Note: Descriptions are shown in the official language in which they were submitted.
1144656
This invention relates to a method of transmittfng an
additional signal via a transmission link which is already used to
transmit a digital signal using a redundant transmission code, and to a
transmission system embodying the method.
It is commonly required to transmit an additional signal via
a digital signal transmission link, for example for protection switch
signalling, remote alarm reporting, and providing a voice channel. It is
known to transmit the additional signal via a separate transmission link,
but thfs involves the disadvantage of providing such a separate link.
Alternatively, additional bits corresponding to the additional signal can
be transmitted with the digital signal vfa the transmission link, but thfs
fnvolves the provision of substantial extra circuitry and results in an
fncrease of the transmftted bit rate, both of which are disadvantageous.
Furthermore, it fs known to modify predetermfned bfts of the digital
signal in dependence upon the additfonal signal, but again thfs requires
substantial additional cfrcuitry and results in degradation of the
transmitted digital signal.
An object of this invention, therefore, fs to provide a
method of transmftting an additional signal by means of which these
disadvantages of the prior art are largely or entirely avoided.
According to one aspect of thfs invention there fs provided
a method of transmfttfng an additional signal vfa a transmisslon link vfa
whfch a digftal signal is transmftted from a transmitter to a receiver
using a redundant transmfssion code, comprising:- at the transmitter, in
dependence upon said additional sfgnal, selectfvely modffyfng a
predetermfned code combinatfon, which occurs randomly in the encoded
digital signal, to produce a predetermined code violation, and
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114465*
transmitting the encoded selectively modified digital signal; and at the
receiver, detecting and correcting each such predetermined code violation
to reproduce said additional signal and said digital signal.
According to another aspect of this invention there is
provided a method of transmitting an additional signal via a transmission
link via which a digital signal is transmitted from a transmitter to a
receiver using a redundant transmission code, comprising:- at the
transmitter, in dependence upon said additional signal, selectively
modifying each of two predetermined complementary code combinations, each
of which occurs randomly in the encoded digital signal, to produce
respective predetermined complementary code violations, and transmitting
the encoded selectively modified digital signal; and at the receiver,
detecting and correcting each of said predetermined code violations to
reproduce said additional signal and said digital signal.
Thus in the present invention at least one randomly
occurring code combination is effectively modulated, by modifying it to a
code violation or leaving it unmodified, by the additional signal which is
to be transmitted. Thus part of the existing redundancy of the
transmission code is made use of to transmit the additional slgnal, and
there is no increase in bit rate or requirement for complicated circuitry
or a separate transmission link. Furthermore, there is very little
degradation of the quality of the transmitted digital signal.
At the receiver, demodulation is effected by detecting and
correcting each predetermined code violation that occurs. If the receiver
includes a separate code violation detector, this can be supplied with the
corrected encoded digital signal, or its violation-indicating output can
be inhibited for each predetermined code violation. Alternatively, the
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1144656
code violation detector and circuitry for detecting and correcting the
predetermined code violation(s) can be combined with one another. The
occurrence of each predetermined code violation, and the absence of such
an occurrence for a predetermined length of time, correspond to two states
of the additional signal. The predetermined length of time is dependent
upon the statistical probability of the occurrence of the predetermined
code combination~s) in the encoded digital signal, and hence is dependent
upon the code combination(s) selected and upon the nature of the digital
signal. Instead of relying on the absence of an occurrence of the
predetermined code violation(s) to determine one of the states of the
additional signal, this can be determined by detecting the unmodified
predetermined code combination(s) in the received signal.
It is particularly convenient if only one bit of the
predetermined code combination need be changed to produce the
predetermined code violation.
The particular forms of the predetermined code
combination~s) and code violation(s) are dependent upon the particular
transmission code which is used. Any one of a variety of redundant
transmission codes, such as 2AMI and similar lB2B codes, UMI, 2B3B, and
3B4B, may be used. Because 2AMI is increasingly being used as a
transmission code, in particular for transmission of digital signals via
fiber optic systems in telephony, the application of the invention to this
code is described in detail below. However, it should be appreciated that
the invention is similarly applicable to other redundant transmission
codes.
2AMI (2-level alternate mark inversion) encoding is
particularly suited to fiber optic digital transmisslon systems ~ecause
~14~6S6
the redundancy of the code permits simple detection of code violations,
the encoded signal has adequate timing energy for signal regeneration,
simple encoding and decoding circuitry can be used, and there is
negligible low frequency content in the encoded signal spectrum. 2AMI
encoding is effected in accordance with the following table:-
Bipolar (AMI) Signal 2AMI Signal Sequence
1 1
_ O Q
O ~ 1 if previous bit was 1
1 0 if previous bit was O
As will be appreciated from this table, a 2AMI encodedsignal has twice the bit rate of a bipolar signal from which it can be
derived, but this doubling of the bit rate presents no problem in low and
medium capacity optic fiber transmission systems. A code violation of the
2AMI code is constituted by three consecutive similar bits, i.e. 111 or
000, this being used as a criterion for operation of a code violation
detector of a transmission system using 2AMI encoding. In order that such
simple code violation detection can be maintained, with 2AMI encoding
preferably the or each predetermined code combination comprises a
predetermined sequence o~ at least four bits.
The selection of the predetermined code combination(s) and
code violation(s) which are used is dependent upon the transmission code,
error monitoring requirements, and other factors, and is best illustrated
by the following example for 2AMI encoding in which each code combination
comprises a four bit sequence in which one bit is changed to produce the
code ~iolation. The following table shows the various possible code
combinations (excluding those which are themselves code violations~ and
the corresponding code violations.
~1446S6
Code Combination Code Violation
1. 001~ l 0000
2. 0100 J
3. 0011 1
4. 0101 ~ 0001
~. 1001 J
6. 0011 ~
7. OlOl ~ 0111
8. 0110 J
9. 1001 ~
10. 1010 ~ 1000
11. 1100 J
12. 0110 ~
13. 1010 ~ lllQ
14. 1100 J
15. 1011 l
16. 1101 J
The selections in lines 1, 4, 13, and 16 of this table are
not desirable if simple unframed modification and detection circuitry is
used, because they can lead to multiple modifications. For example, in
the case of line 1 the valid 2AMI sequence ...001010... could be
modified to ...000010... which contains a false (i.e. not originally
occurring) sequence 0010, resulting in further modification to
...000000.... Similarly, in the case of line 4 the valid 2AMI
sequence .~.010101... could be modified to ...000101... and then
further modified to ...000001.... Similar comments apply in respect
of lines 13 and 1~. Such undesired further modifications can, however, be
avoided by using framing techniques.
1144656
The code combinations in lines 7 and 10 are undesirable
because they correspond to the bipolar sequence 00. Because 2AMI encodes
all bits in a bipolar sequence of zeros with the same code word, using
either of these combinations could produce large peak disparities in the
modified code, especially in low mark density situations.
The code combinations in lines 3, 5, 6, 8, 9, 11, 12, and
14 can be used singly, but complementary code combinations and violations,
such as in lines 8 and 9 for example, can not be used together because
they can interfere with the detection and modification of one another.
The use of two complementary code combinations and corresponding code
violations is advantageous because a greater number of modifications can
be effected and because there is generally less increase of the low
frequency content of the transmitted signal than in the case of one-bit
modification of a single code combination. Furthermore, the use of two
complementary code combinations facilitates the separate transmission of
two additional signals, as described further below.
For the various reasons discussed above, the preferred 4-bit
code combinations for 2AMI encoding, without using framing techniques, are
those in lines 2 and 15 of the above table, i.e. 0100 modifiable to 0000,
and 1011 modifiable to 1111.
A problem with these preferred code combinations can arise
in cases where the 2AMI code is produced from a zero-substituted signal,
for example a B6ZS encoded signal. In such a signal, a sequence of six
zeros is replaced by a sequence OYBOVB, where Y is a violation and B is a
+ or - bit. For example, the bipolar sequence ~000000 would be encoded in
B6ZS encoding as +0+-0-+, which when encoded with 2AMI encoding results in
the sequence 11011100100011, which contalns both of the code combinations
- 1 ~L4~t;~
0100 and loll. If these code combinations were modified, the resulting
sequence would be ~ looooooll, which is not readily detectable at the
receiver to reproduce the original bipolar sequence. In order to avoid
this problem, it is convenient to use as the predetermined code
combinations the sequences 0100 and 1011 each with an initial qualifying
bit, 1 and 0 respectively. Thus the predetermined code combinations
become the five-bit sequences 10100 and 01011, neither of which occurs in
a 2AMI sequence which is derived from a zero sequence in a B6ZS signal.
The particular choice of the predetermined code
combination(s) predominantly determines the rate at which these will occur
in the encoded digital signal, and hence determines the signalling speed
which is available for transmission of the additional signal. In a fiber
optic transmission system in which the digital signal is a 12.624 Mb/s
2AMI encoded signal, derived from a 6.312 Mb/s bipolar signal which
may comprise 96 64 kb/s digital voice channels, with the predetermined
code combinations discussed above the signalling speed available for the
transmission of the additional signal can be at least 64 kb/s with
adequate accuracy, so that the additional signal can itself comprise a
digital voice channel signal provided, for example, for maintenance
purposes.
According to another aspect, this invention provides a
digital transmission system in which data is transmitted from a
transmitter to a receiver using a redundant transmission code,
comprising:- at the transmitter, means responsive to a signal to
selectively mod~fy a predetermined code combination, which occurs randomly
in the encoded data, to produce a predetermined code violation, the
encoded selectively modified data being transmitted; and at the receiver,
114~656
means for detecting and correcting such predetermined code violations to
reproduce said signal and said data. At each of the transmitter and the
receiver, the relevant means can conveniently comprise a shift register
together with logic means such as gating circuitry or a programmable read
only memory (PROM).
The invention will be further understood from the following
description with reference to the accompanying drawings, in which:-
Fig. 1 illustrates an arrangement which is provided at atransmitter of a 2AMI data transmission system for inserting an additional
signal into the 2AMI data stream for transmission;
Fig. 2 illustrates a modification of the arrangement of Fig.
1 for transmitting two additional signals individually via the data
stream;
Fig. 3 illustrates an arrangement which is provided at a
receiver of the transmission system for extracting the additional signal,
inserted by the arrangement of Fig. 1, from the data stream and for
correcting the data stream; and
Fig. 4 illustrates a modification of an arrangement of Fig.
3 for extracting the two additional signals inserted in the data stream by
the modification of Fig. 2 .
The arrangement illustrated in Fig. 1 comprises five D
flip-flops 11 to 15, an Exclusive-OR gate 16, AND gates 17, 18, and 19,
and an OR gate 20. The flip-flops 11 to 15 are coupled in series,
directly and via the gate 16 as shown, to form a shift register via which
a 2AMI da~a stream is passed. For example, the shift register is provided
in the data path from a bipolar-to-2AMI encoder to an optical transmitter
for transmitting the 2AMI data via a fiber optic transmission link.
1144656
Accordingly all of the flip-flops 11 to 15 are clocked via their clock
inputs CK with a clock signal at the data bit rate, for exa~ple 12.624
MHz, the 2AMI data being supplied to a D input of the flip-flop 11 and
the data output being derived from the Q output, Q5, of the flip-flop 15.
No framing of the data is involved in this arrangement.
The inputs of the gates 17 and 18 are coupled as shown to
the outputs Q1, Ql, ....Q5 of the flip-flops 11 to 15, and their
outputs are connected to the inputs of the OR gate 20. Accordingly, the
output of the OR gate 20 is a logic 1 whenever the bits of the data stream
contained in the shift register have the sequence 01011 or the sequence
10100. When the OR gate output is a logic 1 the AND gate 19 is enabled to
supply the binary additional signal to an input of the gate 16, which
couples the Q ou~put of the flip-flop 13 to the D input of the flip-flop
14. Consequently, if the additional signal is a logic 1 each 2AMI data
bit sequence 01011 is changed to 01111 and each 2AMI data bit sequence
10100 is changed to 10000, the modified sequences being transmitted. If
the additional signal is a logic O these sequences are unchanged.
Similarly, other bit sequences of the 2AMI data are unchanged. The
modification shown in Fig. 2, in which lines marked A, B, and C correspond
to similarly marked lines in Fig. 1 so that the AND gates 21 and 22 and
the OR gate 23 of Fig. 2 are used to replace the gates 19 and 20 of Fig.
1, enables the transmission of two individual additional signals instead
of only one as in Fig. 1. Thus with the modification of Fig. 2, the data
se~uence OlQll is selectively modified to 01111 in dependence upon one
additional signal, and the data sequence 10tOO is selectively modified to
10000 in dependence upon the other additional signal, the two selective
modifications taking place completely independently of one another.
l~g~656
Fig. 3 illustrates an arrangement, provided at a receiver
for example following a timing and data recovery circuit and preceding a
2AMI-to-bipolar decoder including a code violation detector, for
extracting the additional signal inserted by the arrangement of Fig. 1 and
for correcting the 2AMI data stream. As in Fig. 1, the arrangement of
Fig. 3 comprises D flip-flops 31 to 35 forming a shift register through
which the 2AMI data stream is clocked. The arrangement also comprises an
Exclusive-OR gate 36, AND gates 37 to 40, OR gates 41 and 42, and a
flip-flop 43 having a set input S and a reset input R.
The gates 37 and 38 have their inputs connected to outputs
of the flip-flops 31 to 35 as shown, and have their outputs connected to
inputs of the OR gate 41. Accordingly, the output of the gate 41 is a
logic 1 whenever the bits of the received data stream contained in the
shift register have the sequence 01111 or 10000, these sequences being
produced by modification of the original data stream as described above
with reference to Fig. 1. The output of the gate 41 is connected to the
set input of the flip-flop 43 and to an input of the gate 36, which
couples the Q output of the flip-flop 33 to the D input of the flip-flop
34. Accordingly, a logic 1 at the output of the gate 41 sets the
flip-flop 43, which consequently reproduces the additional signal as a
logic 1 at its output, and via the gate 36 changes the relevant data
sequence back to its original form, 01011 or 10100. The output of the
gate 41 may also be used as indicated to inhibit a code violation output
signal of a code Yiolation detector.
In the same manner as described above for the gates 17, 1~,
and 20 of Fig. 1, the gates 39, 40, and 42 in Fig. 3 serve to supply a
logic 1 to the reset input of the flip-flop 43, to reproduce the logic O
11446S6
value of the additional signal at its Q output, whenever the unmodified
sequence 01011 or the unmodified sequence 10100 occurs in the data bits in
the shift register formed by the flip-flops 31 to 35. Thus the additional
signal and the original data are reproduced at the ~ outputs of the
flip-flops 43 and 35 respectively.
The modification shown in Fig. 4, in which lines marked A to
E correspond to similarly marked lines in Fig. 3 so that the flip-flops 44
and 45 and the OR gate 46 of Fig. 4 are used to replace the gates 41 and
42 and the flip-flop 43 of Fig. 3, enables reproduction of the two
additional signals inserted by the arrangement of Fig. 2. Thus the
flip-flop 44 is set and reset by the gates 37 and 39 in response to the
data sequences 01111 and 01011 respectively to reproduce at its Q output
the additional signal supplied to the gate 21 in Fig. 2, and the flip-flop
45 is set and reset in response to the data sequences 1DOOO and 10100
respectively to reproduce at its Q output the additional signal supplied
to the gate 22 in Fig. 2. The gate 46 produces a logic 1 in response to
the sequences 01111 and 10000 to inhibit a code violation detector and via
the gate 36 to correct the sequences to 01011 and 10100 respectively in
the same manner as the gate 41 in Fig. 3.
Although particular embodiments of the invention have been
described in detail, it should be appreciated that many modifications,
adaptations, and variations thereof may be made without departing from the
scope of the invention as defined in the claims.
For example, Figs. 1 and 3 illustrate in broken lines inputs
to the gates 17, 18, and 37 to 40 which may be omitted. The flip-flops 1
and 35 and the corresponding AND gate inputs can also be omitted if the
2AMI data is produced from data which is not ~6~S encoded. Additional
11446S6
stages of the shift registers may be provided in order to improve the
reliability of encoding and decoding the additional signal(s), and/or
specific bit sequences in the data may be detected in association with the
sequences which are actually modified in order to facilitate transmission
of more than two additional signals, different additional signals being
identified by different specific bit sequences. Alternati~ely, a single
additional signal can be transmitted using only one of the se~uences 01011
and 10100, with consequent simplification of the gating circuitry
illustrated. The circuitry at the receiver can also be simplified by
replacing each flip-flop 43, 44, or 4~ by a retriggerable monostable
circuit, for example, which is triggered repetitively to indicate one
state of the relevant additional signal and, in the absence of such
repetitive triggering, times out after a predetermined period to indicate
the other state of the additional signal. Obviously in this case the
monostable circuit could be triggered upon detection of either the
modified sequence (code violation) or the unmodified sequence. In such an
arrangement the ~onostable circuit period is selected in dependence upon
the expected lowest recurrent rate of the unmodified sequences in the data
stream. The output of the monostable cirsuit can be filtered in a low
pass filter to reproduce the additional signal; the filter removes the
effects of spurious time-outs of the monostable circuit so that the
monostable circuit period can be reduced.
Furthermore, the described arrangements using flip-flops and
gates may be replaced by various other arrangements which perform an
equivalent function. In particular the described arrangements can be
replaced by arrangements each comprising a shift register and a
programmable read-only memory (RROM). At the transmitter, the original
1144~;56
data can be clocked into the shift register, parallel outputs of which
are connected together with the additional signal(s) to inputs of the
PROM, whose memory contents provide the selectively modified data stream
at an output thereof. The arrangement at the receiver can similarly
comprise a shift register into which the received data is clocked,
parallel outputs of the shift register being connected to inputs of a PROM
which is programmed to provide the corrected data stream and the
additional signal(s) at outputs thereof. In this case the arrangement at
the receiver can conveniently also incorporate a code violation detector
by suitable programming of the PROM.
In addition to the above changes, it is again observed that
the invention is not limited to the 2AMI code or to the specific code
combinations referred to above, but is also applicable to other code
combinations and to other redundant transmission codes.
