Abstract:
The present invention has been made to realize an optical router capable of preventing optical signals from conflicting with each other and of efficiently allocating an optimum optical path.  
     The optical router is configured to select optical paths for optical signals transmitting through an optical transmission line disposed on an optical node basis, output the optical signals, and thereby form an optical communications network, and is comprised of:  
     optical delay means for delaying a plurality of the optical signals containing routing information added to the header parts thereof;  
     an optical switch provided with a plurality of input ports to which a plurality of optical outputs are input from the optical delay means and a plurality of output ports for outputting the optical signals;  
     optical-electrical conversion means for converting a plurality of the optical signals to electrical signals;  
     memory wherein path control information is stored; and  
     a controller for finding the path control information according to the routing information extracted from the output of the optical-electrical conversion means, determining the destinations of the optical signals based on the output requests therefrom, and selecting and assigning one input port in an arbitrary order from a single or plural input ports having output requests to a plurality of the output ports.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an optical router which is disposed on an optical node basis, selects the optical path of an optical signal transmitting through such an optical transmission line as an optical fiber, and outputs the optical signal through the optical path, whereby an optical communications network is formed. More specifically, the present invention relates to an optical router capable of preventing optical signals from conflicting with each other and of efficiently allocating an optimum optical path.  
           [0003]    2. Description of the Prior Art  
           [0004]    An optical router is disposed on an optical node basis, selects the optical path of an optical signal transmitting through such an optical transmission line as an optical fiber, and outputs the optical signal through the optical path, whereby an optical communications network is formed. Literatures on the prior art related to optical routers include the following:  
           [0005]    Japanese Laid-open Patent Application 1996-163610  
           [0006]    Japanese Laid-open Patent Application 1996-204675  
           [0007]    * Photonic Network Revolution—Technologies for realizing the world&#39;s most advanced information technology nations: published by the Secretariat, Photonic Internet Forum, within The Support Center for Advanced Telecommunications Technology Research, Foundation (January 2002): 95-98  
           [0008]    [0008]FIG. 1 is a block diagram illustrating an example of such conventional optical routers (optical burst switches, in particular) as mentioned above.  
           [0009]    In FIG. 1, optical switch  1  having three input ports and three output ports is controlled by controller  2  in terms of optical path selection. Optical switch  1  and controller  2  compose optical router  50 . The main signals, which are the optical input signals (optical burst signals) indicated by MS 01  in FIG. 1, are input to the three input ports of optical switch  1 . The three optical output signals indicated by MS 02  in FIG. 1 are output from the three output ports of optical switch  1  to a subsequent-stage optical router or the like.  
           [0010]    An optical control signal carrying routing information indicated by CS 01  in FIG. 1 is input to controller  2 . The electrical output signal of controller  2  indicated by SS 01  is coupled with the control terminal of optical switch  1 . The optical output signal of controller  2  is output to a subsequent-stage optical router or the like, as the optical control signal indicated by CS 02 .  
           [0011]    Now the behavior of an example of a conventional optical router illustrated in FIG. 1 is described by referring to FIG. 2, wherein FIG. 2 is a block diagram explaining the behavior of the example of the conventional optical router shown in FIG. 1. In FIG. 2, optical switch  1 , controller  2  and optical router  50  are the same as those shown in FIG. 1. The details of interconnection are also the same as those of the example of the conventional optical router illustrated in FIG. 1.  
           [0012]    First, an optical control signal indicated by CS 11  in FIG. 2 is transferred prior to such a main signal (optical input signal) as indicated by SG 11  in FIG. 2, and is input to controller  2 .  
           [0013]    Controller  2  then converts the optical control signal thus input, to an electrical control signal, as well as produces a control signal as indicated by SS 11  in FIG. 2, according to the routing information that the electrical control signal carries, in order to select the optical path of optical switch  1 .  
           [0014]    Now assume, for example, that the abovementioned routing information contains the information that the main signal is input from the input port of optical switch  1  indicated by PT 11  in FIG. 2, and is output from the output port of optical switch  1  indicated by PT 12 . Then, controller  2  controls optical switch  1  to select an optical path as indicated by SR 11  in FIG. 2.  
           [0015]    When such optical path selection as discussed above is completed, such a main signal as indicated by SG 11  in FIG. 2 is input to the input port of optical switch  1  indicated by PT 11 . Consequently, the main signal is output from an output port indicated by PT 12 , as indicated by SG 12  in the figure.  
           [0016]    As a result, by previously selecting the optical path of optical switch  1  that composes optical router  50  using an optical control signal, then transmitting an optical input signal which is the main signal, it is possible to select and control the paths of optical signals.  
           [0017]    However, the example of the conventional optical router illustrated in FIG. 1 has been problematic in that an optical control signal with a shifted transmission time must be separately transmitted in addition to the optical signal that is the main signal.  
           [0018]    For this reason, the patent application 2002-284970 filed by the applicant of the application concerned describes an example of an optical router wherein an optical signal being transmitted is split into a header part and a data part and routing information, such as a destination address, is added to the header part according to the routing information, thereby permitting a selection to be made from given optical paths.  
           [0019]    [0019]FIG. 3 is a block diagram illustrating an example of a prior art optical router described in the patent application 2002-284970.  
           [0020]    Indicated in FIG. 3 are optical delay means  3  for delaying an optical input signal by a specific length of time by transmitting the signal through an optical loop or the like; optical-electrical conversion means  4 , such as a photodiode or phototransistor; optical switch  5  provided with three input ports and three output ports; controller  6  for controlling optical path selection made by optical switch  5 ; and memory  7  wherein path control information, such as routing tables, is stored. In addition, optical delay means  3 , optical-electrical conversion means  4 , optical switch  5 , controller  6  and memory  7  compose optical router  51 .  
           [0021]    In FIG. 3, the three optical input signals (optical burst signals) indicated by SG 21  are input to the three input ends of optical delay means  3 , as well as to the three input ends of optical-electrical conversion means  4 .  
           [0022]    Optical output signals from the three output ends of optical delay means  3  are input to the three input ports of optical switch  5 , and the three optical output signals indicated by SG 22  in FIG. 3 are output from the three output ports of optical switch  5 .  
           [0023]    The electrical output signal of optical-electrical conversion means  4  is coupled with controller  6 , and the electrical output signal of controller  6  indicated by SS 21  in FIG. 3 is coupled with the control terminal of optical switch  5 . In addition, the electrical output signal of controller  6  is mutually coupled with memory  7 .  
           [0024]    Now the behavior of the example of the prior art optical router illustrated in FIG. 3 is described. Optical input signals, which are indicated by SG 21  in FIG. 3 and contain routing information, such as destination addresses, added to the header parts thereof, are delayed by specific lengths of time at optical delay means  3 .  
           [0025]    The optical input signals indicated by SG 21  in FIG. 3, which contain routing information, such as destination addresses, added to the header parts thereof, are converted to electrical signals at optical-electrical conversion means  4  and input to controller  6 .  
           [0026]    Controller  6  extracts the routing information from the electrical signal being input from optical-electrical conversion means  4 , finds path control information stored in memory  7  according to the routing information, specifies a subsequent-stage optical router (output port) appropriate for the entered optical signals to transmit to the destination through the shortest path, and accordingly selects from the optical paths of optical switch  5 .  
           [0027]    For example, controller  6  controls optical switch  5  so that an optical path is selected in such a manner that an optical input signal is input to the input port of optical switch  5  indicated by PT 21  in FIG. 3, and is output from the output port of optical switch  5  indicated by PT 22 .  
           [0028]    If such an optical input signal as is properly delayed by optical delay means  3  after the completion of such optical path selection as described above is input to the input port of optical switch  5  indicated by PT 21  in FIG. 3, the optical output signal will be output from the output port indicated by PT 22 .  
           [0029]    This means that by adding routing information, such as a destination address, to the header part of an optical signal, it is possible to make optical path selections according to the routing information. In this case, it is not necessary to separately transmit an optical control signal as shown in the example of the prior art optical router illustrated in FIG. 1.  
           [0030]    In such examples of the prior art optical router as illustrated in FIGS. 1 and 3, however, no consideration is given as to how to avoid a conflict between optical signals when a plurality of optical signals are input simultaneously. Consequently, a conflict between these optical input signals will occur if an attempt is made to output the simultaneously entered plural optical input signals onto the same output port. This example of the prior art optical router has been problematic since failures may occur such as a specific optical signal or signals not being transmitted at all (or being blocked).  
           [0031]    Let us take FIG. 4 as an example, which is a block diagram explaining problems inherent in the example of the prior art optical router illustrated in FIG. 1, where optical switch  1 , controller  2  and optical router  50  are identical with those shown in FIG. 1.  
           [0032]    If two optical input signals indicated by SG 31  and SG 32  in FIG. 4 are simultaneously input to the input ports of optical switch  1  indicated by PT 31  and PT 32  and an attempt is made to output the two signals to the one and the same output port indicated by PT 33 , such a conflict as indicated by CL 31  occurs, causing the abovementioned failure.  
           [0033]    Let us take FIG. 5 as another example, which is a block diagram explaining problems inherent in the example of the prior art optical router illustrated in FIG. 3, where optical delay means  3 , optical-electrical conversion means  4 , optical switch  5 , controller  6 , memory  7  and optical router  51  are identical with those shown in FIG. 3.  
           [0034]    If two optical input signals indicated by SG 41  and SG 42  in FIG. 5 are simultaneously input to the input ports of optical switch  5  indicated by PT 41  and PT 42  and an attempt is made to output the two signals to the one and the same output port indicated by PT 43 , such a conflict as indicated by CL 41  occurs, causing the abovementioned failure.  
         SUMMARY OF THE INVENTION  
         [0035]    An object of the present invention is to realize an optical router whereby it is possible to avoid a conflict between optical signals and to efficiently allocate an optimum optical path. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0036]    [0036]FIG. 1 is a block diagram illustrating an example of a prior art optical router;  
         [0037]    [0037]FIG. 2 is a block diagram explaining the behavior of the example of the prior art optical router;  
         [0038]    [0038]FIG. 3 is another block diagram illustrating an example of a prior art optical router;  
         [0039]    [0039]FIG. 4 is a block diagram explaining problems inherent in the example of the prior art optical router;  
         [0040]    [0040]FIG. 5 is another block diagram explaining problems inherent in the example of the prior art optical router;  
         [0041]    [0041]FIG. 6 is a block diagram illustrating one embodiment of an optical router in accordance with the present invention;  
         [0042]    [0042]FIG. 7 is a flowchart explaining the conflict avoidance action of a controller;  
         [0043]    [0043]FIG. 8 is a schematic view explaining an example of conflict avoidance action when optical input signals are simultaneously input to the three input ports of an optical switch;  
         [0044]    [0044]FIG. 9 is another schematic view explaining an example of conflict avoidance action when optical input signals are simultaneously input to the three input ports of an optical switch;  
         [0045]    [0045]FIG. 10 is yet another schematic view explaining an example of conflict avoidance action when optical input signals are simultaneously input to the three input ports of an optical switch;  
         [0046]    [0046]FIG. 11 is yet another schematic view explaining an example of conflict avoidance action when optical input signals are simultaneously input to the three input ports of an optical switch;  
         [0047]    [0047]FIG. 12 is yet another schematic view explaining an example of conflict avoidance action when optical input signals are simultaneously input to the three input ports of an optical switch;  
         [0048]    [0048]FIG. 13 yet is another schematic view explaining an example of conflict avoidance action when optical input signals are simultaneously input to the three input ports of an optical switch;  
         [0049]    [0049]FIG. 14 is yet another schematic view explaining an example of conflict avoidance action when optical input signals are simultaneously input to the three input ports of an optical switch; and  
         [0050]    [0050]FIG. 15 is yet another schematic view explaining an example of conflict avoidance action when optical input signals are simultaneously input to the three input ports of an optical switch. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0051]    Preferred embodiments of the present invention are described hereinafter in detail by referring to the accompanying drawings, wherein FIG. 6 is a block diagram illustrating one embodiment of an optical router in accordance with the present invention.  
         [0052]    In FIG. 6, optical delay means  3 , optical-electrical conversion means  4 , optical switch  5  and memory  7  are identical with those shown in FIG. 3, wherein FIG. 6 further includes controller  8  for controlling the optical path selection of optical switch  5 . In addition, optical delay means  3 , optical-electrical conversion means  4 , optical switch  5 , memory  7  and controller  8  compose optical router  52 .  
         [0053]    Three optical input signals (optical burst signals) indicated by SG 51  in FIG. 6 are input to the three input ends of optical delay means  3 , as well as to the three input ends of optical-electrical conversion means  4 .  
         [0054]    Optical output signals from the three output ends of optical delay means  3  are input to the three input ports of optical switch  5 , and the three optical output signals indicated by SG 52  are output from the three output ports of optical switch  5 .  
         [0055]    The electrical output signal of optical-electrical conversion means  4  is coupled with controller  8 , and the electrical output signal of controller  8  indicated by SS 51  is coupled with the control terminal of optical switch  5 . In addition, the electrical input-output signal of controller  8  is mutually coupled with memory  7 .  
         [0056]    Now the behavior of the embodiment illustrated in FIG. 6 is described. The embodiment illustrated in FIG. 6 is the same as the example of the prior art optical router illustrated in FIG. 3 in terms of basic behaviors. That is, optical input signals which are indicated by SG 51  in FIG. 6 and contain routing information, such as destination addresses, added to the header parts thereof, are delayed by specific lengths of time at optical delay means  3 .  
         [0057]    On the other hand, the optical input signals which are indicated by SG 51  in FIG. 6 and contain routing information, such as destination addresses, added to the header parts thereof are converted to electrical signals at optical-electrical conversion means  4  and input to controller  8 .  
         [0058]    Controller  8  extracts the routing information from an electrical signal being input from optical-electrical conversion means  4 , finds path control information stored in memory  7  according to the routing information, specifies a subsequent-stage optical router (output port) appropriate for the entered optical signals to transmit to the destination through the shortest path, and accordingly selects from the optical paths of optical switch  5 .  
         [0059]    For example, controller  6  controls optical switch  5  so that an optical path is selected in such a manner that the optical input signal is input to the input port of optical switch  5  indicated by PT 51  in FIG. 6, and is output from the output port of optical switch  5  indicated by PT 52 .  
         [0060]    If such an optical input signal as is properly delayed by optical delay means  3  after the completion of such optical path selection as described above is input to the input port of optical switch  5  indicated by PT 51 , the optical output signal will be output from the output port indicated by PT 52 .  
         [0061]    Furthermore, the avoidance of conflict between optical signals when a plurality of optical signals are simultaneously input is explained by referring to FIGS.  7  to  15 .  
         [0062]    [0062]FIG. 7 is a flowchart explaining the conflict avoidance action of controller  8 , FIGS.  8  to  11  are schematic views explaining examples of conflict avoidance action when optical input signals are simultaneously applied to the three input ports of optical switch  5 , and FIGS.  12  to  15  are schematic views explaining other examples of conflict avoidance action when optical input signals are simultaneously applied to the three input ports of optical switch  5 .  
         [0063]    In step S 001  of FIG. 7, controller  8  sorts the output ports of optical switch  5  in ascending order of the number of output requests from the input ports of optical switch  5 .  
         [0064]    Let us take FIG. 8 as an example, which illustrates the relationship of output requests between the input ports indicated by IP 61 , IP 62  and IP 63  and the output ports indicated by OP 61 , OP 62  and OP 63 . Each arrow in FIG. 8 represents an output request to such an output port to which an optical signal can be output from the input port of optical switch  5 , or in other words, an output request to such an output port with which the shortest path can be established.  
         [0065]    More specifically, the arrangement of optical routers differs depending on the configuration of an optical communications network. Therefore, there is a case where there are a plurality of subsequent-stage optical routers appropriate for an entered optical signal to transmit to the destination through the shortest path. Consequently, there may be a case where an output request is made from a single input port to a plurality of output ports.  
         [0066]    In the example shown in FIG. 8, the output port indicated by OP 61  has three output requests from the three input ports indicated by IP 61 , IP 62  and IP 63 . Likewise, the output ports indicated by OP 62  and OP 63  have output-requests one each from the two input ports indicated by IP 62  and IP 63 .  
         [0067]    Consequently, sorting the output ports of optical switch  5  in ascending order of the number of output requests from the input ports of optical switch  5  in step S 001  of FIG. 7 results in the order of OP 62  (one output request), OP 63  (one output request) and OP 61  (three output requests), as shown in FIG. 8.  
         [0068]    In step S 002  of FIG. 7, controller  8  identifies the first output port from the results of sorting to determine whether the number of output requests to the first output port is “one”.  
         [0069]    If the number of requests to the abovementioned output port proves to be “one” in step S 002  of FIG. 7, controller  8  assigns the only input port that has an output request to this output port, in step S 003 , then goes to step S 005 .  
         [0070]    If the number of output requests to the output port proves to be “not one,” or in other words “more than one”, in step S 002  of FIG. 7, controller  8  selects one input port from a plurality of input ports having output requests to the output port in step S 004 , assigns that input port to the output port, then goes to step S 005 .  
         [0071]    In step S 005  of FIG. 7, controller  8  cancels the output request from the assigned input port, and also cancels the output request to the output port to which the input port has been assigned, in step S 006 .  
         [0072]    In step S 007  of FIG. 7, controller  8  determines whether or not input ports have been assigned to all of the output ports. If not, controller  8  goes back to step S 002 . If the assignment is complete, controller  8  goes to step S 008 .  
         [0073]    Finally, in step S 008  of FIG. 7, controller  8  determines whether all output requests from the input ports are cancelled. If not, controller  8  takes measures, such as sending information indicating an abnormal end of conflict avoidance, to the source optical router in step S 009 .  
         [0074]    For example, sorting the output ports in step S 001  of FIG. 7 results in the order of OP 62  (one output request), OP 63  (one output request) and OP 61  (three output requests), as shown in FIG. 8. Thus, the number of outputs to the first output port indicated by OP 62  in FIG. 8 proves to be “one.” 
         [0075]    Consequently, in step S 003  of FIG. 7, controller  8  assigns the only input port indicated by IP 62  in FIG. 9 that has an output request to the output port indicated by OP 62 , as indicated by SL 71  in FIG. 9.  
         [0076]    Then, in steps S 005  and S 006  of FIG. 7 controller  8  cancels the output request from the assigned input port indicated by IP 62  in FIG. 9, as indicated by DL 71  in FIG. 9. By going through these steps, the relationship of output requests illustrated in FIG. 8 is changed to the one shown in FIG. 10.  
         [0077]    Likewise, the number of output requests to the output port indicated by OP 63  in FIG. 10, which is the second output port, is also “one.” 
         [0078]    Consequently, in step S 003  of FIG. 7, controller  8  assigns the only input port indicated by IP 63  in FIG. 10 that has an output request to the output port indicated by OP 63 , to that output port, as indicated by SL 81 .  
         [0079]    Then, in steps S 005  and S 006  of FIG. 7, controller  8  cancels the output request from the assigned input port indicated by IP 63  in FIG. 10, as indicated by DL 81  in FIG. 10. By going through these steps, the relationship of output requests illustrated in FIG. 10 is changed to the one shown in FIG. 11.  
         [0080]    Finally, the number of output requests to the output port indicated by OP 61  in FIG. 11, which is the third output port, has been decreased from “three” to “one” due to steps S 005  and S 006  of FIG. 7 taken for the output ports indicated by OP 62  and OP 63  in FIG. 11.  
         [0081]    Consequently, in step S 003  of FIG. 7, controller  8  assigns the only input port indicated by IP 61  in FIG. 11 that has an output request to the output port indicated by OP 61 , to that output port, as indicated by SL 91 .  
         [0082]    As an ultimate result, the input ports indicated by IP 61 , IP 62  and IP 63  in FIG. 11 are efficiently assigned to the output ports indicated by OP 61 , OP 62  and OP 63  in FIG. 11, thus making it possible to prevent optical signals from conflicting with each other.  
         [0083]    Let us take FIG. 12 as another example, which is a schematic view illustrating the relationship of output requests between the input ports indicated by IP 101 , IP 102  and IP 103  and the output ports indicated by OP 101 , OP 102  and OP 103 . Each arrow in FIG. 12 represents an output request to such an output port to which an optical signal can be output from the input port of optical switch  5 , or in other words, an output request to such an output port with which the shortest path can be established.  
         [0084]    In this relationship of output requests, sorting the output ports in step S 001  of FIG. 7, for example, results in the order of OP 101  (two output requests), OP 102  (two output requests) and OP 103  (two output requests), as shown in FIG. 12. Therefore, the number of output requests to the output port indicated by OP 101  in FIG. 12, which is the first output port, is “two.” 
         [0085]    Consequently, in step S 004  of FIG. 7, controller  8  assigns the only input port indicated by IP 101  in FIG. 13 that has an output request to the output port indicated by OP 101 , to that output port, as indicated by SL 111 .  
         [0086]    Then, in steps S 005  and S 006  of FIG. 7, controller  8  cancels the output request from the assigned input port indicated by IP 101  in FIG. 13 as indicated by DL 111  in FIG. 13, as well as the output request to the output port indicated by OP 101  in FIG. 13 as indicated by DL 112 . By going through these steps, the relationship of output requests illustrated in FIG. 13 is changed to the one shown in FIG. 14.  
         [0087]    Similarly, the number of output requests to the output port indicated by OP 102  in FIG. 14, which is the second output port, has been decreased from “two” to “one” due to steps S 005  and S 006  of FIG. 7 taken for the output port indicated by OP 101  in FIG. 14.  
         [0088]    Consequently, in step S 003  of FIG. 7, controller  8  assigns the only input port indicated by IP 102  in FIG. 14 that has an output request to the output port indicated by OP 102 , to that output port, as indicated by SL 121 .  
         [0089]    Then, in steps S 005  and S 006  of FIG. 7, controller  8  cancels the output request from the assigned input port indicated by IP 102  in FIG. 14 as indicated by DL 121  in FIG. 14. By going through these steps, the relationship of output requests illustrated in FIG. 14 is changed to the one shown in FIG. 15.  
         [0090]    Finally, the number of output requests to the output port indicated by OP 103  in FIG. 15, which is the third output port, has been decreased from “two” to “one” due to steps S 005  and S 006  of FIG. 7 taken for the output ports indicated by OP 101  and OP 102  in FIG. 15.  
         [0091]    Consequently, in step S 003  of FIG. 7, controller  8  assigns the only input port indicated by IP 103  in FIG. 15 that has an output request to the output port indicated by OP 103 , to that output port, as indicated by SL 131 .  
         [0092]    As an ultimate result, the input ports indicated by IP 101 , IP 102  and IP 103  in FIG. 15 are efficiently assigned to the output ports indicated by OP 101 , OP 102  and OP 103  in FIG. 15, thus making it possible to prevent optical signals from conflicting with each other.  
         [0093]    Consequently, by allowing controller  8  to sort the output ports of optical switch  5  in ascending order of the number of output requests from the input ports of optical switch  5  and to assign the input ports in sequence according to the results of sorting, it is possible to prevent optical input signals from conflicting with each other and to efficiently assign optimum optical paths, even if a plurality of optical input signals are simultaneously applied to the input ports.  
         [0094]    It should be noted that although the embodiment shown in FIG. 6 illustrates a case where optical switch  5  has three input ports and three output ports, it is possible for the optical switch to have an arbitrary number of input pots and an arbitrary number of output ports.  
         [0095]    It should also be noted that although the embodiment shown in FIG. 6 illustrates a case, for the sake of simpler description, where optical switch  5  satisfies the relationship of “m=n”, where m is the number of input ports and n is the number of output ports, the relationship between these two variables can be “m&gt;n” or “m&lt;n”. Although the embodiment shown in FIG. 6 illustrates a case where the electrical output signal, which is the output of optical-electrical conversion means  4 , is represented by a single signal line, it is possible to output routing information for each optical input signal in a parallel mode. Needless to say, the routing information for each optical input signal can be output in a serial mode, as illustrated in the embodiment shown in FIG. 6.  
         [0096]    Although the embodiment shown in FIG. 6 illustrates a case, for the sake of simpler description, where optical delay means  3  has three channels (multi-channel), it is of course possible to provide optical delay means separately for each optical input signal.  
         [0097]    Although the embodiment shown in FIG. 6 illustrates a case, for the sake of simpler description, where optical-electrical conversion means  4  has three channels (multi-channel), it is of course possible to provide optical-electrical conversion means separately for each optical input signal.  
         [0098]    Although the embodiment shown in FIG. 6 illustrates a case where the output ports of optical switch  5  are sorted in ascending order of the number of output requests from the input ports of optical switch  5  and the input ports are assigned in sequence according to the results of sorting, it is possible to select one input port in an arbitrary order from the plurality of input ports having output requests to a given output port and assign that input port to the output port, without carrying out the abovementioned sorting.  
         [0099]    It is also possible to define an order of priority for optical input signals, so that an input port to which an optical input signal of higher priority is applied is preferentially assigned to an output port, according to the priority order of the optical input signal applied to the input port. As a method of defining the priority order, it is possible, for example, to add priority order information to the header part of each optical input signal.  
         [0100]    In that case, preferentially assigning an input port of higher priority when selecting one input port from a plurality of input ports having output requests to the output ports increases the possibility of the higher-priority optical input signal being transmitted to a subsequent-stage optical router. In contrast, an optical input signal of lower priority is blocked and, therefore, the possibility of the optical input signal being transmitted to a subsequent-stage optical router decreases.  
         [0101]    It is also possible to define an order of priority for the destinations of optical input signals, so that an input port to which an optical input signal with a destination of higher priority is applied is preferentially assigned to an output port, according to the priority order of the destinations of the optical input signal applied to the input port. As a method of defining the priority order of destinations, it is possible, for example, to add information on the priority order of destinations to the header part of each optical input signal.  
         [0102]    In that case, preferentially assigning an output port of higher priority, among a plurality of output ports (subsequent-stage optical routers) selected for a given input port to transmit to the destination thereof through the shortest path, increases the possibility of the optical input signal with a higher-priority destination being transmitted to an output port with a higher-priority destination (subsequent-stage optical router). In contrast, the possibility of the optical input signal being transmitted to an output port with a lower-priority destination (subsequent-stage optical router) decreases.  
         [0103]    In the embodiment shown in FIG. 6, a case is also illustrated wherein controller  8  takes such measures as sending information indicating an abnormal end of conflict avoidance, to the destination optical router, if not all of the output requests from input ports are cancelled, thus making it possible to notify a pre-stage optical router that a conflict between optical signals has occurred.  
         [0104]    In that case, it does not matter whether the abovementioned information is sent to source optical routers that have transferred all (three) of the optical input signals or only to the source optical router whose output request has not been cancelled, or in other words, only to the source optical router that has transferred an optical input signal that was not output to any subsequent-stage optical router.  
         [0105]    As is evident from the description given above, the following advantageous effects are provided according to the present invention.  
         [0106]    By selecting one input port in an arbitrary order from a plurality of input ports having output requests to a given output port and assigning that input port to the output port or by sorting the output ports in ascending order of the number of output requests from the input ports of an optical switch and assigning the input ports in sequence according to the results of sorting, it is possible to prevent optical input signals from conflicting with each other and to efficiently assign optimum optical paths, even if a plurality of optical input signals are simultaneously applied to the input ports.  
         [0107]    Another advantageous effect is that preferentially assigning an input port of higher priority when selecting one input port from a plurality of input ports having output requests to the output ports, increases the possibility of the higher-priority optical input signal being transmitted to a subsequent-stage optical router. In contrast, an optical input signal of lower priority is blocked and, therefore, the possibility of the optical input signal being transmitted to a subsequent-stage optical router decreases.  
         [0108]    Yet another advantageous effect is that preferentially assigning an output port of higher priority, among a plurality of output ports (subsequent-stage optical routers) selected for a given input port to transmit to the destination thereof through the shortest path, increases the possibility of the optical input signal with a higher-priority destination being transmitted to an output port with a higher-priority destination (subsequent-stage optical router). In contrast, the possibility of the optical input signal being transmitted to an output port with a lower-priority destination (subsequent-stage optical router) decreases.  
         [0109]    Yet another advantageous effect is that by taking such measures as sending information indicating an abnormal end of conflict avoidance, to the source optical router, if not all of the output requests from input ports are cancelled, it is possible to notify a pre-stage optical router that a conflict between optical signals has occurred.