Abstract:
In a communication system wherein a station transceiver and a plurality of nodes communicate in a TDM system through a signal transmission line, at least apart of the line is shared, a method to judge nodes performing normal operation from the station transceiver comprises a request step to request a return of a test pattern by transmitting a trigger signal for a designated node in the plurality of nodes, a correlation process step to process correlation between a received signal in a timeslot assigned to the designated node and a reference pattern corresponding to the designated node, and a judging step to judge whether the designated node is a normal node according to the correlation process result.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application is based upon the benefit of priority from the prior Japanese Patent Application No. 2001-357010, filed on Nov. 22, 2001, the entire contents of which are incorporated herein by reference.  
         FIELD OF THE INVENTION  
         [0002]    This invention relates to a node judging method, a communication system, and a node measuring apparatus, and more specifically relates to a method for detecting a disturbing node and its communication system, and a node measuring apparatus for detecting the disturbing node.  
         BACKGROUND OF THE INVENTION  
         [0003]    [0003]FIG. 7 shows a schematic diagram of a passive optical subscriber network of TDM (time division multiplexing) system, which connects a single station transceiver and a plurality of subscriber nodes.  
           [0004]    A station transceiver  110  connects to a common port #C of optical multiplexer/demultiplexer  114  through an optical fiber  112 . The optical multiplexer/demultiplexer  114  comprises an optical element to demultiplex an input light from the common port #C into N portions and output each demultiplexed light through ports #1˜#N, and to multiplex input light from each of the ports #1˜#N and output the multiplexed light through the common port #C. Each of the ports #1˜#N of the optical multiplexer/demultiplexer  114  connects to each of optical transceivers  118 - 1 ˜ 118 -N belonging to respective subscribers #1˜#N via optical fibers  116 - 1 ˜ 116 -N.  
           [0005]    A TDM system is used for the communication between the station transceiver  110  and each of the optical transceivers  118 - 1 ˜ 118 -N belonging to the subscribers #1˜#N respectively. Namely, each of the optical transceivers  118 - 1 ˜ 118 -N extracts a signal in a timeslot assigned for itself out of the time division multiplexed signals (down signal) from the station transceiver  110 , receives the extracted signal, and discards the rest of the signal lights in the other timeslots. Each of the optical transceivers  118 - 1 ˜ 118 -N also outputs a signal to be transmitted for the station transceiver  110  onto the respective optical fibers  116 - 1 ˜ 116 -N at timing according to its own assigned timeslot. The station transceiver  110  predeterminedly and continuously synchronizes the station transceiver  110  and the optical transceivers  118 - 1 ˜ 118 -N. By this operation, each of the optical transceivers  118 - 1 ˜ 118 -N is able to know the timing of its assigned timeslot for transmission and reception. Description for the synchronizing procedure between the station transceiver  110  and the optical transceivers  118 - 1 ˜ 118 -N is omitted here.  
           [0006]    Used as a down signal to transmit from the station transceiver  110  to the optical transceivers  118 - 1 ˜ 118 -N belonging to the respective subscribers #1˜#N and an up signal from the optical transceivers  118 - 1 ˜ 118 -N belonging to the respective subscribers #1˜#N to the station transceiver  110  are optical carriers having a wavelength different from each other. In a conventional system, a 1.5 μm band optical carrier is used for the down signal and a 1.3 μm band optical carrier is used for the up signal.  
           [0007]    The operation of a conventional system is explained below. The station transceiver  110  time-division-multiplexes a down optical signal Di (i=1−N) destined for the respective subscribers #1˜#N and outputs onto the optical fiber  112 . The down optical signal Di propagates on the optical fiber  112  and enters a common port #C of the optical multiplexer/demultiplexer  114 . The optical multiplexer/demultiplexer  114  divides the time-division-multiplexed down optical signal Di into N portions and outputs each divided light onto the optical fibers  116 - 1 ˜ 116 -N through the ports #1˜#N. Formatively, all the down optical signals D 1 ˜DN destined for the respective subscribers #1˜#N enter every one of the transceivers  118 - 1 ˜ 118 -N. Each of the optical transceivers  118 - 1 ˜ 118 -N extracts an optical signal in a timeslot assigned for itself out of the input optical signals, receives the extracted signal, and discards the rest of the optical signals in the other timeslots. For instance, the optical transceiver  118 - 1  exclusively receives a down optical signal D 1 , and the optical transceiver  118 - 2  exclusively receives a down optical signal D 2 .  
           [0008]    Each of the optical transceivers  118 - 1 ˜ 118 -N outputs an up optical signal Ui (i=1˜N) according to its own assigned timeslot onto the optical fibers  116 - 1 ˜ 116 -N. The up optical signal Ui (i=1˜N) propagates on the optical fibers  116 - 1 ˜ 116 -N and enters the ports #1˜#N of the optical multiplexer/demultiplexer  114  respectively. The optical multiplexer/demultiplexer  114  multiplexes the respective up optical signals Ui (i=1˜N) from the optical fibers  116 - 1 ˜ 116 -N) and outputs onto the optical fiber  112  through the common port #C.  
           [0009]    When the optical transceivers  118 - 1 ˜ 118 -N output the optical signal Ui (i=1˜N) onto the optical fibers  116 - 1 ˜ 116 -N in the respective assigned appropriate timeslots, the up optical signals Ui on the optical fiber  112  are located on proper timeslots not overlapping each other in the time domain, as shown in FIG. 7. That is, the optical multiplexer/demultiplexer  114  multiplexes the respective up optical signals Ui without adjusting their time locations.  
           [0010]    The up optical signal Ui being output onto the optical fibers  112  from the optical multiplexer/demultiplexer  114  transmits on the optical fiber  112  and enters the station transceiver  110 . Since the station transceiver  110  synchronizes with each of the optical transceivers  118 - 1 ˜ 118 -N, it can accurately separate each up optical signal Ui out of the input optical signals from the optical fiber  112 .  
           [0011]    In the above-described passive optical subscriber network, a plurality of subscribers shares one signal band in the time domain. Therefore, when one of the subscribers&#39; units outputs an up optical signal in a timeslot other than the one assigned to itself due to some fault, the other subscriber&#39;s communication originally using the mistaken timeslot is inhibited.  
           [0012]    For instance, supposing that the optical transceiver  118 - 1  outputs a continuous disturbance light onto the optical fiber  116 - 1 , this disturbance light extremely deteriorates a signal-to-noise power ratio (SNR) of the up optical signals U 2 ˜UN, which are output for the station transceiver  110  by the other optical transceivers  118 - 2 ˜ 118 -N, on the optical fiber  112 . This inhibits signal transmission from the optical transceivers  118 - 2 ˜ 118 -N to the station transceiver  110 . This kind of situation can be occurred when a subscriber is confused of connecting optical fiber codes and connects to a wrong communication device by mistake or a subscriber maliciously outputs an up optical signal that is not permitted.  
           [0013]    When this type of fault occurs, it is most important to eliminate the fault factor as soon as possible. In particular, when the disturbance optical signal is transmitted continuously, all the subscribers&#39; signals are interrupted and thus it is necessary to solve the problem without a moment&#39;s delay. However, conventionally, to identify the one outputting the disturbance light out of all the optical transceivers  1181 ˜ 118 -N was impossible and thus there was no other way but to check every transceiver one by one.  
         SUMMARY OF THE INVENTION  
         [0014]    A node judging method according to the present invention is a method to judge nodes of normal performance from a station transceiver side in a communication system wherein the station transceiver and a plurality of nodes communicate in a TDM system through a signal transmission line in which at least a part of the line is shared and comprises a request step to request a return of a test pattern by transmitting a trigger signal for a designated node in the plurality of nodes, a correlation process step to process correlation between a received signal in a timeslot assigned to the designated node and a reference pattern corresponding to the designated node, and a judging step to judge whether the designated node is a disturbance node according to the correlation process result.  
           [0015]    A communication system according to the present invention is a system wherein a station transceiver and a plurality of nodes communicate in a TDM system through a signal transmission line in which at least a part of the line is shared, and is characterized in that the station transceiver comprises a trigger signal transmitter to transmit a trigger signal including a synchronous pattern signal for the designated node, a reference pattern generator to generate a reference pattern corresponding to the designated node, a correlation processor to process correlation between a received signal in a timeslot assigned to the designated node and the reference pattern, and a judging apparatus to judge whether the designated node is a normal node according to a correlation processed result by the correlation processor and each of the plurality of nodes comprises an apparatus to output a predetermined test pattern signal according to the trigger signal.  
           [0016]    A node measuring apparatus according to the present invention comprises a trigger transmitter to transmit a trigger signal for a designated node in the plurality of nodes to request the designated node to return a test pattern signal, a reference patter generator to generate a reference pattern corresponding to the designated node, correlation processor to process correlation between a received signal in a timeslot assigned to the designated node and the reference pattern, and a judging apparatus to judge whether the designated node is a normal node according to the correlation process result from the correlation processor. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0017]    The above and other objects, features and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments of the invention in conjunction with the accompanying drawings, in which:  
         [0018]    [0018]FIG. 1 is a schematic block diagram of an embodiment according to the present invention;  
         [0019]    [0019]FIG. 2 is a schematic block diagram of a station transceiver  10 ;  
         [0020]    [0020]FIG. 3 shows waveform examples of correlation result and integration result when a received test pattern and a reference pattern coincide;  
         [0021]    [0021]FIG. 4 shows waveform examples of correlation result and integration result when a received test pattern and a reference pattern do not coincide;  
         [0022]    [0022]FIG. 5 is a schematic block diagram of another configuration example of the station transceiver  10 ;  
         [0023]    [0023]FIG. 6 is a schematic block diagram of an optical transmitter  18 - 1 ; and  
         [0024]    [0024]FIG. 7 is a schematic block diagram of a passive optical subscriber network of a conventional TDM (time division multiplexing) system. 
     
    
     DETAILED DESCRIPTION  
       [0025]    Embodiments of the invention are explained below in detail with reference to the drawings.  
         [0026]    [0026]FIG. 1 is a schematic block diagram of an embodiment of the present invention.  
         [0027]    A station transceiver  10  connects to a common port #C of an optical multiplexer/demultiplexer  14  through an optical fiber  12 . The optical multiplexer/demultiplexer  14  comprises an optical element to divide an input light through the common port #C into N portions and output each divided light for ports #1˜#N, and to multiplex input lights from the ports #1˜#N and output through the common port #C. The ports #1˜#N of the optical multiplexer/demultiplexer  14  connect to optical transceivers  18 - 1 ˜ 18 -N belonging to respective subscribers #1˜#N through optical fibers  16 - 1 ˜ 16 -N respectively.  
         [0028]    Communication between the station transceiver  10  and the optical transceivers  18 - 1 ˜ 18 -N of the subscribers #1˜#N is identical to that of the conventional system shown in FIG. 7. That is, a TDM system is used in communication between a station transceiver  10  and the optical transceivers  18 - 1 ˜ 18 -N belonging to the subscribers #1˜#N, and an up signal and a down signal is distinguished by a wavelength of an optical carrier to be used. In this embodiment, an optical carrier of wavelength λd is used for the down signal from the station transceiver  10  to the optical transceivers  18 - 1 ˜ 18 -N belonging to the respective subscribers #1˜#N, and an optical carrier of wavelength λu different to the wavelength λd is used for the up signal from the optical transceivers  18 - 1 ˜ 18 -N of the respective subscribers #1˜#N to the station transceiver  10 . Similarly to the conventional system, λd is 1.5 Am band and λu is 1.3 μm.  
         [0029]    In FIG. 1, the transceiver  18 - 2  plays a role of a disturbance node to regularly output disturbance lights onto the optical fiber  16 - 2 . The operation of this embodiment to identify the disturbance node (the optical transceiver  18 - 2 ) is explained below in detail.  
         [0030]    The station transceiver  10  comprises a node measuring apparatus to measure disturbance nodes and fault occurrences. In the explanation below, the operation of the station transceiver  10  to measure whether a node is normal or not is the operation of the node measuring apparatus built in the station transceiver  10 .  
         [0031]    First, the station transceiver  10  instructs all the optical transceivers  18 - 1 ˜ 18 -N to be tested or one or some of the test objects to shift to a test mode.  
         [0032]    The station transceiver  10  periodically outputs an optical trigger signal  20  (wavelength λd) for the optical fiber  12  so that the respective optical transceivers  18 - 1 ˜ 18 -N transmit a test pattern according to the predetermined timing. The optical trigger signal  20  comprises a node designator  20   a  that designates an optical transceiver to return a test pattern and an idle pattern signal  20   b  to synchronize the object optical transceiver with the station transceiver  10 . The node designator  20   a  can designate a single specific one in the optical transceivers  18 - 1 ˜ 18 -N and also can specify all the optical transceivers  18 - 1 ˜ 18 -N at once. In FIG. 1, the node designator  20   a  designates the optical transceiver  18 - 1 . The idle pattern signal  20   b  can be identical to the test pattern to be returned from the optical transceivers  18 - 1 ˜ 18 -N and also can be a fixed pattern made from a constant repetition of mark and space.  
         [0033]    An optical multiplexer/demultiplexer  14  divides the optical trigger signal  20  input from the station transceiver  10  through the optical fiber  12  into N portions and outputs the respective divided light for the optical transceivers  18 - 1 ˜ 18 -N through the optical fibers  16 - 1 ˜ 16 -N. Only the optical transceiver  18 - 1  designated by the node designator  20   a  in the optical trigger signal  20  outputs a test pattern optical signal  22  onto the optical fiber  16 - 1 . At this time, the optical transceiver  18 - 1  synchronizes with an idle pattern signal  20   b  in the input optical trigger signal  20  and outputs an optical test pattern signal  22  (wavelength λu) for the optical fiber  16 - 1 . Through this operation, the synchronization between the station transceiver  10  and the optical transceiver  18 - 1  is confirmed.  
         [0034]    The test pattern carried by the test pattern optical signal  22  (wavelength λu) can be any pattern as far as the station transceiver  10  recognizes it. For instance, it can be identical to the idle pattern  20   b  or an encoded idle pattern and also can be identical or different in the respective optical transceivers  18 - 1 ˜ 18 -N. However, it is preferable that the test pattern comprises a pseudo random pattern.  
         [0035]    When all the optical transceivers  18 - 1 ˜ 18 -N are tested at the same time, it is preferable that the optical transceivers  18 - 1 ˜ 18 -N return a test pattern optical signal comprising a pattern different from each other to the station transceiver  10 . Because, when some of the optical transceivers locate on the same distance from the station transceiver  10 , the station transceiver  10  cannot detect the test pattern optical signals from those transceivers individually even if it uses a correlation method to be described later.  
         [0036]    When a single optical transceiver is tested one by one, each of the optical transceivers  18 - 1 ˜ 18 -N can either returns the same test pattern optical signal  22  to the station transceiver  10  or returns a test pattern optical signal  22  different from each other to the station transceiver  10 .  
         [0037]    The test pattern optical signal  22  being output from the optical transceiver  18 - 1  propagates on the optical fiber  16 - 1  and enters the optical multiplexer/demultiplexer  14 . Also, the disturbance light  24  (wavelength λu) being output from the optical transceiver  18 - 2  onto the optical fiber  16 - 2  propagates on the optical fiber  16 - 2  and enters the optical multiplexer/demultiplexer  14 . The optical multiplexer/demultiplexer  14  applies its test pattern optical signal  22  (wavelength λu) and the disturbance light  24  (wavelength λu) to the station transceiver  10  through the optical fiber  12 . On the optical fiber  12 , an SNR of the test pattern optical  22  greatly deteriorates due to the disturbance light  24 .  
         [0038]    The station transceiver  10  converts the light consisted of the test pattern optical signal  22  and the disturbance light  24  input from the optical fiber  12  to an electric signal, processes the correlation between the output and the reference pattern, and integrates the correlation result. The reference pattern comprises a pattern identical to the test pattern carried by the test pattern optical signal  22 . From this correlation process, the test pattern optical signal  22  can be detected. Even when the SNR of the test pattern optical signal  22  is greatly deteriorated, the test pattern optical signal  22  can be certainly detected by integrating the correlation result.  
         [0039]    Although the details are described later, when the test pattern is detected, the reference pattern is applied to a correlation process circuit after the trigger optical signal  20  is output onto the optical fiber  12 , the time lag equals to the time needed for the roundtrip distance between the optical transceiver to be tested (here, the apparatus  18 - 1 ) and the station transceiver  10  plus the return time in the apparatus  18 - 1 . By applying the reference pattern to the correlation process circuit at the timing where the test pattern does not exist, a cross-correlation value with the signal input from the optical fiber  12  can be calculated. This cross-correlation value shows an index of background noise.  
         [0040]    If the integration value of the correlation result is larger than the predetermined value, it proves that the subject optical transceiver being tested is normally operated. The station transceiver  10  outputs the trigger optical signal  20  on to the optical fiber  12  taking a next optical transceiver, for instance the apparatus  18 - 2 , as a testing object. According to this manner, the station transceiver  10  tests each of the optical transceivers  18 - 1 ˜ 18 -N one by one.  
         [0041]    The optical transceiver  18 - 2  outputting the disturbance light  24  does not send a test pattern optical signal in return to the trigger optical signal  20 . Therefore, when the optical transceiver  18 - 2  has been tested, the integration value of correlation process in the station transceiver  10  is lower than the predetermined value. From this, it is clear that the station transceiver  10  recognizes a fault in the optical transceiver  18 - 2 .  
         [0042]    When each of the optical transceivers  18 - 1 ˜ 18 -N outputs a test pattern optical signal different from each other, the station transceiver  10  can identify an optical transceiver having a fault, e.g. fault node, quicker by performing correlation process in parallel. Needless to say, the station transceiver  10  can perform correlation process of the test pattern optical signal from each of the optical transceivers  18 - 1 ˜ 18 -N sequentially. This requires less time to identify the fault node compared to the method that outputs the trigger optical signal  20  for the optical transceivers  18 - 1 ˜ 18 -N individually.  
         [0043]    Generally, when a disturbance light exists, it is expected that all the up signals are being disturbed. However, sometimes it happens that a disturbance light is transmitted in a timeslot other than the predetermined timeslot due to a defect of timing circuits etc. In this case, the transmission timing is likely to have periodicity, and thus although it affects one or some of other subscribers, it does not affect the communication of the rest of the subscribers. Under the circumstance, to avoid the influence to the nodes performing normal communication, the above detecting process is performed in the situation that only the node having communication fault is enforced to send test pattern data instead of communication data for a station to which the node having fault has been assigned. In this case, it is necessary to perform integral process only when the test pattern data exists.  
         [0044]    The internal configuration of the station transceiver  10  is explained below. FIG. 2 shows a schematic block diagram of an embodiment of the station transceiver  10 . However, it mainly shows a configuration of a node measuring apparatus to measure whether the transceivers  18 - 1 ˜ 18 -N operate normally. A control circuit  30  applies a subscriber ID to identify an optical transceiver to be tested and idle pattern to an optical transmitter  32  and applies a trigger signal and delay time obtained by considering roundtrip distance between the optical transceiver to be tested and the station transceiver  10  to a reference pattern generating circuit  34 .  
         [0045]    The optical transmitter  32  converts the subscriber ID and idle pattern signal into an optical signal of wavelength λd to generate a trigger optical signal  20  whose retrieval object is identified by the subscriber ID. The trigger optical signal  20  is applied to the optical fiber  12  through a WDM optical multiplexer/demultiplexer  36  and sent for the optical transceivers  18 - 1 ˜ 18 -N as previously explained.  
         [0046]    The WDM optical multiplexer/demultiplexer  36  is a wavelength-selective optical coupler to couple the λd light output from the optical transmitter  32  to the optical fiber  12  and input the light of wavelength λu from the optical fiber  12  to an optical receiver  58 .  
         [0047]    The WDM optical multiplexer/demultiplexer  36  applies the light of wavelength λu including the test pattern optical signal  22  and disturbance light  24  from the optical fiber  12  into the optical receiver  38 . The optical receiver  38  converts the input light into an electric signal and applies to a correlation processor  40 . A test pattern carried by the test pattern optical signal  22  enters the correlation processor  40 .  
         [0048]    On the other hand, the reference pattern generator  34  generates a reference pattern when the delay time set by the controller  30  passed from inputting the trigger signal from the controller  30  and applies the reference pattern to the correlation processor  40 . The reference pattern generator  34  also applies a gate signal to the integrator  42 , the gate signal showing a timeslot used by the optical transceiver to output a test pattern optical signal for the correlation result from the correlation processor  40 . The gate signal shows the timing to integrate the correlation process result for the test pattern signal from the tested optical transceiver.  
         [0049]    The correlation processor  40  processes the correlation between the received test pattern from the optical receiver  38  and the reference pattern from the reference pattern generator  34  and applies the correlation result to the integrator  42 . The integrator  42  integrates the correlation result from the correlation processor  40  in a timeslot assigned by the gate signal from the reference pattern generator  34 .  
         [0050]    The controller  30  specifies one or more optical transceivers generating the disturbance light according to the integration result for each of the optical transceivers  18 - 1 ˜ 18 -N by the integrator  42 .  
         [0051]    [0051]FIG. 3 shows waveforms of correlation result and integration result when a received test pattern and a reference pattern coincide, and FIG. 4 shows waveforms of correlation result and integrated result when the received test pattern and the reference pattern do not coincide. When the received test pattern and the reference pattern coincide, the integration result of the integrator  42  increases with time. This means that the tested optical transceiver is operating normally and not outputting the disturbance light. Conversely, when the received test pattern and the reference pattern do not coincide, the integration result of the integrator  42  only varies around zero value or within minus values. This means that the tested optical transceiver is not operating normally and outputting the disturbance light. As described above, it is possible to judge whether the tested optical transceiver is operating normally or not, namely outputting disturbance light or not, according to the integration result output from the integrator  42 .  
         [0052]    When the reference pattern generator  34 , correlation processor  40 , and integrator  42  are designed as a digital processor, an analog/digital (A/D) converter should be disposed between the optical receiver  38  and the correlation processor  40 .  
         [0053]    As explained above, the disturbance node can be specified more quickly owing to the configuration in which the test pattern optical signals are output in the timeslots assigned respectively by the plurality of optical transceivers  18 - 1 ˜ 18 -N, and the station transceiver  10  performs correlation process of the plurality of received test pattern signals spontaneously.  
         [0054]    [0054]FIG. 5 shows a schematic block diagram of an embodiment of the station transceiver  10  to perform parallel processing of a plurality of received test pattern signals.  
         [0055]    A trigger generator  50  applies an identifier to show all or some of the optical transceivers  18 - 1 ˜ 18 -N and an idle pattern to an optical transmitter  52 . The optical transmitter  52  generates a trigger optical signal  20  (wavelength λd) to be broadcasted or multicasted to all or some of the optical transceivers  18 - 1 ˜ 18 -N, and, at the same time, applies a trigger signal to show test-start timing to the correlation judging circuits  54 - 1 ˜ 54 -N to judge correlation of the test patterns returned from each of the optical transceivers  18 - 1 ˜ 18 -N. Delay time is applied to each of the correlation judging circuits  54 - 1 ˜ 54 -N, each delay time is determined considering roundtrip distance from the station transceiver  10  to the corresponding optical transceivers  18 - 1 ˜ 18 -N. The trigger optical signal  20  (wavelength λd) generated by the optical transmitter  52  is applied to the optical fiber  12  through the WDM optical multiplexer/demultiplexer  56  and, as previously explained, enters the optical transceivers  18 - 1 ˜ 18 -N. The WDM optical multiplexer/demultiplexer  56  comprises an optical element identical to the WDM multiplexer/demultiplexer  36 .  
         [0056]    The WDM optical multiplexer/demultiplexer  56  applies the light of wavelength λu from the optical fiber  12  to the optical receiver  58 . The optical receiver  58  converts the light of wavelength λu from the WDM optical multiplexer/demultiplexer  56  into an electric signal and applies to each of the correlation judging circuits  54 - 1 ˜ 54 -N. The output from the optical receiver  58  includes the test pattern signal returned from the optical transceiver assigned by the trigger optical signal  20  and the disturbance light from the disturbance node.  
         [0057]    Each of the correlation judging circuits  54 - 1 ˜ 54 -N comprises a configuration identical to the circuit block diagram of the embodiment shown in FIG. 1, which consists of the reference pattern generator  34 , the correlation processor  40  and the integrator  42 . Performing the same operation to the one explained for the embodiment in FIG. 1, each correlation judging circuit  54 - 1 ˜ 54 -N judges according to the output from the optical receiver  58  whether the corresponding optical transceiver  18 - 1 ˜ 18 -N normally returns a test pattern signal. Judged result of each correlation judging circuit  54 - 1 ˜ 54 -N corresponds to the output from the integrator  42 . The judged result of each correlation judging circuit  54 - 1 ˜ 54 -N is applied to a judging circuit  60 . The judging circuit  60  specifies which optical transceiver  18 - 1 ˜ 18 -N outputs the disturbance signal or light according to the judged output from each correlation judging circuit  54 - 1 ˜ 54 -N.  
         [0058]    In the embodiment shown in FIG. 5, since it is possible to process in parallel the test pattern signals returned from the plurality of optical transceivers  18 - 1 ˜ 18 -N, the disturbance node can be detected quickly.  
         [0059]    In the embodiment shown in FIG. 5, it is also possible to convert the analog output from the optical receiver  38  to a digital signal and applies to each correlation judging circuit  54 - 1 ˜ 54 -N.  
         [0060]    There are two kinds of methods for generating a test pattern signal in each optical transceiver  18 - 1 ˜ 18 -N. The first method is to return the pattern from the station transceiver  10  as it is or after coding it. In this method, the internal configuration of the optical transceiver  18 - 1 ˜ 18 -N can be simplified. The second method is to use the idle pattern  20   b  from the station transceiver  10  only for synchronizing with the station transceiver  10  and each optical transceiver  18 - 1 ˜ 18 -N generates an original test pattern. In this case, each optical transceiver  18 - 1 ˜ 18 -N requires a pattern generator, and the generated pattern must be identical to the reference pattern generated in the station transceiver  10 .  
         [0061]    [0061]FIG. 6 shows a schematic block diagram of an embodiment of the optical transceiver  18 - 1  capable of generating an original test pattern. The other optical transceivers  18 - 2 ˜ 18 -N also have the same configuration.  
         [0062]    A WDM optical multiplexer/demultiplexer  70  applies the light input from the optical fiber  16 - 1  to an optical receiver  72 . The optical receiver  72  converts the timeslot part assigned to the optical transceiver  18 - 1  in the input light into an electric signal and outputs for a transmission/reception circuit  74 .  
         [0063]    The transmission/reception circuit  74  transmits/receives data to/from the station transceiver  10  in communication mode. In test mode, the transmission/reception circuit  74  identifies ID showing return test pattern from the signal carried by the optical trigger signal from the station transceiver  10  and applies to the test pattern generator  78 , and applies a synchronous signal for synchronizing the test pattern with the idle pattern to the test pattern generator  78 . The test pattern generator  78  generates a test pattern having a pattern-content according to the ID identified by the ID identifier  76  in synchronization with the synchronous signal from the transmission/reception circuit  74  and applies to a b-contact of switch  80 .  
         [0064]    The transmission/reception circuit  74  also receives control command for the optical transceiver  18 - 1  and controls each part of the optical transceiver  18 - 1  according to the received command. For instance, although a switch  80  normally connects to a-contact (the output of the transmission/reception circuit  74 ), the transmission/reception circuit  74  connects the switch  80  to the b-contact (the output of the test pattern generator  78 ) when it receives a command to instruct shifting to the test mode from the station transceiver  10 .  
         [0065]    The signal selected at the switch  80  is applied to the optical transmitter  82  and converted to an optical signal of wavelength λu. The optical signal (wavelength λu) output from the optical transmitter  82  is transmitted onto the optical fiber  16 - 1  by the WDM optical multiplexer/demultiplexer  70  and enters the station transceiver  10  through the optical fiber  16 - 1 , the optical multiplexer/demultiplexer  14 , and the optical fiber  12 .  
         [0066]    In the test mode, since the switch  80  connects to the b-contact, the test pattern optical signal  22  to carry the test pattern output from the test pattern generator  78  is transmitted from the optical transceiver  18 - 1  to the station transceiver  10 .  
         [0067]    Although explained above is an embodiment in which a node measuring apparatus is built in the station transceiver  10 , a configuration such that a node measuring apparatus with the above-described function is disposed outside the station transceiver is obviously applicable.  
         [0068]    As readily understandable from the aforementioned explanation, according to the invention, even if communication of a TDM optical network is inhibited due to a fault of a specific node apparatus, it is possible to check normally operating nodes easily and precisely. That is, the node having the fault is identified quickly.  
         [0069]    While the invention has been described with reference to the specific embodiment, it will be apparent to those skilled in the art that various changes and modifications can be made to the specific embodiment without departing from the spirit and scope of the invention as defined in the claims.