA SIGNALS
This invention relates to a method of and apparatus for
synchronizing an asynchronous data signal to produce a synchronized
data signal.
It has long been known to use stuffing techniques in order
to produce a data signal, which is synchronized to a local clock
frequency, from an incoming data signal which is asynchronous to the
local clock frequency. The synchronized data signal can then
conveniently be switched or multiplexed and transmitted with other,
similarly synchronized, data signals.
The most frequently used stuffing technique is positive
stuFfing, in which it is assured that the frequency of the
synchronized data signal is equal to or greater than the highest
possible frequency of the asynchronous data signal, and frequency
differences are made up by the insertion of stuff bits. For
example, an asynchronous DS1 data signal has a frequency of
1.544Mb/s +/-20t)b/s, and may be converted by positive stuffing into
a synchronized data signal with a frequency oF at least 1.5442Mb/s.
Generally, a higher frequency than this is used for the synchronized
data slgnal in order to enable walt~ng time jltter, which arises as
a result of the stuffing process ancl typically has a Frequerlcy
component equal to the stuffing frequency, to be subsequently
filtered out from the synchronized data signal.
Recently, synchronous communications networks, such as that
using the so-called SONET format, have become of increasing
importance in the communication of data signals. In such a network
various proposals for synchronizing asynchronous data signals,
especially asynchronous DS1 data signals, have been made.
In one such proposal, an incoming asynchronous data signal
can have a frequency which is either lower or higher than the
synchronized data signal frequency, and a synchronizing arrangement
is required in order to effect positive or negative stuffing,
respectively, to produce a synchronized data signal from the
asynchronous data signal. Whereas positive stuffing comprises
providing a stuff bit in the synchronized data signal to compensate
For a relatively lower asynchronous data signal frequency, negative
stuffing comprises using a "spare' bit of the synchronized data
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signal for transmitting data to compensate for a relatively higher
asynchronous data signal frequency.
With this positive/negative stuffing, the synchronized data
signal has a waiting time jitter component at a frequency which is
equal to the rate of positive or negative stuffing. The closer the
asynchronous data signal frequency is to the synchronized data
! signal frequency, the lower will be the stuffing rate and hence the
jitter component frequency, rendering it more difficult to filter
out the jitter from the synchronized data signal. This problem can
be avoided in the manner described and claimed in McEachern et al.
Canadian patent application No. 510,260 filed May 29, 1986 and
entitled "Synchronization of Asynchronous Data Signals". In the
invention of that application, threshold values, with which phase
differences between writing to and reading from an elastic store are
compared to produce stuff requests, are modified in such a manner
that additional positive and negative stuffing takes place whereby
the stuffing frequency is increased, enabling the resulting jitter
to be filtered out using a phase-locked loop.
In another SONET proposal, described in detail below, an
incomlng asynchronous data signal is subjected only to positive
stuFfing. In the prior art, it would be appropriate to select an
appropriate stuff ratio for this positive stuFfing, in well-known
manner for achieving an acceptably low level of jitter, and to use
this without further concern for jitter. It has been found,
however, that following such prior art procedures for the SONET
proposal does not result in the anticipated low level of jitter9 but
rather in an unacceptable higher jitter level. It is believed that
this difference stems from the distribution of probabilities that
stuffing will take place in particular stuffing opportunities; the
prior art techniques inherently assume (although it does not appear
to be discussed in the prior art) an equal probability of stuffing
for all stuffing opportunities, whereas such an equal probability
distribution does not apply to stuffing in the proposed SONET
format,
An object of this invention, therefore, is to provide an
improved synchronizing method and apparatus in which this problem is
reduced or substantially avoided.
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According to one aspect this invention provides a method of
synchronizing an asynchronous signal to produce a synchronized
signal by stuffing the asynchronous signal in dependence upon a
stuff request signal produced from a comparison of phases of the
asynchronous and synchronized signals, wherein the synchronized
signal comprises time division multiplexed frames each including
data bits, at least one stuffing opportunity bit, and at least one
overhead bit, and a plurality of said frames constitute a superframe
in which at least two frames comprise di-fferent numbers of data
and/or overhead bits, comprising the step of modifying the
production of the stuff request signal in dependence upon a waveform
having a period equal to that of the superframe thereby to
distribute stuffing among said frames in the superframes.
Thus in accordance with the invention a waveform is used to
modify the production of stuff requests in such a manner as to
compensate for the biassed timing of such requests which would
otherwise occur, relative to the frame timing in the superframes,
due to the unequal dlstribut-ion of data and/or overhead bits among
the frames. To this end, the waveform is preferably a function of
the differences in the number of data and/or overhead bits in the
plurality of frames in the superframes, and the number of frames in
each superframe,
In an embodiment of the invention described below the frames
of the synchronized signal contain, on average, the same number of
data bits as the frames of the asynchronous signal. In such a case
typically all of the frames contain the same number of overhead bits
~e.g. framing information and stuff indication bits), and there are
different numbers of data bits in individual frames of the
superframe. Considered alternatively, there are a fixed number of
overhead bits and a variable number of data bits between successive
stuffing opportunity bits.
However, the data bits of the asynchronous signal may be
mapped into the synchronized signal format in many different ways.
In particular, it may be mapped so that there are a variable number
of overhead bits and a fixed or variable number of data bits between
successive stuffing opportunity bits, so that different frames in
the stuffing superframe comprise different numbers of overhead bits,
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and possibly also different numbers of data bits.
In the embodiment of the invention described below there is
a difference of one between the number of data bits in one of the
frames of the superframe and the number of data bits in each other
frame of the superframe, and the waveform comprises a generally
sawtooth waveform having an amplitude which changes between
successive frames by an amount substantially equal to a phase
difference of one bit divided by the number of frames in each
superframe. As described below there are four frames in each
superframe, one of the frames containing one more data bit than each
other frame. The waveform is in this case a stepped sawtooth
waveform having a period of 4 frames and naving an amplitude which
is stepped between consecutive frames by an amount equivalent to a
phase difference of one quarter of a bit~
Conveniently a phase difference between the asynchronous and
synchronized signals is compared with a threshold value and the step
of modifying the production of the stuff request signal comprises
modifying the threshold level or the phase difference in dependence
upon said waveform.
This invention also provides synchronizing appara-tus
comprising: means responsive to a stufF request signal for stufflng
an asynchronous data si~qnal to produce a synchronized data signal
comprising time division multiplexed frames each including data
bits, at least one stuffing opportunity bit, and at least one
overhead bit, a plurality of said frames constituting a superframe
in which at least two frames contain different numbers of data
and/or overhead bits; means for producing the stuff request signal
in dependence upon the relative phases of the asynchronous and
synchronized data signals; and modifying means for modifying the
production of the stuff request signal in dependence upon a waveform
having a period equal to that of the superframe thereby to
distribute stuffing among said frames in the superframes.
The invention will be further understood from the following
description with reference to the accompanying drawings, in which:
Fig. 1 is a diagram illustrating a proposed SONET format for
accommodating an asynchronous DS1 data signal in a synchronized
tributary;
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Fig. 2 is a block diagram illustrating a synchronizing
arrangement in accordance with an embodiment of this invention; and
Figs. 3 and 4 are diagrams illustrating phase relationships
and stuffing, with reference to which the problem addressed by this
invention and its solution by the embodiment of Fig. 2 are explained.
Referring to Fig. 19 a proposed SONET format for mapping an
asynchronous DS1 data signal into a synchronized tributary data
signal, referred to below for simplicity as a tributary, is
illustrated. Such a tributary consists of 26 8~bit words which are
numbered 0 to 25 in Fig. 1, and may be multiplexed with other such
tributaries in a synchronous, word-interleaved manner for example as
described in Graves et al. Canadian patent application No. 494,466
filed November 1, 1985 and entitled "Method of Multiplexing Digital
Signals". More particularly, a plurality of such tributaries may be
multiplexed together to form one frame of a so-called STS-1 signal
in the SONET format. Accordingly, and for consistency, the contents
of the 26 words represented in Fig. 1 are referred to as one frame
of tributary data.
The tributary frame contalns overhead informat~on including
a pointer as word 0, and other overhead inFormation 0ll as bits 1 and
2 of word 1, with which the present invention is not concerned and
accordingly which are not further discussed here. The third bit of
; word 1, referred to as an I bit, is an information bit which
identifies tributary frames. For example the I bit is a binary 1
for three consecutive tributary frames and is a binary 0 for the
next, or fourth, tributary frame, this pattern repeating cyclically
for successive groups of tributary frames thereby defining a ~-frame
superframe. This 4-frame stuffing superframe format is assumed for
the remaining description, but it should be understood that other
formats may similarly be used.
The first bit, marked D/X, of word 21 in each tributary
frame is used as a data bit in every fourth tributary frame, when
-the bit I is a binary 0, and is discarded or is used for other
purposes in the other three frames of each stuffing superframe. The
second bit, marked S0, of word 21 is a stuffing opportunity bit,
which is used as a data bit or a stuffed data bit to provide the
desired positive stuffing. The use of the S0 bit in each tributary
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frame is represented by the first bit of words 6, 11, and 16 of the
frame, which are redundantly provided for reliability and which are
referred to as stuff indication or C bits corresponding to
nomenclature used for DS-3 signals. The C bits all have the same
binary value (so that a majority decision can be reliably reached
after transmission even in the presence of single bit errors)
representing that the S0 bit is a data bit or a stuffed bit.
Fig. 1 illustrates the number of bits remaining in each of
words 1, 6, 11, 16, and 21, which ~ogether with the eight bits of
each of the remaining 20 words constitute a total of 192 bits. If
the S0 bit is used as a data bit in each frame, there are thus 193
data bits in each of 3 frames, and 194 data bits in every fourth
frame, for an average of 193.25 data bits per frame. If the S0 bit
is used as a stuffed data bit in each frame, there are 192 data bits
in each of 3 frames, and 193 data bits in every fourth frame, For
an average of 192.25 data bits per frame. In practice, nominally
the 193 bits per frame of an asynchronous DS1 data signal will be
mapped lnto the tributary Frame, wlth an arbitrary orien-tation, with
one S0 bit in every four tributary frames used as a stuffed bit and
the other three S0 bits in every four tributary frames used as data
bits, to achieve an average of 193 data bits per frame and a nominal
stuff ratio of 0.25.
Fig. 2 illustrates a synchronizing arrangement which may be
used in accordance with the invention for converting an asynchronous
DS1 data signal incoming on a line 10 into the synchronized
tributary data on a line 12. A multiplexer 14 effects positive
stuffing under the control of stuff control signals supplied thereto
by a timing and control circuit 16, and also multiplexes the other
overhead information shown in Fig. 1 into the tributary data on the
line 12.
The asynchronous data is written into a cyclic or elastic
store 18 at addresses supplied by a write address generator 20
supplied with a recovered clock signal produced by a clock recovery
circuit 22 from the asynchronous data bit stream on the line 10, and
is read from the store 18 to the multiplexer 14 under the control of
a read address generator 24 supplied with a gapped clock signal from
the circuit 16. In order to determine when a stuff is necessary, a
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phase comparator 26~ when enabled by the circuit 16 via a line 28,
compares the relative phases of writing to and reading from the
; store 18, and supplies a comparison signal to a stuff control
decision circuit 30. Because the relative writing and reading
phases will vary considerably during each frame in view of the
presence of the overhead information on the tributary data as shown
in Fig. 1, the phase comparator 26 is enabled by the circuit 16 to
make a phase comparison for example only once in each frame, at the
same point in consecutive frames.
10The stuff control decision circuit 30 produces stuff
requests on the basis of the phase comparison relative to a
threshold level. Traditionally, this would have been a fixed
threshold level as described below. In the present invention,
however, this threshold level is cyclically varied or modulated with
a waveform having the same period as the stuffing superframe, for
the reasons explained fully below. Accordingly, a threshold
modulation waveform generator 32 is provided, synchronized by the
circuit 16, to generate this waveform, which is summed with, and
hence modlfles, a Fixed threshold in a summer 3~, the resultant
modulated threshold bein~ supplied to the circuit 30 for comparison
with the phase comparator output signal.
The problem which the present invention addresses is now
described with reference to Fig. 3, which is a diagram illustrating
as a function of time a net relative phase signal which corresponds
to the phase difference detected by the phase comparator 26, shown
as a solid line 36, for the case of a fixed, constant amplitude,
threshold level, as would traditionally have been used in
synchronizing arrangements, shown as a broken line 38. It should be
appreciated that the line 36 does no-t represent the actual output
signal of the phase comparator in view of the cyclical relative
phase changes which take place within each frame and the consequent
cyclical enabling of the comparator, but the line 36 does represent
the relative phase difference between writing to and reading from
the store 18 from one frame to the next, hence the term "net
relative phase" is used.
At the bottom of Fig. 3, frame numbers in the stuffing
superframe are shown, thereby illustrating the time scale oF the
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drawing, and those frames in which a stuff takes place are
identified by a letter S in a line marked "STUFFING". Frame 4 in
each stuffing superframe sequence is assumed to contain the data bit
as the first bit in word 21, and accordingly there is a positive
phase change of 1 bit shown in the line 36 at the end of each frame
number 4, at times marked P in Fig. 3. In response to each stuffed
bit, a negative phase change of 1 bit is shown in the line 36 at the
end of the frame in which the stuff takes place, at times marked Q.
The line 36 has an overall (much exaggerated) positive slope,
corresponding to the asynchronous DS1 data signal having an actual
frequency which is slightly less than its nominal frequency of
1.544Mb/s, so that for synchronization the actual stuff ratio,
averaged over a long period of time, is slightly more than the
nominal stuff ratio of 0.25.
Each time that the line 36 crosses above the threshold level
38, the stuff con-trol decision circuit produces a stuff request to
cause a stuffed bit at the next stuffing opportunity, i.e. in the
next frame, to cause the line 36 to cross back below the level 38.
With the exception of the crosslng at a tinle t, due to the slope oF
the line 36, each crossing above the level 38 occurs at a time P due
to the extra data bit in each fourth frame, so that stuffing takes
place in each case in frame number 1. Only in the frame following
the time t does this situation change, with a stuff taking place in
this example in frame number 3.
Thus Fig. 3 illustrates that the long-term average stuff
ratio of nominally 0.25 is misleading, and that in fact the nominal
stuff ratio is substantially 1 for each frame number 4 and O for
each other frame. The slope of the line 36 gives rise to crossings
such as at the time t, with consequent extra (or, in the case of an
incoming asynchronous signal having a frequency above the nominal
frequency, omitted) stuffs at a low frequency, giving rise to an
unacceptable jitter component at such a low frequency.
Fig. 4 shows, in a manner and using references similar to
Fig. 3, how the invention avoids this problem. In this case the
threshold level is modulated with a stepped waveform having a
period equal to that of the stuffing superframe, the steps
corresponding to relative phase differences of three-quarters, half,
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one-quarter, and zero bits in frame numbers 1, 2, 3, and 4
respectively.
As in Fig. 3, at times marked P and Q there are positive and
negative, respectively, phase changes each of 1 bit. At one time,
marked P,Q, these changes cancel one another so that the line 36 is
not stepped at this time. In contrast to the situation shown in
Fig. 3, in Fig. 4 the stepped nature of the threshold level 38 means
that the line 36 does not consistently cross above the level 38 at
each time P, and so stuffing does not consistently take place in
frame number 1. On the contrary, stuffing now takes place in a
variable manner in various frames of the stuffing superframes, but
at the same average rate. The redistribution of stuffing among the
stuffing opportunities more accurately corresponds to the
traditionally accepted characteristics for the chosen nominal stuff
; 15 ratio of 0.25.
More particularly, whereas the stuffing representecl ln ~ig.
3 produces a ji-tter componen-t at d relatively high frequency, easlly
removed by filtering in a phase locked loop, due to the periodlc
stuffing in each frame number 1, and a jitter component at a
relatively low frequency, which is very difficult to remove, due to
the overall slope of the line 36, in contrast the stuffing
represented in Fig. 4 produces significant jitter components only at
relatively high frequencies, which are easily removed to leave an
acceptably jitter-free signal.
From the above description, it can be seen that the waveform
yenerator 32 can easily produce the stepped waveform shown in Fig. 4
in synchronism with the stuffing superframe, and can simply be in
the form of a counter, with a digital-to-analog converter if the
summer 34, comparator 26 and circuit 30 are implemented using analog
components (these may instead be implemented using digital
techniques). Alternatively, however, the generated waveform may be
a sawtooth waveform, or some other waveform consistent with the
object to vary the frames in which stuffing takes place in the
manner described above. Furthermore, the amplitude of the waveform
may be changed frorn that described above. Different waveforms may,
in particular, be appropriate for different synchronized data
formats, the waveform period in any event corresponding to that of
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the stuffing superframes.
Although the invention has been described above in relation
to positive stuffing of an asynchronous DS1 data signal, it is
also applicable to negative stuffing and to the synchronization of
signals with other bit rates and formats, in particular DSlC,
DS2, and European 2.048Mb/s data signalsO
Furthermore, it should be appreciated that the principles of
this invention may be applied either alone or in combination,
especially in the case of +J-/0 stuffing, with the principles
described in the McEachern et al. patent application already
referred to. If the principles are combined, it should be noted
that the threshold level with which the monitored phase difference
is compared to produce stuff requests will be subject to the sum of
two modifying waveforms, namely one as described above with a period
equal to that of the stuffing superframes, and one as described in
the McEachern et al, application which typically will have a
different frequency.
It should also be no-ted that, although as described above
the threshold level is modified by the described waveforrn,
alternatively the output of the phase comparator may be modified in
a complementary manner to achleve the same resultsO
Numerous other modifications, variations, ancl adaptations
may be made to the described embodiment without departing from the
scope of the invention as defined in the claims.
