Abstract:
A bidirectional optical communication system employing a living line and a standby line. One of a master unit and a slave unit has a photoswitch for switching between the living line and the standby line and the other has a photocoupler for collecting the living line and the standby line. When deterioration in communication state of the living line is detected on the side having the photoswitch, the photoswitch on that side is switched to use the standby line.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a communication system using an active line and a standby line for improving reliability of communication.  
         [0002]     In order to transmit large amounts of information, for example, a communication system by means of optical communication is utilized. An example of a large amount of information includes image information, and that of a live telecast, the above communication system is utilized. It is important in a live telecast to avoid an incident causing interruption of image. For this purpose, the communication cable of the communication system is multiplexed. For example, the active line and the standby line are used for communication (Jpn. unexamined patent publication No. 8-293854).  
         [0003]      FIG. 1  is a diagram exemplifying the conventional communication system by means of the active line and the standby line used for optical communication. The communication system comprises the master apparatus  101 , and the slave apparatus  102 . The master apparatus  101  comprises the first transmitter/receiver unit  103 , and the first optical switching unit  104 , and the slave apparatus  102  comprises the second transmitter/receiver unit  105 , and the second optical switching unit  106 .  
         [0004]     The first optical switching unit  104  comprises the terminals  109 ,  110 , and  111 . The terminal  109  is connected to the first transmitter/receiver unit  103 , the terminal  110  is connected to the active line  107 , and the terminal  111  is connected to the standby line  108 . The first optical switching unit  104  is able to connect the terminal  109  to the terminal  110  or  111 . Similar to the above configuration, the second optical switching unit  106  comprises the terminal  102 ,  113 , and  114 . The terminal  112  is connected to the second transmitter/receiver unit  105 , the terminal  113  is connected to the active line  107 , and the terminal  114  is connected to the standby line  108 . The second optical switching unit  106  is able to connect the terminal  102  to the terminal  113  or  114 .  
         [0005]     As shown in  FIG. 1 , the terminal  109  is connected to the terminal  110  in the first optical switching unit  104 , and the terminal  112  is connected to the terminal  113  in the second optical switching unit  106 , so that it becomes possible to the communication by means of the active line  107  is carried out. If interruption occurs in the active line  107 , the terminal  109  and the terminal  111  are connected in the first optical switching unit  104 , and the terminal  112  and the terminal  114  are connected in the second optical switching unit  106 , so that communication by means of the standby line  108  is carried out.  
         [0006]     Further,  FIG. 2  is a diagram exemplifying a configuration of the optical switching unit used for the first optical switching unit  104  and the second optical switching unit  106 . The optical switching unit comprises the input/output terminal  201 ,  202 ,  203 , and the mirror  204 . When the mirror  204  is inserted, optical transmission/reception between the input/output terminal  202  and  203  becomes possible. Accordingly, for example, as a result of the communication between the input/output terminal  202  and the terminal  110 , between the input/output terminal  201  and the terminal  111 , and between the input/output terminal  203  and the terminal  109 , the first optical switching unit  104  can be configured. Similarly, the above can be applied to the second optical switching unit  106 .  
         [0007]     Note that, although selection of two-to-one optical input/output is possible in  FIG. 2 , selection of two-to-two optical input/output is also possible.  
         [0008]      FIG. 3  is a diagram exemplifying a configuration of the optical switching unit, which is able to select two-to-two optical input/output. The optical switching unit shown in  FIG. 3  comprises the input/output terminal  201 ,  202 ,  203  and  205 . When the mirror  204  is inserted, optical transmission/reception between the input/output terminal  201  and  205 , and between the input/output terminal  202  and  203  become possible. Meanwhile, when the mirror  204  is not inserted, optical transmission/reception between the input/output terminal  201  and  203 , and between the input/output terminal  202  and  205  become possible.  
         [0009]     The cited document: Jpn. unexamined patent publication No. 8-293854  
         [0010]     According to the configuration the master apparatus and the slave apparatus comprise the optical switching unit, it is necessary to perform switching in the first optical switching unit of the master apparatus and in the second optical switching unit of the slave apparatus at the same time. However, it is difficult to perform the switching at the same time. For example, for some reason, if the master apparatus detects the occurrence of temporary interruption such as instantaneous interruption of the active line, and the slave apparatus does not detect it, and the switching only in the first optical switching unit of the master apparatus is carried out, and communication between the master apparatus and the slave apparatus becomes impossible. Further, in cases where the switching are respectively performed in the master apparatus and the slave apparatus, if the timings of the switching are not same, the communication is impossible.  
       BRIEF SUMMARY OF THE INVENTION  
       [0011]     It is an objective of the present invention to carry out smooth switching between an active line and a standby line in a communication system using the active line and the standby line.  
         [0012]     In order to achieve the above objective, according to the present invention, a communication system, wherein one of the master apparatus or the slave apparatus comprises an optical switching unit, the other comprises an optical coupler unit, and if bad communication status of the active line is detected in the apparatus comprising the optical switching unit, the standby line is used by causing the optical switching unit to perform switching to the standby line, is provided.  
         [0013]     The optical switching unit exists only in the master apparatus or the slave apparatus, so that it enables smooth switching between the active line and the standby line. Accordingly, the above deficiency is overcome.  
         [0014]     The apparatus comprising the optical switching unit monitors the optical signal transmitted from said slave apparatus via said standby line, and if level of said optical signal is lower than a predetermined level or is undetectable, the connection to said optical switching unit may be maintained.  
         [0015]     According to the above configuration, even if the apparatus, which does not comprise the optical switching unit, becomes unable to transmit the signal for some reason such as a maintenance problem, the active line remains in use, so that it becomes possible to avoid interruption of signals due to unnecessary switching in the optical switching unit.  
         [0016]     In addition, in cases where the standby line is used, the apparatus comprising the optical switching unit may be able to acquire an approval signal for returning to the active line.  
         [0017]     According to the above configuration, in the case of recovery of the active line from interruption, it becomes possible to return to the active line at the appropriate moment.  
         [0018]     As described above, according to the present invention, it becomes possible to carry out smooth switching between an active line and a standby line in a communication system using the active line and the standby line. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     Accompanying the specification are figures which assist in illustrating the embodiments of the invention, in which:  
         [0020]      FIG. 1  is a diagram exemplifying the conventional communication system;  
         [0021]      FIG. 2  is a diagram exemplifying a configuration of the optical switching unit;  
         [0022]      FIG. 3  is a diagram exemplifying a configuration of the optical switching unit;  
         [0023]      FIG. 4  is a functional block diagram of the bidirectional optical communication system of the first embodiment of the present invention;  
         [0024]      FIG. 5  is a diagram exemplifying a configuration of the first receiver/transmitter  403 ;  
         [0025]      FIG. 6  is a diagram exemplifying a configuration of the second receiver/transmitter  407 ;  
         [0026]      FIG. 7  is a schematic diagram of the configuration of the optical coupler unit;  
         [0027]      FIG. 8  is a diagram exemplifying a configuration of the optical coupler unit;  
         [0028]      FIG. 9  is a diagram illustrating driving of the optical switching unit  404  by the optical switch driving unit  405 ;  
         [0029]      FIG. 10  is a flow chart explaining processing flow in the master apparatus;  
         [0030]      FIG. 11  is a functional block diagram of the master apparatus of the bidirectional optical communication system of the second embodiment of the present invention;  
         [0031]      FIG. 12  is a diagram illustrating connection between the first receiver/transmitter and the level monitoring unit;  
         [0032]      FIG. 13  is a flow chart explaining processing flow in the master apparatus;  
         [0033]      FIG. 14  is a functional block diagram of the master apparatus of the bidirectional optical communication system of the second embodiment of the present invention;  
         [0034]      FIG. 15  is a functional block diagram of the master apparatus of the bidirectional optical communication system of the third embodiment of the present invention;  
         [0035]      FIG. 16  is a flow chart explaining processing flow in the master apparatus;  
         [0036]      FIG. 17  is a functional block diagram of the master apparatus of the bidirectional optical communication system of the fourth embodiment of the present invention;  
         [0037]      FIG. 18  is a functional block diagram of the master apparatus of the bidirectional optical communication system of the fifth embodiment of the present invention;  
         [0038]      FIG. 19  is a functional block diagram of the master apparatus of the bidirectional optical communication system of the sixth embodiment of the present invention;  
         [0039]      FIG. 20  is a graph in which the horizontal axis  2001  indicates wavelength, and the vertical axis  2002  indicates optical signal level. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0040]     Embodiments of the present invention will be described hereinbelow with reference to the drawings. The present invention is not to be limited to the above embodiments and able to be embodied in various forms without departing from the scope thereof.  
         [0041]     As the first embodiment of the present invention, a communication system, wherein one of the master apparatus or the slave apparatus comprises a switching unit, which is able to select an active line or a standby line, another comprises a coupler unit, which couples said active line and standby line, and if bad communication status of the active line is detected in the apparatus comprising the switching unit, the standby line is used by causing the switching unit to perform switching to the standby line, will be described. In addition, hereinbelow, the bidirectional optical communication system as an example of the above communication system, which carries out bidirectional optical communication by means of cables or fibers passed by optical signals, will be described.  
         [0042]      FIG. 4  is a functional block diagram of the bidirectional optical communication system of the first embodiment of the present invention. The bidirectional optical communication system comprises the master apparatus  401  and the slave apparatus  402 .  
         [0043]     The master apparatus  401  comprises the first transmitter/receiver unit  403 , and the optical switching unit  404 , and the optical switch driving unit  405 . The slave apparatus  402  comprises the optical coupler unit  406 , and the second transmitter/receiver unit  407 .  
         [0044]     The master apparatus  401  and the slave apparatus  402  are able to carry out bidirectional communication by means of the active line  408  and the standby line  409 . Note that the active line  408  and the standby line  409  consist of optical cable or optical fiber.  
         [0045]     The ‘first transmitter/receiver unit’  403  receives an optical signal. Therefore, it comprises transmission means for optical signal and reception means for optical signal.  
         [0046]      FIG. 5  is a diagram exemplifying a configuration of the first receiver/transmitter  403 . The first transmitter/receiver unit  403  comprises the transmission means  501 ,  502 ,  503  etc., which transmit optical signals, and further comprises at least one reception means  504 , which receives the optical signal. The communication path for transmitting the optical signal used by the transmission means  501 ,  502 ,  503 , and the reception means  504  are finally assembled, and extracted to the outside of the first receiver/transmitter  403 . Note that, for example, it is preferable that wavelengths of the optical signals transmitted by the respective transmission means are different, so that the signals transmitted by the transmission means  501 ,  502 , and  503  are distinguishable by the receiver. Therefore, it is preferable that signals of multiple wavelengths are multiplexed and transmitted/received.  
         [0047]     For example, a dielectric multilayer filter (TFF) transmits only an optical signal of any one wavelength, and reflects signals of all other wavelengths. Further, when an optical signal of any one wavelength is irradiated to the one side of the dielectric multilayer filter, and optical signals of multiplexed wavelengths are irradiated to the other side of it, all optical signals of multiplexed wavelengths are reflected, and the optical signal of any one wavelength irradiated to the one side is added. By means of this property, the transmission means  501 ,  502 , and  503  etc. and the reception means  504  are linked together through TFF, so that it becomes possible to carry out multiplexing and transmission/reception of lights of multiple wavelengths.  
         [0048]     The ‘optical switching unit’  404  is configured by a switch such as optical switches shown in  FIG. 2  or  3 , which is able to select the active line  408  or the standby line  409  to be connected to the first transmitter/receiver unit  403 . Note that, distinction between the active line  408  and the standby line  409  may be relative. Therefore, a line currently connected to the first transmitter/receiver unit  403  may be named as an active line, and the other may be named as a standby line. Alternatively, distinction between the active line  408  and the standby line  409  may be absolute. Therefore, the line mainly used may be named as an active line.  
         [0049]     The ‘optical switch driving unit’ 405  is for driving the optical switching unit  404 . In cases where the optical switching unit  404  is configured by the optical switch shown in  FIG. 2  or  3 , it controls insertion/removal of the mirror  204 . For example, by controlling an actuator, insertion/removal of the mirror  204  is carried out.  
         [0050]     The ‘optical coupler unit’  406  couples the active line  408  and the standby line  409 . Therefore, it couples optical signals to be transmitted to the slave apparatus  402  via the active line  408  and the standby line  409 , and separates the optical signal transmitted from the second transmitter/receiver unit  407 , which will be described hereinbelow, and outputs them via the active line  408  and to the standby line  409 .  
         [0051]      FIG. 7  is a schematic diagram of the configuration of the optical coupler unit. In the optical coupler unit  406 , the terminal  701 ,  702 , and  703  exist, the terminal  701  is connected to the second transmitter/receiver unit  407 , the terminal  702  is connected to the active line  408 , and the terminal  703  is connected to the standby line  409 . The optical signals transmitted to the slave apparatus  402  via the active line  408  and the standby line  409  are coupled and transmitted to the terminal  701 , and outputted to the second transmitter/receiver unit  407 . Meanwhile, the optical signal transmitted to the terminal  701  by the second transmitter/receiver unit  407  is separated and transmitted to the terminal  702  and the terminal  703 , and outputted to the active line  408  and the standby line  409 .  
         [0052]      FIG. 8  is a diagram exemplifying a configuration of the optical coupler unit  406 . In  FIG. 8 , the optical coupler is configured by two optical fibers  801  and  802 , which have been fusion bonded at the portion  803 . By means of this configuration, the optical signal inputted to the edge point  804  is separately outputted to the edge point  805  and  806 . Meanwhile, the optical signals inputted to the edge point  805  and  806  are coupled and are outputted to the edge point  804 . Accordingly, by relating the edge point  804  to the terminal  701 , and by relating the edge point  805  and  806  to the terminal  702  and  703 , respectively, the optical coupler  406  can be configured.  
         [0053]     The ‘second transmitter/receiver unit’  407  receives an optical signal. Further, as described above, it is connected to the optical coupler unit  406 . The second transmitter/receiver unit  407  comprises transmission means for optical signal and reception means for optical signal.  
         [0054]      FIG. 6  is a diagram exemplifying a configuration of the second receiver/transmitter  407 . The second transmitter/receiver unit  407  comprises the reception means  601 ,  602 ,  603  etc., which receive optical signals, and further comprises at least one transmission means  604 , which transmits the optical signal. The communication path for transmitting the optical signal used by the reception means  601 ,  602 ,  603 , and the transmission means  604  are finally assembled, and extracted to the outside of the second receiver/transmitter  407 . Note that, for example, it is preferable that wavelengths of the optical signals received by the respective reception means are different, so that the signals received by the reception means  601 ,  602 , and  603  are transmitted from different transmission means. For example, as described above, multiplexing the wavelengths is carried out by means of TFF. Therefore, it is preferable that the reception means  601  receives the light of the wavelength transmitted by the transmission means  501 , the reception means  602  receives the light of the wavelength transmitted by the transmission means  502 , and the reception means  603  receives the light of the wavelength transmitted by the transmission means  503 .  
         [0055]     In the first embodiment, the optical switching unit  404  is driven by the optical switch driving unit  405  in the master apparatus  401  as described hereinbelow. Normally, the active line  408  and the first transmitter/receiver unit  403  are connected. If bad communication status of the active line  408  is detected through the optical signal received by the first transmitter/receiver  403 , the standby line  409  and the first transmitter/receiver unit  403  are connected. The terms ‘bad communication status’ means, for example, the level of the optical signal transmitted from the slave apparatus decreases, so that communication becomes impossible or almost impossible. Further, a state, in which the interruption in communication occurs, for example, an error concerning parity increases, or a state, in which occurrence of the interruption is expected, is caused. This bad communication status is, for example, detected by the reception means of the first transmitter/receiver unit  403 . Alternatively, a monitoring unit for communication status of the active line other than the reception unit of the first transmitter/receiver unit  403  may be provided.  
         [0056]      FIG. 9  is a diagram illustrating driving of the optical switching unit  404  by the optical switch driving unit  405 .  FIG. 9  ( a ) shows a normal state, in which the active line  408  and the first transmitter/receiver unit  403  are connected. For example, in cases where the optical switching unit  404  is configured by means of the optical switch shown in  FIG. 3 , the terminal  901 ,  902 ,  903 , and  904  are related to the input/output terminal  201 ,  203 ,  202 , and  205 , respectively, and the mirror  204  is removed.  FIG. 9  ( b ) shows a connection state in cases where bad communication status of the active line  408  is detected. The terminal  901  and  904  are connected, and the standby line  409  and the first transmitter/receiver unit  403  are connected. This state corresponds to the state shown in  FIG. 3 , in which the mirror  204  is inserted into the optical switch.  
         [0057]      FIG. 10  is a flow chart explaining processing flow in the master apparatus. In step S 1001 , the active line and the first transmitter/receiver unit are connected (the connection step in a normal state). This is a normal connection. After that, in step S 11002 , detection of bad communication status of the active line is awaited. If bad communication status of the active line is detected, in step S 1003 , the standby line and the first transmitter/receiver unit are connected (the connection step in abnormal state).  
         [0058]     Note that, after the standby line and the first transmitter/receiver unit are connected, the names of the standby line and the active line may be exchanged. Therefore, this is the case where distinction between the active line and the standby line is relative, so that the communication path, to which the code  408  is attached, may be named as active line. In such case, after the processing of step S 1003 , names of the standby line and the active line are exchanged and step S 1002  is carried out.  
         [0059]     According to the first embodiment, the optical switching unit exists only in the master apparatus, so that it enables smooth switching between the active line and the standby line.  
         [0060]     As the second embodiment of the present invention, the bidirectional optical communication system, wherein the master apparatus monitors the optical signal transmitted from said slave apparatus via said standby line, and maintains the connection to the optical switching unit if said level monitoring unit detects that level of the optical signal is lower than a predetermined level, or is undetectable.  
         [0061]      FIG. 11  is a functional block diagram of the master apparatus of the bidirectional optical communication system of the second embodiment of the present invention. The master apparatus  1101  comprises the first transmitter/receiver unit  403 , and the optical switching unit  404 , the optical switch driving unit  405 , and the level monitoring unit  1102 , and the optical switch driving unit  405  comprises the maintaining means upon detecting insufficient level  1103 . The configuration of the slave apparatus is the same as that of the first embodiment. Therefore, the bidirectional optical communication system of the second embodiment is the bidirectional optical communication system according to the first embodiment, wherein the master apparatus comprises the level monitoring unit, and the optical switch driving unit comprises the maintaining means upon detecting insufficient level.  
         [0062]     The ‘level monitoring unit’  1102  monitors the optical signal transmitted from the slave apparatus  402  via the standby line  409 . For example, the monitoring is carried out by measuring intensity of the optical signal or frequency of occurrence of errors in information in the optical communication. Therefore, the level monitoring unit  1102  is connected to the standby line.  
         [0063]      FIG. 12  is a diagram illustrating a connection between the first receiver/transmitter and the level monitoring unit.  FIG. 12  ( a ) corresponds to the normal state. The optical switching unit  404  connects the active line  408  and the first transmitter/receiver unit  403 , and the standby line  409  and the level monitoring unit  1102 .  FIG. 12  ( b ) corresponds to the state in which bad communication status of the active line is detected through the optical signal received by the first transmitter/receiver. The optical switching unit  404  connects the standby line  409  and the first transmitter/receiver unit  403 , and may connect the active line  408  and the level monitoring unit  1102 . For example, in cases where names of the active line and the standby line are exchanged due to the bad communication status of the active line, the communication line, to which the code  408  is attached, is connected to the level monitoring unit  1102 .  
         [0064]     The ‘maintaining means upon detecting insufficient level’  1103  maintains the connection to the optical switching unit  404  if the level monitoring unit  1102  detects that level of said optical signal is lower than a predetermined level. Here, the term ‘said optical signal’ corresponds to the optical signal transmitted from the stave apparatus  402  via the standby line  409 . Further, the terms ‘maintains the connection to the optical switching unit  404 ’ means that the connection between the active line  408  and the first transmitter/receiver unit  403  is maintained.  
         [0065]     In the present invention, since the optical signal transmitted from the first transmitter/receiver unit is divided by the optical coupler unit of the slave apparatus, basically, the same optical signal is transmitted from the slave apparatus  402  via the active line  408  and the standby line  409 . If an interruption such as disconnection of the standby line  409  occurs, connection between the standby line  409  and the first transmitter/receiver unit  403  is worthless, so that connection between the active line  408  and the first transmitter/receiver unit  403  is maintained. Further, in cases where a portion or all of the transmission means of the second transmitter/receiver unit  407  stop due to a maintenance of the second transmitter/receiver unit  407  etc., level of the optical signal received by the first transmitter/receiver unit  403  decreases or the optical signal received by the first transmitter/receiver unit  403  disappears even if any interruption occurs in the active line  408 , so that there is a possibility that bad communication status of the active line is detected. However, there is no actual interruption in the active line  408 , and switching between the active line and the standby line is unnecessary, so that connection between the active line  408  and the first transmitter/receiver unit  403  is maintained.  
         [0066]      FIG. 13  is a flow chart explaining processing flow in the master apparatus. In step S 1301 , the active line  408  and the first transmitter/receiver unit  403  are connected (the connection step in normal state). In step S 1302 , detection of bad communication status of the active line is awaited. If bad communication status of the active line is detected, in step S 1303 , it is determined whether the level monitoring unit  1102  has detected an optical signal, in other terms, whether the level of the optical signal transmitted via the standby line  409  is lower than a predetermined level. If the optical signal has been detected, step S 1304  is carried out. If not, processing returns to step S 1302 . In step S 1304 , the standby line  409  and the first transmitter/receiver unit  403  are connected (the connection step in abnormal state).  
         [0067]     According to the second embodiment, it becomes possible to prevent unnecessary switching between the active line and the standby line in cases where the standby line is unavailable or it appears that interruption occurs in the active line. Specifically, in case where it appears that interruption occurs in the active line even if there is no actual interruption, it becomes possible to prevent instantaneous interruption of communication, which occurs upon switching from the active line to the standby line.  
         [0068]     As the third embodiment of the present invention, the bidirectional optical communication system, wherein the master apparatus is able to acquire an approval signal for returning to a state of using the active line, will be described.  
         [0069]      FIG. 14  is a functional block diagram of the master apparatus of the bidirectional optical communication system of the third embodiment of the present invention. The master apparatus  1401  comprises the first transmitter/receiver unit  403 , and the optical switching unit  404 , the optical switch driving unit  405 , and the monitoring unit for communication status of active line  1402 , and the optical switch driving unit  405  may comprise the acquisition means for approval signal  1403 . In addition, as shown in  FIG. 15 , the master apparatus may comprise the level monitoring unit, and the optical switch driving unit may comprise the maintaining means upon detecting insufficient level. The configuration of the slave apparatus is the same as that of the first or second embodiment. Therefore, the bidirectional optical communication system of the third embodiment is the bidirectional optical communication system according to the first or second embodiment, wherein the master apparatus comprises the monitoring unit for communication status of active line, and the optical switch driving unit comprises the acquisition means for approval signal.  
         [0070]     The ‘monitoring unit for communication status of active line’  1402  is able to monitor a communication status of the active line  408 . The monitoring unit for communication status of active line  1402  is able to monitor the communication status of the active line  408  even when the standby line  409  and the first transmitter/receiver unit  403  are connected. Therefore, as shown in  FIG. 14  or  15 , the monitoring unit for communication status of active line  1402  acquires the optical signal of the active line  408  from the optical switching unit  404 . Alternatively, the optical signal transmitted from the slave apparatus via the active line  408  may be divided by an optical coupler etc., and may be acquired.  
         [0071]     The ‘acquisition means for approval signal’  1403  acquires an approval signal if the optical switching unit  404  of the master apparatus  1401  connects the standby line  409  to the first transmitter/receiver  403 , and the monitoring unit for communication status of active line  1402  detects good communication status of the active line  408 . Here, the term ‘approval signal’ corresponds to a signal for causing the optical switching unit  404  to connect the active line  408  and the first transmitter/receiver  403 . For example, this signal is inputted from the input apparatus connected to the master apparatus  1401 , and is acquired by the acquisition means for approval signal  1403 . Alternatively, the signal, which indicates that the monitoring unit for communication status of active line  1402  detects good communication status of the active line  408  after bad communication status, may be an approval signal, and may be acquired by the acquisition means for approval signal  1403 .  
         [0072]      FIG. 16  is a flow chart explaining processing flow in the master apparatus. In step S 1601 , the active line  408  and the first transmitter/receiver unit  403  are connected (the connection step in normal state). In step S 1602 , detection of bad communication status of the active line is awaited. If bad communication status of the active line is detected, in step S 1603 , the standby line  409  and the first transmitter/receiver unit  403  are connected. After that, in step S 1604 , detection of good communication status of the active line  408  by the monitoring unit for communication status of active line  1402  is awaited. If the good communication status of the active line  408  is detected, in step S 1605 , acquisition of the approval signal by the acquisition means for approval signal  1403  is awaited. When the approval signal is acquired, in step S 1605 , the active line  408  and the first transmitter/receiver unit  403  are connected.  
         [0073]     Note that, in cases where the master apparatus may comprise the level monitoring unit, and the optical switch driving unit may comprise the maintaining means upon detecting insufficient level as in the second embodiment, a step, which is for determining whether the level monitoring unit has detected the optical signal, is added between step S 1602  and S 1603 . If the optical signal has not been detected, processing returns to step S 1602 , and if the optical signal has been detected, step S 1603  is carried out.  
         [0074]     According to the third embodiment, it becomes possible to return to a state of using the active line at appropriate timing in cases where interruption once occurs in the active line and is later recovered.  
         [0075]     As the fourth embodiment of the present invention, the bidirectional optical communication system, wherein the ‘predetermined level’ described in the second embodiment is stored, will be described.  
         [0076]      FIG. 17  is a functional block diagram of the master apparatus of the bidirectional optical communication system of the fourth embodiment of the present invention. The master apparatus  1701  comprises the first transmitter/receiver unit  403 , and the optical switching unit  404 , and the optical switch driving unit  405 , and the optical switch driving unit  405  may comprise the maintaining means upon detecting insufficient level  1103 , and the level monitoring unit  1102  comprises the storing means for predetermined level  1702 . The configuration of the slave apparatus is the same as that of the second embodiment. Therefore, the bidirectional optical communication system of the fourth embodiment is the bidirectional optical communication system according to the second embodiment, wherein the level monitoring unit of the master apparatus comprises the storing means for predetermined level.  
         [0077]     The ‘storing means for predetermined level’  1702  stores said predetermined level. For example, in cases where a portion including the level monitoring unit  1102  is implemented by means of microcomputer etc., the storing means for predetermined level is configured by means of a flash memory, and the predetermined level is stored in the flash memory. Note that, the storing means for predetermined level may be configured by means of an ordinary semiconductor memory, instead of the memory such as a flash memory, which performs permanent storage. Accordingly, in the fourth embodiment, the level monitoring unit  1102  reads a predetermined level from the storing means for predetermined level  1702  according to necessity, and compares the read level and the level of the optical signal transmitted from the slave apparatus via the standby line. The terms ‘according to necessity’ refers to such as the case where the level monitoring unit  1102  becomes operatable after turning on or reset of the master apparatus is carried out, the case where passage of a certain period of time or of a predetermined time is detected, or the case where a predetermined level stored by the storing means for predetermined level is changed.  
         [0078]     According to the fourth embodiment, it becomes possible to remove botheration in setting a predetermined level of the level monitoring unit  1102  with respect to each turning on etc. of the master apparatus, and to change the predetermined level as described hereinbelow.  
         [0079]     As the fifth embodiment of the present invention, the bidirectional optical communication system, wherein the ‘predetermined level’ described in the second embodiment is changeable, will be described.  
         [0080]      FIG. 18  is a functional block diagram of the master apparatus of the bidirectional optical communication system of the fifth embodiment of the present invention. The master apparatus  1801  comprises the first transmitter/receiver unit  403 , and the optical switching unit  404 , and the optical switch driving unit  405 , and the optical switch driving unit  405  may comprise the maintaining means upon detecting insufficient level  1103 , and the level monitoring unit  1102  comprises the storing means for predetermined level  1702 , the acquisition means for predetermined level  1802 , and the changing means for predetermined level  1803 . The configuration of the slave apparatus is the same as that of the fourth embodiment. Therefore, the bidirectional optical communication system of the fifth embodiment is the bidirectional optical communication system according to the fourth embodiment, wherein the level monitoring unit of the master apparatus comprises the acquisition means for predetermined level and the changing means for predetermined level  
         [0081]     The ‘acquisition means for predetermined level’  1802  acquires the predetermined level to be stored by the storing means for predetermined level. This acquisition may be carried out, for example, by acquiring a value inputted to the input apparatus connected to the master apparatus  1801 . For example, the input apparatus may be a personal computer, which comprises a keyboard, a mouse, and a display, and to which an operator of the master apparatus can input the value.  
         [0082]     In addition, the acquisition means for predetermined level  1802  may appropriately generate the level. For example, in cases where the operation of an amplifier of the signal in the standby line changes according to passage of time, a different level according to the passage of time may be generated.  
         [0083]     The ‘changing means for predetermined level’  1803  changes the predetermined level stored by the storing means for predetermined level  1702  to the level acquired by the acquisition means for predetermined level  1802 .  
         [0084]     Accordingly, in the fifth embodiment, when a level is acquired by the acquisition means for predetermined level  1802 , the changing means for predetermined level  1803  changes the predetermined level stored by the storing means for predetermined level  1702  to the level acquired by the acquisition means for predetermined level  1802 . After that, the level monitoring unit  1102  reads a predetermined level stored by the storing means for predetermined level according to necessity, and compares the read level and the level of the optical signal transmitted from the slave apparatus via the standby line.  
         [0085]     According to the fifth embodiment, it becomes possible to change the level of the optical signal, at which the maintaining means upon detecting insufficient level  1103  operates. For example, in cases where the number of transmission means of the second transmitter/receiver unit, or in cases where setting of an amplifier for the standby line is changed, it becomes possible to set an appropriate level.  
         [0086]     As the sixth embodiment of the present invention, the bidirectional optical communication system, which detects number of transmission means of the second transmitter/receiver unit, and changes the ‘predetermined level’, will be described.  
         [0087]      FIG. 19  is a functional block diagram of the master apparatus of the bidirectional optical communication system of the sixth embodiment of the present invention. The master apparatus  1901  comprises the first transmitter/receiver unit  403 , and the optical switching unit  404 , and the optical switch driving unit  405 , and the optical switch driving unit  405  may comprise the maintaining means upon detecting insufficient level  1103 , and the level monitoring unit  1102  comprises the storing means for predetermined level  1702 , the acquisition means for predetermined level  1802 , the changing means for predetermined level  1803 , and the detection means for number of transmission means  1902 . The configuration of the slave apparatus is the same as that of the fifth embodiment. Therefore, the bidirectional optical communication system of the sixth embodiment is the bidirectional optical communication system according to the fifth embodiment, wherein the level monitoring unit of the master apparatus comprises the detection means for number of transmission means  1902 .  
         [0088]     The ‘the detection means for number of transmission means’  1902  detects number of transmission means, which transmits the optical signal, of the second transmitter/receiver unit  407 . For example, at least one transmission means of the second transmitter/receiver unit  407  transmits information indicating a configuration of the second transmitter/receiver unit  407 , which includes the number of the transmission means etc., and based on the transmitted information, the detection means for number of transmission means  1902  detects the number of the transmission means. Alternatively, in cases where transmission is carried out by means of different wavelengths with respect to each transmission means, by measuring intensity of the optical signal for the wavelength, and by detecting number of maximal points of the intensity, it becomes possible to detect the number of transmission means.  
         [0089]      FIG. 20  is a graph in which the horizontal axis  2001  indicates wavelength, and the vertical axis  2002  indicates optical signal level. In this case, three maximal points exist, so that the number of transmission means is three.  
         [0090]     In the sixth embodiment, the acquisition means for predetermined level  1802  acquires the level based on the number detected by the detection means for number of transmission means  1902 . For example, the value acquired by multiplying a preliminary determined value for one transmission means by the number of transmission means is generated as a level.  
         [0091]     According to the sixth embodiment, level according to the number of transmission means of the second transmitter/receiver unit is a predetermined level, so that it becomes possible to appropriately determine whether interruption occurs in the standby line.  
         [0092]     The bidirectional optical communication system of the present invention is effective in carrying out smooth switching between an active line and a standby line in the case of interruption, and is beneficial in industrial use.