OPTICAL COMMUNICATIONS SYSTEM

Abstract: A line monitoring arrangement for an optical communications system launches a sequence of test pulses of comparable energy to system data pulses into a live optical fibre transmission line (5), and backscattered light is monitored. The test sequence is separated from other signals by a correlation process, the test sequence being encoded with a Barker or Golay code. Sequential backscattered pulse sequences are integrated over an extended period, and monitored for change.

Description: OPTICAL COMMUNICATIONS SYSTEM This invention relates to an optical communications system, and in particular to a line monitoring arrangement for early detection of faults in such a system. If an installed optical fibre is to be tested for an apparent or suspected fault, the way in which this is presently performed is by utilising an Optical Time Domain Reflectometer (OTDR). An OTDR comprises a pulse source, usually a high power laser, from which a single pulse is launched into the fibre to be tested, and backscattered light returning to the launch end of the fibre is monitored. Peaks in the backscattered light are indicative of faults in the fibre, and the distance of a given fault from the launch end of the fibre is known from the time interval between launch and return of the respective backscattered peak. Once a period of time sufficient to receive all detectable backscattered light has passed, a further pulse may be launched into the fibre. The pulse width may be varied for difference dynamic range or resolution requirements. Thus, for a given amplitude, " an increase in the pulse width enables a greater length of fibre to be monitored, that is to say it increases the dynamic range. Unfortunately, however, increasing the pulse width decreases the spatial resolution (that is to say the minimum distance between which events can be distinguished). A particular limitation of using an OTDR of this type is that the optical communications system on the fibre has to be disconnected, or at least discontinued, both to permit the connection of the OTDR, and also to prevent system light from affecting the OTDR trace. The result of this limitation is that an OTDR tends to be utilised only after a failure occurs, and cannot be used for constant line surveillance simultaneously with data transmission. The aim of the present invention is to provide a line monitoring arrangement that may be used simultaneously with data transmission. The present invention provides a line monitoring arrangement of an optical fibre communications system carrying system data, the monitoring arrangement comprising a test sequence generator for generating a sequence of test pulses, means for repeatedly launching the sequence of test pulses into a first end of a transmission line forming part of the communications system, a correlator for correlating signals received at the first end of the transmission line with the sequence of test pulses to identify backscattered test pulse sequences, and an integrator for integrating backscattered test pulse sequences over a predetermined time interval. Thus, this line monitoring arrangement can be operated while data is being transmitted, which is not possible with present Optical Time Domain Reflecto eters. This is achieved by superimposing, or slotting in, test pulses along with the system data. However, because there is data transmission, the energy of the test pulses launched into the transmission line has to be compatible with the associated equipment, so as to minimise the possibility of damage, and also to minimise interference with the transmitted data. This results in having to use comparatively low energy pulses compared-to usual OTDR pulse energy, for example by limiting the pulse width of the test pulses to that compatible with the system data, which may typically be at 20 megabits with a pulse width of 50ns. However, by using a sequence of test pulses rather than a single pulse, the energy of the test signal can be increased to compensate for this requirement to use low energy OTDR pulses. In a preferred embodiment, the test pulse sequence is launched into the transmission line at the system data rate. Preferably, the test pulses each have a pulse width and height of the same order as the system data pulse width and height. Backscattered light is monitored continuously, from the launch end of the fibre, for changes in the backscattered test sequence intensity that would be indicative of changes in the fibre. Advantageously, the test pulse sequence is included in a respective time multiplexed transmission frame, and the test pulse sequence is in a Barker or Golay code. The test sequence in the backscattered light has to be detected against a background not only of noise, as with normal OTDR, but also against backscattered light from the system data, and, in the case of a duplex system, incoming system data transmitted from the distant end of the fibre link. To extract the returning backscattered test sequence from the background, a correlation technique is utilised in which the correlator compares the incoming (returning) signal with a duplicate of the test sequence to establish the position of the test sequence in the backscattered light. From establishment of the position of the returning test sequence, and the repeat pattern or interval, a summation of successive returning sequences is made. Since the intensity of the returning test sequence will be low, and it is not envisaged that a single test sequence will be utilised to locate a fault, the returning test sequences may be integrated over an extended period, possibly of several hours or even several days, and compared with an earlier integrated pattern for the same time period. In this way, modification to the pattern due to stress, degradation or splice loss, which in general exhibits a progressive rather than instantaneous failure, can be detected at an early stage before the fault is sufficient to interupt normal system data. The invention also provides an optical fibre communications system comprising a transmission line, means for launching data signals into a first end of the transmission line, and a line monitoring arrangement as defined above. Advantageously, a frame compiler control comprises the means for launching data signals into the first end of the transmission line, system data being fed to an input of the frame compiler control, and test pulse sequences being fed to another input of the frame compiler control. Preferably, the frame compiler control is such as to multiplex each test pulse sequence into respective frame in such a manner that the test pulse sequence is inserted into a slot provided in that frame for supervisory and maintenance signals. This invention further provides a method of monitoring an optical fibre transmission line without disrupting data transmission, the method comprising the steps of generating a sequence of test pulses, repeatedly launching the sequence of test pμlses into a first end of the transmission line, receiving optical signals at the first end of the line, and extracting backscattered test pulse sequences from the received signals. A duplex optical communications system incorporating a line monitoring arrangement constructed in accordance with the invention will now be described in detail, by way of example, with reference to the accompanying drawing, the single figure of which is a schematic representation of the system. Referring to the drawing, the line monitoring arrangement includes a test sequence generator 1 which provides test sequence pulses along a line 2 to an input of a frame compiler control 3. The pulse width of the test pulses is limited to be the same as that of the system data - typically 50 nanoseconds for a 20 megabit data signal. System data is also input to the frame compiler control 3 along with any other elements to be transmitted, The frame compiler control 3 time multiplexes the components of each frame (typically of 10 milliseconds duration) in a predetermined order. The bulk of each frame contains data, but also includes a slot for supervisory and maintenance signals. In the event that all the time slots are not filled, random data is inserted. The test sequence of pulses can be inserted into that part of the frame allocated for maintenance signals. The combined system data and test sequence is then input to an optical transmitter 4, and launched along a fibre link 5, via couplers 6 and 6 ! , to an optical receiver 7 at the far end. System data in the return direction (which could include a different test sequence) is transmitted from an optical transmitter 8 at the far end of the fibre link 5. This return data is launched onto the fibre link 5 via the coupler 6 1 . Thus, signals from the optical transmitter 8 and the backscattered light from the signals outbound from the optical transmitter 4, travel towards the first end of the fibre link 5. This light passes through the coupler 6 to an optical receiver 9. The light entering the optical receiver 9 includes relatively high power system data signals transmitted from the far end. Superimposed on these high intensity signals, with no particular synchronism, is the relatively low intensity backscattered light originating from the transmitter 4. The backscattered light produces a voltage ripple superimposed on the data signal voltage at the receiver 9. This ripple voltage is of small magnitude compared with the incoming data voltage levels. Thus, the comparator (which forms part of the receiver 9) effectively ignores the ripple voltage, and provides a digital output of the data signal. An analogue output of the receiver signal voltage (including the ripple voltage) is fed along a line 10 to a correlator 11. The test sequence is also input to the correlator 11, where the start point of the test sequence in the received signal is determined by a rolling comparison. A code for the test sequence such as a Barker or Golay code is used, such a code exhibiting a high correlation when exactly synchronised with itself, but exhibiting a low correlation when out of synchronism with itself or with random or system data. Thus, the position of the test sequence in the returning backscattered signal can be located, and the test sequence separated from the backscattered system data. The output ofthe correlator 11 is fed to one input of an integrator 12, a frame synchronism signal from the frame compiler control 3 being fed to another input of the integrator. Using the frame synchronism signal to superimpose the test sequences output from the correlator 11, the integrator 12 provides a summation of the backscattered test signals. Noise and system data from the transmitter 8 will tend to average out, leaving backscattered test sequence peaks that can be monitored for change.

Claims: CLAIMS 1. A line monitoring arrangement of an optical fibre communications system carrying system data, the monitoring arrangement comprising a test sequence generator for generating a sequence of test pulses, means for repeatedly launching the sequence of test pulses into a first end of a transmission line forming part of the communications system, a correlator for correlating signals received at the first end of the transmission line with the sequence of test pulses to identify backscattered test pulse sequences, and an integrator for integrating backscattered test pulse sequences over a predetermined time interval. 2. An arrangement as claimed in claim 1, wherein the test pulse sequences are launched into the transmission line at the system data rate. 3. An arrangement as claimed in claim 1 or claim 2, wherein the test pulses each have a pulse width and height of the same order as the system data pulse width and height. 4. An arrangement as claimed in any one of claims 1 to 3, wherein each test pulse sequence is included in a respective time multiplexed transmission frame. 5. An arrangement as claimed in any one of claims 1 to 4, wherein the test pulse sequence is in a Barker or Golay code. 6. An optical fibre communications system comprising a transmission line, means for launching data signals into a first end of the transmission line, and a line monitoring arrangement as claimed in any one of claims 1 to 5. 7. A system as claimed in claim 6, wherein a frame compiler control comprises the means for launching data signal into the first end of the transmission line, system data being fed to an input of the frame compiler control, and test pulse sequences being fed to another input of the frame compiler control. 8. A system as claimed in claim 7 when appendant to claim 4, wherein the frame compiler control is such as to multiplex each test pulse sequence into respective frame in such a manner that the test pulse sequence is inserted into a slot provided in that frame for supervisory and maintenance signals. 9. A method of monitoring an optical fibre transmission line without disrupting data transmission, the method comprising the steps of generating a sequence of test pulses, repeatedly launching the sequence of test pulses into a first end of the transmission line, receiving optical signals at the first end of the line, and extracting backscattered test pulse sequences from the received signals. 10. A method as claimed in claim 9, wherein backscattered test pulse sequences are extracted by a correlation and integration procedure. 11. A method as claimed in claim 9 or claim 10, wherein the width and height of each of the pulses in the test pulse sequence is comparable with the width and height of the data pulses being transmitted. 12. A method as claimed in any one of claims 9 to 11, wherein each test pulse sequence is- incorporated into a respective time multiplexed transmission frame. 13. A method ' as claimed in any one of claims 9 to 12, wherein the test pulse sequence is encoded in a Barker or Golay code.

Inventors: DROUET DOMINIK, GOLD STEPHEN ERIC, JAMES SIMON MARK, DROUET, DOMINIK, GOLD, STEPHEN, ERIC, JAMES, SIMON, MARK
Applicants: BRITISH TELECOMM, BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1990
Main classification: G01M11
IPC codes: G01M11