Method for the additional transmission of information via a digital auxiliary channel, in an optical transmission system

A method for the additional transmission of information items via a digital auxiliary channel in an optical wave guide system for digital signal transmission, in which system electric signals, which are coded in a pseudo-ternary bipolar code, are converted into a redundant, binary line code and are transmitted in converted form via an optical wave guide. In each case one information unit of the information items is transmitted by modification of a selected bit pattern from the data stream present in line code. According to the method, the pseudo-ternary bipolar code is the HDB-3 code, the MCMI code is used as the line code, the bit sequences 110011 and 001100 are used as selected bit patterns and at least one of the bit sequences 011011, 100111 and 101101 is allocated as a modification to the bit sequence 110011, and at least one of the bit sequences 001001, 000110 and 010010 is allocated as a modification to the bit sequence 001100. The method is applied to the remote monitoring of an optical data transmission system including repeaters, which permits such monitoring to be implemented in an inexpensive and functionally reliable manner.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for the additional transmission 
of information via a digital auxiliary channel such as is known, for 
example, from the article "Optical Line Codes Bearing Ancillary Channels" 
by K. W. Cattermole and W. D. Grover, IEE Colloquium on Data Transmission 
Codes, London, November, 1980. The present invention also relates to such 
a method as applied to an optical transmission system as described in 
telcom report 6 (1983), supplement "message transmission by light", pages 
127-132. 
2. Discussion of Background 
In the field of line-conducted digital signal transmission, for example of 
PCM transmission in telephone channels, the use of optical wave guides is 
becoming more and more significant because of the large band width 
becoming available by this means. 
In this connection, the replacement of electrical conductors by optical 
wave guides initially refers to the main transmission path via which the 
data present are transmitted from one station to the next. But in addition 
to the main transmission path, auxiliary and service channels are also 
needed for controlling and monitoring the operation of the line network 
and, if necessary, to pass on instructions or alarm signals. In the 
conventional electrical coaxial line systems, such auxiliary channels are 
provided by additional pairs of lines in parallel with the coaxial lines. 
Transferring this principle of the separatelyconstructed auxiliary channels 
to the technology of optical wave guides leads either to combining an 
optical wave guide (main transmission path) with an electrical pair of 
lines, auxiliary channel) or to a plurality of optical wave guides having 
separate tasks. 
Both solutions are costly because of the additional conductors needed. In 
the case of the first solution, the advantages of freedom from 
interference and of direct-current isolation of optical fibers are lost 
due to the use of the electrical pair of conductors. In the case of the 
second solution, channel capacity is wasted because only a fraction of the 
high transmission capacity of an optical wave guide is utilized due to the 
low signal rate in the service channel. 
For these reasons, people have searched for some time for solutions in 
which, by taking special measures, the same optical wave guide can be used 
both as the main transmission path and as the auxiliary channel. Of these 
solutions, those are of particular interest in which the auxiliary channel 
can be used without interrupting the main transmission path. Thus, for 
example, it has been proposed to implement an independent auxiliary 
channel by transmitting the corresponding information by modulating the 
clock frequency (Electronics Letters 16 (16), July, 1980, pages 624-626). 
Another proposal relates to analog transmission by amplitude or frequency 
modulation within a frequency range below the traffic band (Philips 
Telecommunication Review 40 (2), July, 1982, pages 79-86). 
Such "analog" auxiliary channels, however, require elaborate signal 
processing (modulation, demodulation), necessitate changes and 
interventions at the optical transmitter and receiver and have a 
disadvantageous influence on the transmission quality in the main 
transmission path. 
For these reasons, it has been proposed in the article by K. W. Cattermole 
and W. D. Grover initially mentioned, to utilize the redundancy of the 
line code, used in optical data transmission, for transmitting information 
in binary form by modifying selected bit patterns of the data stream in 
the main transmission path, in which arrangement one information unit is 
represented by the presence or absence of a modification ("digital" 
auxiliary channel). 
As selected bit patterns (SOP: Signalling Opportunity Pattern), bit 
patterns are used which occur with sufficient frequency in the present 
data stream and can be converted in simple manner into modified bit 
patterns which do not occur in the original data stream and can thus be 
easily detected at the receiver side. 
Starting with a transmission system at the electric interfaces of which the 
data are present in a pseudo-ternary bipolar code and in which these data 
are converted into the redundant, binary line code 2 AMI (Alternate Mark 
Inversion) for optical transmission, Cattermole and Grover specify as 
selected bit patterns (SOP) the sequences 1011 and 0100 which are 
associated with the sequences 1111 and 0000 as modified patterns. 
The utility of the bit patterns specified is restricted to transmission 
systems in which the special codes are used which have been assumed. 
A different optical wave guide system having a transmission rate of about 2 
and 8 Mbits/s, as described in the printed supplement "message 
transmission by light", pages 127-132 of the telcom report 6 (1983), is 
based on interface signals which are coded in so-called HDB-3 code (High 
Density Bipolar) according to CCITT Recommendation G 703, Annex A, and are 
converted into the MCMI code (Modified Coded Mark Inversion) for optical 
transmission. 
SUMMARY OF THE INVENTION 
Since optical wave guide systems of the type last described play an 
increasingly important role for digital signal transmission but an 
additional transmission of information via a digital auxiliary channel for 
these systems has hitherto not been known, the present invention has the 
objective of specifying a method by means of which the concept of a 
digital auxiliary channel for optical wave guide systems can be realised, 
using HDB-3/MCMI code conversion. 
In a method of the initially-mentioned type, the providing a novel method 
for the additional transmission of information items via a digital 
auxiliary channel in an optical wave guide system for digital signal 
transmission, in which system electric signals, which are coded in a 
pseudo-ternary bipolar code, are converted into a redundant, binary line 
code and are transmitted in converted form via an optical wave guide. In 
each case one information unit of the information items is transmitted by 
modification of a selected bit pattern from the data stream present in 
line code. According to the method, the pseudo-ternary bipolar code is the 
HDB-3 code, the MCMI code is used as the line code, the bit sequences 
110011 and 001100 are used as selected bit patterns and at least one of 
the bit sequences 011011, 100111 and 10111 is allocated as a modification 
to the bit sequence 110011, and at least one of the bit sequences 001001, 
000110 and 010010 is allocated as a modification to the bit sequence 
001100. 
The method according to the invention is characterised by the following 
advantages: 
the unmodified bit patterns 110011 and 001100 occur relatively frequently 
(with a probability p=1/8 in the case of statistical data) so that the 
auxiliary channel has a sufficiently high capacity; 
the corresponding modified bit patterns 011011, 100111, 101101, 001001, 
000110 and 010010 are the shortest sequences which can again be 
unambiguously detected as modiciations in the MCMI-coded data stream 
without synchronization at the receiver side because they cannot occur in 
the original data stream; 
the modifications do not produce any additional DC component because the 
ratio of "1" to "0" is not changed by the modifications; 
the running digital sum (RDS) is not impaired by the modification so that 
bit error measuring the MCMI-coded data stream by means of the RDS is 
still possible; and 
in particular, the selected bit patterns also occur in the AIS (Alarm 
Indication Signal) established by the CCITT which corresponds to a 
continuous sequence of binary "1" so that the transmission in the 
auxiliary channel is ensured even during the transmission of an AIS. 
The use of the method according to the invention in an optical wave guide 
system in which data are transmitted via two optical wave guides in two 
opposite directions by means of at least one repeater with interposition 
of an HDB-3-to-MCMI code conversion between two line terminals, provides 
for the respective digital auxiliary channels to be used for remote 
monitoring of the system and, particularly, of the repeaters. 
This application also has considerable advantages: 
in each direction, a "transparent" auxiliary channel for the telemetry data 
of the remote monitoring system exists which goes from line terminal 
through to line terminal and passes unimpeded through all repeaters like 
the main transmission path; 
the telemetry data can be inserted into and extracted again from the 
MCMI-coded data stream in a simple manner by means of conventional digital 
circuits; 
since the telemetry data are accommodated as modified bit patterns in the 
main data stream, no interference with the optical transmitters and 
receivers is necessary; 
the digital auxiliary channel has no effects on the transmission quality in 
the main transmission path; 
in the case of a failure of a repeater or interruption of an optical wave 
guide, the remote monitoring remains intact up to the last repeater before 
the fault location so that the fault can be unambiguously located; 
if the data signal in the main transmission path fails, the AIS transmitted 
in that case can be used without difficulties as a carrier; and 
all assemblies can be produced as digital circuits.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, wherein like reference numerals designate 
identical or corresponding parts throughout the several views, and more 
particularly to FIG. 1 thereof shows a block diagram of a known optical 
transmission system comprising a digital auxiliary channel. On the 
left-hand transmitter side of the system, available data present in a 
pseudo-ternary bipolar code are converted in a line encoder 1 into a 
redundant binary line code (for example AMI). The line-coded data stream 
passes via an additional encoder 2, the operation of which will be 
described in greater detail in the text following, to an optical 
transmitter 3 which converts the electric signals into optical signals and 
transmits them via an optical wave guide 10 to an optical receiver 4. 
Receiver 4 reconverts the optical signals into electric signals and passes 
them via a reconverter 5 to a line decoder 6 which carries out the 
re-conversion from the line code to the bipolar code. 
The optical wave guide 10, the optical transmitter 3 and receiver 4 and the 
line encoder 1 and line decoder 6 form the main transmission path for the 
main data stream. 
The consequence of the redundancy feature of the line code used is that 
certain bit patterns cannot occur in the data stream of the main 
transmission path. If, therefore, such a bit pattern is inserted into the 
data stream on the transmitter side by the additional encoder 2, it can be 
detected on the receiver side by means of an appropriate additional 
decoder 8 which continuously monitors the data stream. 
If, in addition, a bit pattern of the same extent, which frequently occurs 
in the data stream, is selected and a bit pattern of the type described, 
which does not occur, is allocated to this selected bit pattern as a 
modification, a binary information item can be transferred by transmitting 
in the data stream, instead of the selected bit pattern, its associated 
modification. 
In this manner, a digital auxiliary channel is implemented in known manner, 
which channel extends via the additional encoder 2, the optical 
transmitter 3 and receiver 4, the optical wave guide 10 and the additional 
decoder 8. To eliminate the possibility of effects of the modifications on 
the further processing of the data, the reconverter 5, which replaces each 
modification by the associated selected bit pattern of the original data 
stream, is suitably provided at the receiver side. 
In addition, in each case one temporary memory 7 and 9 (data buffer) is 
suitably provided at the input and output of the digital auxiliary channel 
so that the data passing via the auxiliary channel can be temporarily 
stored and, when required, called up for transmission when one of the 
selected bit patterns, irregularly occurring in the data stream is 
present. 
The method according to the invention is also based on a system of the type 
shown in FIG. 1. In contrast to the known method, however, a data stream 
coded in HDB-3 code, which is converted to an MCMI code for the optical 
transmission, is processed in this case. 
The individual steps of this conversion can be explained with the aid of 
the table shown in FIG. 2, starting from an illustrative binary starting 
sequence. 
In the top row of the table, a binary starting sequence of zeros and ones 
is shown which can be obtained, for example, as an excerpt from a 
PCM-coded speech signal. During the conversion to the HDB-3 code, this 
starting sequence leads to a sequence of three values of B+, B- and 0, 
shown in the next-lower row of the table. Further conversion from 
pseudo-ternary HDB-3 code to the MCMI line code allocates a sequence of 
two ones to each (B+) value, a sequence of two zeros to each (B-) value 
and a sequence of one zero and one one to each zero value. The resultant 
bit sequence with twice the bit rate and corresponding redundancy is shown 
in the third row of the table. 
According to the invention, the two framed sequences 110011 and 001100, in 
each case corresponding to a sequence of three ones in the binary starting 
sequence, are then used as a the selected bit patterns of this 
MCMI-encoded data stream. 
One of the three sequences 011011, 100111 and 101101 is optionally 
allocated to the one sequence 110011 and one of the sequences 001001, 
000110 and 010010 is optionally allocated to the other sequence 001100 as 
modification. In the table of FIG. 2, the two bit patterns 011011 and 
001001, for example, have been selected as modifications resulting in the 
modified data stream (MCMI (mod.)) in the bottom row of the table if the 
framed sequences are replaced by their modifications. If the other 
possible modifications are used, similar data streams will result. 
The binary "1" of an information unit is then transmitted via the digital 
auxiliary channel by replacing, for example, a selected bit pattern of the 
110011 type in the data stream by one of the three modifications 
mentioned. Correspondingly, the binary "0" is transmitted by replacing a 
selected bit pattern of the other type 001100 in the data stream by one of 
the three modifications allocated to it. Naturally, the roles of the bit 
patterns can also be interchanged so that the selected bit pattern 001100 
and its modifications are allocated to the binary "1". 
A preferred application of this transmission method by means of a digital 
auxiliary channel is found particularly in the remote monitoring of an 
optical transmission system as shown in FIG. 3. 
Such a transmission system contains two line terminals 12 and 14 in which 
the incoming electric signals are converted into optical signals, and 
conversely. Between the two line terminals 12 and 14, the data streams are 
transmitted in opposite directions by two optical wave guides 10 and 11. 
For compensating damping losses in the optical wave guides 10 and 11, one 
or several repeaters 13, which regenerate the data stream, are provided on 
the transmission path, depending on the distance between the line 
terminals 12 and 14. The method according to the invention is used for 
exchanging instructions and remote monitoring (telemetry) data between one 
of the line terminals 12 and 14 and the repeaters 13 and the other one of 
the line terminals via the digital auxiliary channels of the two optical 
wave guides 10 and 11 without having to interrupt the operation on the 
main transmission oath (so-called in-service or on-line monitoring). 
According to a preferred illustrative embodiment, an address is allocated 
in each case to each repeater 13 and the line terminal to be monitored. If 
then, for example, the line terminal 12 is monitoring the operation of the 
remaining system, this monitoring line terminal 12 will successively call, 
by addressing, the repeaters 13 and the line terminal 14, to be monitored, 
via the digital auxiliary channel of the optical wave guide 10. 
The functional unit called in each case then returns the appropriate 
telemetry data via the digital auxiliary channel of the second optical 
wave guide 11 to the monitoring line terminal 12. For continuous remote 
monitoring, this serial polling of the repeaters 13 and of the line 
terminal 14 to be monitored is cyclically repeated. 
A block diagram of a preferred illustrative embodiment of the line terminal 
12 designed for remote monitoring is shown in FIG. 4. 
The data coming in HDB-3 code from an electric interface are converted 
inside line terminal 12 by the line encoder 1 into MCMI line code in a 
manner known per se and are converted into optical signals in the optical 
transmitter 3 and fed into the outgoing optical wave guide 10. The optical 
transmitter 3 contains, for example, a regulated laser diode which emits a 
light signal with constant swing. Technical details of such a transmitter 
can be found, for example, in the above-mentioned printed supplement 
"message transmission by light", pages 129 and following, of the telcom 
report 6 (1983). 
Analogously, the optical signals produced by the incoming optical fiber 11 
pass to the optical receiver 4 which, for example, is equipped with an 
avalanche photo diode (APD) and the technical design of which is also 
known from the above-mentioned printed document. 
The pulse shape of the electric data signals generated by the optical 
receiver 4 is preferably regenerated in a subsequent regenerator circuit 
19, after which the signals are reconverted from the MCMI line code to the 
HDB-3 code in the line decoder 6 and supplied to an electric interface, 
not shown, at the receiver. 
To construct the two digital auxiliary channels for the remote monitoring 
system, an additional encoder 2 is arranged between the line encoder 1 and 
the optical transmitter 3 and a corresponding additional decoder 8 is 
arranged between the optical receiver 4 and the line decoder 6. 
The additional encoder 2 is designed for modifying the selected bit 
patterns of the MCMI-encoded data stream as already described. For this 
purpose, for example, the incoming bit sequence of the data stream is 
shifted through a shift register and the register contents are compared at 
each clock cycle with the contents of two read-only memories in which the 
selected bit patterns 110011 and 001100 are stored. 
When a binary "1" is to be transmitted via the digital auxiliary channel, 
one of the two selected bit patterns is replaced by one of its three 
modifications allocated in each case as soon as it has been detected in 
the data stream by the comparison. 
If, in contrast, a binary "0" is to be transmitted, the other one of the 
two selected bit patterns is modified. In this manner, an information unit 
can be transmitted via the digital auxiliary channel whenever one of the 
selected bit patterns appears in the data stream. 
The additional decoder 8 monitors the incoming modified MCMI data stream 
which contains both the selected bit patterns (with missing transmission) 
and the modifications, not normally occurring (if a binary "1" or "0" is 
transmitted), and detects these bit patterns. 
For each modification of the one selected bit pattern detected, the 
additional decoder 8 outputs a "0" at its output and for each modification 
of the other bit pattern detected, correspondingly, a "1" so that the 
information items transmitted on the digital auxiliary channel are 
extracted again from the data stream. 
The remote monitoring system is controlled by a fault location unit 15 
(FLU) which is connected to an input and output unit (16). In the normal 
case, the fault location unit 15 cyclically and in a predetermined 
sequence (serially) requests all repeaters 13 and the opposite line 
terminal 14 by direct addressing via the digital auxiliary channel 
associated with the optical wave guide 10 to send the alarm signals and 
error messages occurring in the form of a message via the digital 
auxiliary channel associated with the optical wave guide 11 back to the 
fault location unit 15. 
All repeaters 13 and the line terminal monitored monitor the MCMI data 
stream modified by the additional encoder 2 in the line terminal 12 and 
only respond when they detect their own address. The fault location unit 
15 evaluates the messages extracted by the additional decoder 8 from the 
returning data stream and displays corresponding messages at the input and 
output unit 16. 
Illustrative embodiments of the repeaters 13 and of the line terminal 14 
monitored are reproduced in the block diagram in FIGS. 5 and 6. In 
addition to the functional blocks of the optical transmitters 3, the 
optical receivers 4, the regenerator circuits 19, the additional encoder 2 
and the additional decoder 8, the repeater 13 of FIG. 5 contains a 
monitoring unit 17. 
The monitoring unit 17 collects the important operating data from the 
repeater 13. When the additional decoder 8 detects the address of "its" 
repeater in the data stream of the optical wave guide 10, the instructions 
connected with the address are passed on to the monitoring unit 17. Within 
the normal polling cycle, the monitoring unit then sends the operating 
data in the form of a message via the additional encoder 2 and the digital 
auxiliary channel of the wave guide 11 to the fault location unit 15 in 
the line terminal 12. 
In addition, it is possible to use a special instruction, which is detected 
in the monitoring unit 17, for commanding a so-called loop closure. In 
this case, the monitoring unit of the repeater selected actuates a loop 
switch 18 which shorts the two opposite data lines inside the repeaters 
selected, thus forming a closed line loop which starts from the monitoring 
line terminal. In this manner, the individual sections of the transmission 
system can be successively checked for their viability from the line 
terminal. 
Analogously to the repeater 13 of FIG. 5, the monitored line terminal 14 of 
FIG. 6 is also equipped with a monitoring unit 17 and a loop switch 18 
which have the same functions as in a repeater. 
According to a preferred embodiment of the system, both line terminals 12 
and 14 are of identical construction, that is to say the line terminal 12 
of FIG. 4 also contains a monitoring unit and a loop switch which can be 
selected and actuated from the other line terminal 14. 
For this purpose, the other line terminal 14 is also equipped with a fault 
location unit so that the remote monitoring of the transmission system can 
be carried out with equal priority and optionally from each of the line 
terminals 12 and 14. 
Correspondingly, the repeaters 13 according to FIG. 5 are in this 
embodiment equipped with another monitoring unit 17', another loop switch 
18' and another additional encoder 2' and additional decoder 8' which are 
drawn in dashed lines in the Figure. 
The various loops of the data streams for a double-sided remote monitoring 
of the type described are shown in FIG. 7. In one monitoring mode, the 
fault location unit 15 of the one line terminal 12 operates in conjunction 
with all monitoring units 17 in the repeaters 13 and of the "passive" 
monitored line terminal 14. 
The fault location unit 15' of the other line terminal operates in 
conjunction with all monitoring units 17' in the repeaters 13 and the line 
terminal 12 which is passive in this case. 
Overall, the method according to the invention creates the possibility of 
transferring, with comparatively small effort and on the basis of digital 
engineering, telemetry data via a digital auxiliary channel in an optical 
wave guide system and, in this manner, achieves effective remote 
monitoring without interruption of operations. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described. 
LIST OF DESIGNATIONS 
1: Line encoder 
2, 2': Additional encoder 
3: Optical transmitter 
4: Optical receiver 
5: Reconverter 
6: Line decoder 
7, 9: Temporary memory 
8, 8': Additional decoder 
10, 11: Optical wave guide 
12, 14: Line terminal 
13: Repeater 
15, 15': Fault location unit 
16: Input and output unit 
17, 17': Monitoring unit 
18, 18': Loop switch 
19: Regenerator circuit