fig2 shows in block form a scheduler according to a first embodiment of the present invention , for use as scheduler 3 of the packet switch shown in fig1 . as shown in fig2 , scheduler 3 comprises n output schedulers 11 - 1 through 11 - n and n input schedulers 12 - 1 through 12 - n . output scheduler 11 - 1 comprises request counters 13 - 1 - 1 through 13 - 1 - n for counting requests from all input lines # 1 through # n to output line # 1 for the respective input lines , grant pointer g 1 ( 14 - 1 ) for indicating a leading end of the round - robin rules , and controller 17 - 1 for controlling request counters 13 - 1 - 1 through 13 - 1 - n and grant pointer g 1 ( 14 - 1 ). output schedulers 11 - 2 through 11 - n comprise request counters 13 - 2 - 1 through 13 - n - n for counting requests from all input lines # 1 through # n to corresponding output lines for the respective input lines , grant pointers g 2 through gn ( 14 - 2 through 14 - n ) for indicating leading ends of the round - robin rules , and controllers 17 - 2 through 17 - n for controlling request counters 13 - 2 - 1 through 13 - n - n and grant pointers g 2 through gn ( 14 - 2 through 14 - n ). input scheduler 12 - 1 comprises grant registers gs ( 15 - 1 - 1 through 15 - 1 - n ) for extracting grants for input line # 1 from selected results of all output schedulers 11 - 1 through 11 - n and holding the extracted grants for the respective output lines , accept pointer a 1 ( 16 - 1 ) for indicating a leading end of the round - robin rules , and controller 18 - 1 for controlling grant registers gs ( 15 - 1 - 1 through 15 - 1 - n ) and accept pointer a 1 ( 16 - 1 ). input schedulers 12 - 2 through 12 - n comprise grant registers gs ( 15 - 2 - 1 through 15 - n - n ) for receiving selected results of all output schedulers 11 - 1 through 11 - n , accept pointers a 2 through an ( 16 - 2 through 16 - n ) for indicating leading ends of the round - robin rules , and controllers 18 - 2 through 18 - n for controlling grant registers gs ( 15 - 2 - 1 through 15 - n - n ) and accept pointers a 2 through an ( 16 - 2 through 16 - n ). controller 17 - 1 of output scheduler 11 - 1 extracts requests whose destinations are designated as output line # 1 from the scheduling requests input from the voq managers of all input lines # 1 through # n , and counts requests for the respective input lines by request counters 13 - 1 - 1 through 13 - 1 - n . controller 17 - 1 extracts cancels whose destinations are designated as output line # 1 from canceling information input from input schedulers 12 - 1 through 12 - n , and increases the values of request counters 13 - 1 - 1 through 13 - 1 - n for the respective input lines . controller 17 - 1 selects one of the input lines for which the number of requests counted by request counters 13 - 1 - 1 through 13 - 1 - n is 1 or more , according to the round - robin rules with top priority given to grant pointer g 1 ( 14 - 1 ), and decrements by 1 the request counter of the selected input line . controller 17 - 1 outputs the number of the selected input line as granted to input schedulers 12 - 1 through 12 - n , and updates grant pointer g 1 ( 14 - 1 ) to a value indicative of an input line next to the selected input line . for example , if input line # 1 is selected , then controller 17 - 1 updates grant pointer g 1 ( 14 - 1 ) to a value indicative of input line # 2 , and if input line # n is selected , then controller 17 - 1 updates grant pointer g 1 ( 14 - 1 ) to a value indicative of input line # 1 . controller 17 - 2 of output scheduler 11 - 1 extracts requests whose destinations are designated as output line # 2 from the scheduling requests input from the voq managers of all input lines # 1 through # n , and counts requests for the respective input lines by request counters 13 - 2 - 1 through 13 - 2 - n . controller 17 - 2 extracts cancels whose destinations are designated as output line # 2 from canceling information input from input schedulers 12 - 1 through 12 - n , and increases the values of request counters 13 - 2 - 1 through 13 - 2 - n for the respective input lines . controller 17 - 2 selects one of the input lines for which the number of requests counted by request counters 13 - 2 - 1 through 13 - 2 - n is 1 or more , according to the round - robin rules with top priority given to grant pointer g 2 ( 14 - 2 ), and decrements by 1 the request counter of the selected input line . controller 17 - 2 outputs the number of the selected input line as granted to input schedulers 12 - 1 through 12 - n , and updates grant pointer g 2 ( 14 - 2 ) to a value indicative of an input line next to the selected input line . controllers 17 - 3 through 17 - n of output schedulers 11 - 3 through 11 - n also perform the same controlling process as controller 17 - 1 of output scheduler 11 - 1 and controller 17 - 2 of output scheduler 11 - 2 . controller 18 - 1 of input scheduler 12 - 1 extracts grants whose sources are designated as input line # 1 from the grants input from all output schedulers 11 - 1 through 11 - n , and stores the extracted grants in grant registers 15 - 1 - 1 through 15 - 1 - n for the respective output lines . if there are not corresponding grants , then controller 18 - 1 clears the grant registers ( no grants ). controller 18 - 1 selects one of the output lines for which grants are included in the grant registers , according to the round - robin rules with top priority given to accept pointer a 1 ( 16 - 1 ), and outputs the number of the selected output line as a scheduling result to voq manager 7 - 1 of input buffer 1 - 1 . controller 18 - 1 updates accept pointer a 1 ( 16 - 1 ) to a value indicative of an output line next to the selected output line . for example , if output line # 1 is selected , then controller 18 - 1 updates accept pointer a 1 ( 16 - 1 ) to a value indicative of output line # 2 , and if output line # n is selected , then controller 18 - 1 updates accept pointer a 1 ( 16 - 1 ) to a value indicative of output line # 1 . if none of grant registers 15 - 1 - 1 through 15 - 1 - n store grant information , then the scheduling result is output as unconnected , and accept pointer a 1 ( 16 - 1 ) holds the preceding value . if the grant registers store grants , but the grants have not been selected in the above selecting process , then they are output as canceling information to the corresponding output schedulers . controller 18 - 2 of input scheduler 12 - 2 extracts grants whose sources are designated as input line # 2 from the grants input from all output schedulers 11 - 1 through 11 - n , and stores the extracted grants in grant registers 15 - 2 - 1 through 15 - 2 - n for the respective output lines . if there are no corresponding grants , then controller 18 - 2 clears the grant registers ( no grants ). controller 18 - 2 selects one of the output lines for which grants are included in the grant registers , according to the round - robin rules with top priority given to accept pointer a 2 ( 16 - 2 ), and outputs the number of the selected output line as a scheduling result to voq manager 7 - 1 of input buffer 1 - 2 . controller 18 - 2 updates accept pointer a 2 ( 16 - 2 ) to a value indicative of an output line next to the selected output line . if none of grant registers 15 - 1 - 1 through 15 - 1 - n store grant information , then the scheduling result is output as unconnected , and accept pointer a 2 ( 16 - 2 ) holds the preceding value . if the grant registers store grants , but the grants have not been selected in the above selecting process , then they are output as canceling information to the corresponding output schedulers . controllers 18 - 3 through 18 - n of input schedulers 12 - 3 through 12 - n also perform the same controlling process as controller 18 - 1 of input scheduler 12 - 1 and controller 1 - 8 - 2 of input scheduler 12 - 2 . output schedulers 11 - 1 through 11 - n and input schedulers 12 - 1 through 12 - n operate independently of each other , and a request counting process ( step 1 ), an output scheduling process ( step 2 ), and an input scheduling process ( step 3 ) are performed once in one time slot , as shown in fig3 . fig4 is a flowchart of the request counting process ( step 1 ), the output scheduling process ( step 2 ), and the input scheduling process ( step 3 ). operation of the scheduler according to the first embodiment where the number n of input / output lines is 4 will be described below with reference to fig2 , 4 and 7 ( a )- 7 ( d ). in step 1 , output schedulers 11 - 1 through 11 - 4 count scheduling requests from input buffers 1 - 1 through 1 - 4 to output buffers 4 - 1 through 4 - 4 ( fig5 ( a ) through 5 ( c )). fig1 ( a ) shows the values of request counters 13 - 1 - 1 through 13 - 4 - 4 before step 1 is executed . fig5 ( b ) shows new scheduling requests req # 1 through req # 4 input from input buffers 1 - 1 through 14 . fig5 ( c ) shows canceling information can # 1 through can # 4 input from input schedulers 12 - 1 through 12 - 4 . fig5 ( d ) shows the values of request counters 13 - 1 - 1 through 13 - 4 - 4 after step 1 is executed . with respect to the combination of input line # 1 and output line # 1 , the value “ 1 ” of the request counter ( at the 1st row and the 1st column in fig5 ( a )), the value “ 1 ” of the request ( at the 1st row and the 1st column in fig5 ( b )), and the value “ 0 ” of the cancel ( at the 1st row and the 1st column in fig5 ( c )) are added to each other , thereby updating the value of the request counter ( at the 1st row and the 1st column in fig5 ( d )) to “ 2 ”. with respect to the combination of input line # 1 and output line # 2 , the value “ 0 ” of the request counter ( at the 1st row and the 2nd column in fig5 ( a )), the value “ 0 ” of the request ( at the 1st row and the 2nd column in fig5 ( b )), and the value “ 0 ” of the cancel ( at the 1st row and the 2nd column in fig5 ( c )) are added to each other , thereby updating the value of the request counter ( at the 1st row and the 2nd column in fig5 ( d )) to “ 0 ”. similarly , with respect to all combinations of the input lines and the output lines , the values of the request counters are updated . in step 2 , output schedulers 11 - 1 through 11 - 4 select a request for each output line from the requests whose destinations are designated as output lines # 1 through # 4 , according to the round - robin rules ( fig6 ( a ) through 6 ( d )). fig6 ( a ) shows requests from the input lines to the output lines and to grant pointers before step 2 is carried out , and fig6 ( b ) shows selected requests to the output lines and grant pointers after step 2 is carried out . fig6 ( c ) shows the values of the request counters after step 2 is carried out , and fig6 ( d ) shows grants output from output schedulers 11 - 1 through 11 - 4 after step 2 is carried out . controller 17 - 1 of output scheduler 11 - 1 searches for input lines for which the number of requests counted by the request counter is 1 or more , from input line # 1 indicated by grant pointer g 1 ( 14 - 1 ). since the request counter from input line # 1 has a count of 2 , output scheduler 11 - 1 selects input line # 1 as a candidate that can be connected to output line # 1 . output scheduler 11 - 1 updates the value of the grant pointer to a value indicative of input line # 2 according to the round - robin rules , and decrements by 1 the value of request counter 13 - 1 - 1 from selected input line # 1 , thereby updating it to “ 1 ” ( at the 1st row and the 1st column in fig6 ( c )). output scheduler 11 - 1 makes grants output to input scheduler 12 - 1 valid , i . e ., sets them to “ 1 ” ( at the 1st row and the 1st column in fig6 ( d )), and makes grants output to input schedulers 12 - 2 , 12 - 3 , 12 - 4 invalid , i . e ., sets them to “ 0 ” ( at the 2nd row and the 1st column , the 3rd row and the 1st column , and the 4th row and the 1st column in fig6 ( d )). controller 17 - 2 of output scheduler 11 - 2 searches for input lines for which the number of requests counted by the request counter is 1 or more , from input line # 2 indicated by grant pointer g 2 ( 14 - 2 ). since the request counter from input line # 2 has a count of 2 , output scheduler 11 - 2 selects input line # 2 as a candidate that can be connected to output line # 2 . controller 17 - 2 of output scheduler 11 - 2 updates the value of the grant pointer to a value indicative of input line # 3 according to the round - robin rules , and decrements the value of request counter 13 - 1 - 2 from selected input line # 2 , thereby updating it to “ 1 ” ( at the 2nd row and the 2nd column in fig6 ( c )). output scheduler 11 - 2 makes grants output to input scheduler 12 - 2 valid , i . e ., sets them to “ 1 ” ( at the 2nd row and the 2nd column in fig6 ( d )), and makes grants output to input schedulers 12 - 1 , 12 - 3 , 12 - 4 invalid , i . e ., sets them to “ 0 ” ( at the 1st row and the 2nd column , the 3rd row and the 2nd column , and the 4th row and the 2nd column in fig6 ( d )). controller 17 - 3 of output scheduler 11 - 3 searches for input lines for which the number of requests counted by the request counters is 1 or more , from input line # 3 indicated by grant pointer g 3 ( 14 - 3 ). since the request counter from input line # 3 has a count of 2 , controller 17 - 3 selects input line # 3 as a candidate that can be connected to output line # 3 . controller 17 - 3 updates the value of the grant pointer to a value indicative of input line # 4 according to the round - robin rules , and decrements the value of request counter 13 - 1 - 3 from selected input line # 3 , thereby updating it to “ 1 ” ( at the 3rd row and the 3rd column in fig6 ( c )). controller 17 - 3 makes grants output to input scheduler 12 - 3 valid , i . e ., sets them to “ 1 ” ( at the 3rd row and the 3rd column in fig6 ( d )), and makes grants output to input schedulers 12 - 1 , 12 - 2 , 12 - 4 invalid , i . e ., sets them to “ 0 ” ( at the 1st row and the 3rd column , the 2nd row and the 3rd column , and the 4th row and the 3rd column in fig6 ( d )). controller 17 - 4 of output scheduler 11 - 4 searches for input lines for which the number of requests counted by the request counters is 1 or more , from input line # 4 indicated by grant pointer g 4 ( 14 - 4 ). since the request counter from input line # 4 has a count of 0 , input line # 4 cannot be selected as a candidate that can be connected to output line # 4 . the number of requests counted by the request counter from input line # 1 , as it is referred to , according to the round - robin rules is 1 . output scheduler 11 - 4 thus selects input line # 1 as a candidate that can be connected to output line # 4 . controller 17 - 4 updates the value of the grant pointer to a value indicative of input line # 2 according to the round - robin rules , and decrements the value of request counter 13 - 4 - 1 from selected input line # 1 , thereby updating it to “ 0 ” ( at the 1st row and the 4th column in fig6 ( c )). controller 17 - 4 makes grants output to input scheduler 12 - 1 valid , i . e ., sets them to “ 1 ” ( at the 1st row and the 4th column in fig6 ( d )), and makes grants output to input schedulers 12 - 2 , 12 - 3 , 12 - 4 invalid , i . e ., sets them to “ 0 ” ( at the 2nd row and the 4th column , the 3rd row and the 4th column , and the 4th row and the 4th column in fig6 ( d )). in step 3 , input schedulers 12 - 1 through 12 - 4 select a grant for each input line from valid grants input from output schedulers 11 - 1 through 11 - 4 according to the round - robin rules ( fig7 ( a ) through 7 ( d )). fig7 ( a ) shows valid grants from the input lines to the output lines and accept pointers before step 3 is carried out , and fig7 ( b ) shows selected grants and accept pointers after step 3 is carried out . fig7 ( c ) shows scheduling results after step 3 is carried out , and fig7 ( d ) shows canceling information output from input schedulers 12 - 1 through 12 - 4 after step 3 is carried out . controller 18 - 1 of input scheduler 12 - 1 receives grants input from respective output schedulers 11 - 1 through 11 - 4 by grant registers 15 - 1 - 1 through 15 - 1 - 4 . controller 18 - 1 refers to grant - register 15 - 1 - 2 for output line # 2 indicated by accept pointer a 1 ( 16 - 1 ), and it is invalid , i . e ., it is set to “ 0 ”. controller 18 - 1 refers to grant register 15 - 1 - 3 for next output line # 3 according to the round - robin rules , and it is invalid , i . e ., it is set to “ 0 ”. input scheduler 12 - 1 refers to grant register 15 - 1 - 4 for next output line # 4 , and it is valid , i . e ., it is set to “ 1 ”. therefore , controller 18 - 1 selects output line # 4 as a line to which packets can be output . controller 18 - 1 updates the value of accept pointer a 1 to a value indicative of output line # 1 according to the round - robin rules . controller 18 - 1 makes a scheduling result acc # 1 output to input buffer 1 - 1 valid , i . e ., sets it to “ 1 ”, only for output line # 4 ( at the 1st row and 4th column in fig7 ( c )), and makes it invalid , i . e ., sets it to “ 0 ”, for output lines # 1 , # 2 , # 3 ( at the 1st row and the 1st column , the 1st row and the 2nd column , and the 1st row and the 3rd column in fig7 ( c )). controller 18 - 1 outputs information that though the grant register is valid , the grant of output line # 1 which is not selected is canceled , to output scheduler 11 - 1 ( at the 1st row and the 1st column in fig7 ( d )). controller 18 - 2 of input scheduler 12 - 2 receives grants input from respective output schedulers 11 - 1 through 11 - 4 by grant registers 15 - 2 - 1 through 15 - 24 . controller 18 - 2 refers to grant register 15 - 2 - 1 for output line # 1 indicated by accept pointer a 2 ( 16 - 2 ), and it is invalid , i . e ., it is set to “ 0 ”. controller 18 - 2 refers to grant register 15 - 2 - 2 for next output line # 2 according to the round - robin rules , and it is valid , i . e ., it is set to “ 1 ”. therefore , input scheduler 12 - 2 selects output line # 2 as a line - to which packets can be output . controller 18 - 2 updates the value of accept pointer a 2 to a value indicative of output line # 3 according to the round - robin rules . controller 18 - 2 makes a scheduling result acc # 2 output to input buffer 1 - 2 valid , i . e ., sets it to “ 1 ”, only for output line # 2 ( at the 2nd row and 2nd column in fig7 ( c )), and makes it invalid , i . e ., sets it to “ 0 ”, for output lines # 1 , # 3 , # 4 ( at the 2nd row and the 1st column , the 2nd row and the 3rd column , and the 2nd row and the 4th column in fig7 ( c )). controller 18 - 3 of input scheduler 12 - 3 receives grants input from respective output schedulers 11 - 1 through 11 - 4 by grant registers 15 - 3 - 1 through 15 - 34 . controller 18 - 3 refers to grant register 15 - 3 - 1 for output line # 1 indicated by accept pointer a 3 ( 16 - 3 ), and it is invalid , i . e ., it is set to “ 0 ”. controller 18 - 3 refers to grant register 15 - 3 - 2 for next output line # 2 according to the round - robin rules , and it is invalid , i . e ., it is set to “ 0 ”. controller 18 - 3 refers to grant register 15 - 3 - 3 for next output line # 3 , and it is valid , i . e ., it is set to “ 1 ”. therefore , controller 18 - 3 selects output line # 3 as a line that packets can be output . controller 18 - 3 updates the value of accept pointer a 3 to a value indicative of output line # 4 according to the round - robin rules . controller 18 - 3 makes a scheduling result acc # 3 output to input buffer 1 - 3 valid , i . e ., sets it to “ 1 ”, only for output line # 3 ( at the 3rd row and 3rd column in fig7 ( c )), and makes it invalid , i . e ., sets it to “ 0 ”, for output lines # 1 , # 2 , # 4 ( at the 3rd row and the 1st column , the 3rd row and the 2nd column , and the 3rd row and the 4th column in fig7 ( c )). controller 18 - 4 of input scheduler 12 - 4 receives grants input from respective output schedulers 11 - 1 through 11 - 4 by grant registers 15 - 4 - 1 through 15 - 4 - 4 . controller 18 - 4 refers to grant register 15 - 4 - 3 for output line # 3 indicated by accept pointer a 4 ( 16 - 4 ), and it is invalid , i . e ., it is set to “ 0 ”. controller 18 - 4 refers to grant registers 15 - 4 - 4 , 15 - 4 - 1 , 15 - 4 - 2 for next output lines # 4 , # 2 , # 1 according to the round - robin rules , and all of them are invalid , i . e ., all of them are set to “ 0 ”. therefore , controller 18 - 4 judges that there is no line that packets can be output , and controls accept pointer a 4 ( 36 - 4 ) to hold the preceding value . controller 18 - 4 makes a scheduling result acc # 4 output to input buffer 14 invalid , i . e ., sets it to “ 0 ”, for all output lines # 1 through # 4 ( at the 4th row in fig7 ( c )). according to the first embodiment , the request counting process ( fig5 ( a ) through 5 ( d )), the output scheduling process ( fig6 ( a ) through 6 ( d )), and the input scheduling process ( fig7 ( a ) through 7 ( d )) are performed independently of each other and are carried out in respective time slots . therefore , the amount of processing operations in each of the time slots is reduced . fig8 shows in block form a scheduler according to a second embodiment of the present invention . as shown in fig8 , the scheduler comprises n input schedulers 21 - 1 through 21 - n and n output schedulers 22 - 1 through 22 - n . input scheduler 21 - 1 comprises request counters 23 - 1 - 1 through 23 - 1 - n for counting requests from input line # 1 to output lines # 1 through # n for the respective output lines , grant pointer g 1 ( 24 - 1 ) for indicating a leading end of the round - robin rules , and controller 27 - 1 for controlling request counters 23 - 1 - 1 through 23 - 1 - n and grant pointer g 1 ( 24 - 1 ). input schedulers 21 - 2 through 21 - n comprise request counters 23 - 2 - 1 through 23 - n - n for counting requests from corresponding input lines to output lines # 1 through # n for the respective output lines , grant pointers g 2 through gn ( 24 - 2 through 24 - n ) for indicating leading ends of the round - robin rules , and controllers 27 - 2 through 27 - n for controlling request counters 23 - 2 - 1 through 23 - n - n and grant pointers g 2 through gn ( 24 - 2 through 24 - n ). output scheduler 22 - 1 comprises grant registers 25 - 1 - 1 through 25 - 1 - n for extracting grants for input line # 1 from selected results of all input schedulers - 21 - 1 through 21 - n and for holding the extracted grants for the respective input lines , accept pointer a 1 ( 26 - 1 ) for indicating a leading end of the round - robin rules , and controller 28 - 1 for controlling grant registers 25 - 1 - 1 through 25 - 1 - n and accept pointer a 1 ( 26 - 1 ). output schedulers 22 - 2 through 22 - n comprise grant registers 25 - 2 - 1 through 25 - n - n for receiving selected results of all input schedulers 21 - 1 through 21 - n , accept pointers a 2 through an ( 26 - 2 through 26 - n ) for indicating leading ends of the round - robin rules , and controllers 28 - 2 through 28 - n for controlling grant registers 25 - 2 - 1 through 25 - n - n and accept pointers a 2 through an ( 26 - 2 through 26 - n ). controller 27 - 1 of input scheduler 21 - 1 counts scheduling requests input from the voq manager of input line # 1 for the respective output lines by request counters 23 - 1 - 1 through 23 - 1 - n . controller 27 - 1 extracts cancels whose sources are designated as input line # 1 from canceling information input from output schedulers 22 - 1 through 22 - n , and increases the values of request counters 23 - 1 - 1 through 23 - 1 - n for the respective output lines . controller 27 - 1 selects one of the output lines for which the number of requests counted by request counters 23 - 1 - 1 through 23 - 1 - n is 1 or more , according to the round - robin rules with top priority given to grant pointer g 1 ( 24 - 1 ), and decrements by 1 the request counter of the selected input line . controller 27 - 1 outputs the number of the selected input line as granted to output schedulers 22 - 1 through 22 - n , and updates grant pointer g 1 ( 24 - 1 ) to a value indicative of an input line next to the selected output line . for example , if output line # 1 is selected , then controller 27 - 1 updates grant pointer g 1 ( 24 - 1 ) to a value indicative of input line # 2 , and if output line # n is selected , then controller 27 - 1 updates grant pointer g 1 ( 24 - 1 ) to a value indicative of output line # 1 . controller 27 - 2 of input scheduler 21 - 2 counts scheduling requests input from the voq manager for input line # 2 for the respective output lines by request counters 23 - 2 - 1 through 23 - 2 - n . controller 27 - 2 extracts cancels whose sources are designated as input line # 2 from canceling information input from output schedulers 22 - 1 through 22 - n , and increases the values of request counters 23 - 2 - 1 through 23 - 2 - n for the respective output lines . controller 27 - 2 selects one of the output lines for which the number of requests counted by request counters 23 - 2 - 1 through 23 - 2 - n is 1 or more , according to the round - robin rules with top priority given to grant pointer g 2 ( 24 - 2 ), and decrements by 1 the request counter of the selected input line . controller 27 - 2 outputs the number of the selected input line , as granted , to output schedulers 22 - 1 through 22 - n , and updates grant pointer g 2 ( 24 - 2 ) to a value indicative of an input line next to the selected output line . controllers 27 - 3 through 27 - n of input schedulers 21 - 3 through 21 - n also perform the same controlling process as controller 27 - 1 of input scheduler 21 - 1 and controller 27 - 2 of input scheduler 21 - 2 . controller 28 - 1 of output scheduler 22 - 1 extracts grants whose destinations are designated as output line # 1 from the grants output from all input schedulers 21 - 1 through 21 - n , and stores the extracted grants in grant registers 25 - 1 - 1 through 25 - 1 - n for the respective input lines . if there are no corresponding grants , then controller 28 - 1 clears the grant registers ( no grants ). controller 28 - 1 selects one of the input lines for which grants are included in grant registers 25 - 1 - 1 through 25 - 1 - n , according to the round - robin rules with top priority given to accept pointer a 1 ( 26 - 1 ). controller 28 - 1 outputs the number of the selected input line as a scheduling result to the voq manager of output buffer 4 - 1 , and updates accept pointer a 1 ( 26 - 1 ) to a value indicative of an output line next to the selected output line . for example , if input line # 1 is selected , then controller 28 - 1 updates accept pointer a 1 ( 26 - 1 ) to a value indicative of input line # 2 , and if input line # n is selected , then controller 28 - 1 updates accept pointer a 1 ( 26 - 1 ) to a value indicative of input line # 1 . if all grant registers 25 - 1 - 1 through 25 - 1 - n store no grant information , then the scheduling result is output as unconnected , and accept pointer a 1 ( 26 - 1 ) holds the preceding value . if the grant registers store grants , but the grants have not been selected in the above selecting process , then they are output as canceling information to the corresponding input schedulers . controller 28 - 2 of output scheduler 22 - 2 extracts grants whose destinations are designated as output line # 2 from the grants output from all input schedulers 21 - 1 through 21 - n , and stores the extracted grants in grant registers 25 - 2 - 1 through 25 - 2 - n for the respective input lines . if there are no corresponding grants , then controller 28 - 2 clears the grant registers ( no grants ). controller 28 - 2 selects one of the input lines for which grants are included in grant registers 25 - 2 - 1 through 25 - 2 - n , according to the round - robin rules with top priority given to accept pointer a 2 ( 26 - 2 ). controller 28 - 2 outputs the number of the selected input line as a scheduling result to the voq manager of output buffer 4 - 2 , and updates accept pointer a 2 ( 26 - 2 ) to a value indicative of an output line next to the selected output line . if none of grant registers 25 - 2 - 1 through 25 - 2 - n store grant information , then the scheduling result is output as unconnected , and accept pointer a 1 ( 26 - 2 ) holds the preceding value . if the grant registers store grants , but the grants have not been selected in the above selecting process , then they are output as canceling information to the corresponding input schedulers . controllers 28 - 3 through 28 - n of output schedulers 22 - 3 through 22 - n also perform the same controlling process as controller 28 - 1 of output scheduler 22 - 1 and controller 28 - 2 of output scheduler 22 - 2 . input schedulers 21 - 1 through 21 - n and output schedulers 22 - 1 through 22 - n operate independently of each other , and each of their processing operations is performed once in a one time slot . operation of the scheduler according to the second embodiment where the number n of input / output lines is 4 will be described below with reference to fig8 and 9 ( a )- 9 ( d ) and 11 ( a ) through 11 ( d ). in step 1 , input schedulers 21 - 1 through 214 count scheduling requests from input buffers 1 - 1 through 1 - 4 to output buffers 4 - 1 through 4 - 4 ( fig9 ( a ) through 9 ( d )). fig9 ( a ) shows the values of request counters 23 - 1 - 1 through 23 - 4 - 4 before step 1 is executed . fig9 ( b ) shows new scheduling requests req # 1 through req # 4 input from input buffers 1 - 1 through 1 - 4 . fig9 ( c ) shows canceling information can # 1 through can # 4 input from output schedulers 22 - 1 through 22 - 4 . fig9 ( d ) shows the values of request counters 23 - 1 - 1 through 23 - 4 - 4 after step 1 is executed . with respect to the combination of input line # 1 and output line # 1 , the value “ 1 ” of the request counter ( at the 1st row and the 1st column in fig9 ( a )), the value “ 1 ” of the request ( at the 1st row and the 1st column in fig9 ( b )), and the value “ 0 ” of the cancel ( at the 1st row and the 1st column in fig9 ( c )) are added to each other , thereby updating the value of the request counter ( at the 1st row and the 1st column in fig9 ( d )) to “ 2 ”. with respect to the combination of input line # 1 and output line # 2 , the value “ 0 ” of the request counter ( at the 1st row and the 2nd column in fig9 ( a )), the value “ 0 ” of the request ( at the 1st row and the 2nd column in fig9 ( b )), and the value “ 0 ” of the cancel ( at the 1st row and the 2nd column in fig9 ( c )) are added to each other , thereby updating the value of the request counter ( at the 1st row and the 2nd column in fig9 ( d )) to “ 0 ”. similarly , with respect to all combinations of the input lines and the output lines , the values of the request counters are updated . in step 2 , input schedulers 21 - 1 through 21 - 4 select a request for each output line from the requests whose destinations are designated as output lines # 1 through # 4 , respectively , according to the round - robin rules ( fig1 ( a ) through 10 ( d )). fig1 ( a ) shows requests from the input lines to the output lines and grant pointers before step 2 is carried out , and fig1 ( b ) shows selected requests to the output lines and grant pointers after step 2 is carried out . fig1 ( c ) shows the values of the request counters after step 2 is carried out , and fig1 ( d ) shows grants output from input schedulers 21 - 1 through 21 - 4 after step 2 is carried out . controller 28 - 1 of input scheduler 21 - 1 searches for output lines for which the number of requests counted by the request counters is 1 or more , from output line # 1 indicated by grant pointer g 1 ( 24 - 1 ). since the request counter from output line # 1 has a count of 2 , controller 28 - 1 selects output line # 1 as a candidate that can be connected to input line # 1 . controller 28 - 1 updates the value of the grant pointer to a value indicative of input line # 2 according to the round - robin rules , and decrements by 1 the value of request counter 23 - 1 - 1 to selected output line # 1 , thereby updating it to “ 1 ” ( at the 1st row and the 1st column in fig1 ( c )). controller 28 - 1 makes grants output to output scheduler 22 - 1 valid , i . e ., sets them to “ 1 ” ( at the 1st row and the 1st column in fig1 ( d )), and makes grants output to output schedulers 22 - 2 , 22 - 3 , 22 - 4 invalid , i . e ., sets them to “ 0 ” ( at the 1st row and the 2nd column , the 1st row and the 3rd column , and the 1st row and the 4th column in fig1 ( d )). controller 27 - 2 of input scheduler 21 - 2 searches for output lines for which the number of requests counted by the request counters is 1 or more , from output line # 2 indicated by grant pointer g 2 ( 24 - 2 ). since the request counter from output line # 2 has a count of 2 , controller 27 - 2 selects output line # 2 as a candidate that can be connected to input line # 2 . controller 27 - 2 updates the value of the grant pointer to a value indicative of output line # 3 according to the round - robin rules , and decrements by 1 the value of request counter 23 - 1 - 2 to selected output line # 2 , thereby updating it to “ 1 ” ( at the 2nd row and the 2nd column in fig1 ( c )). controller 27 - 2 makes grants output to output scheduler 22 - 2 valid , i . e ., sets them to “ 1 ” ( at the 2nd row and the 2nd column in fig1 ( d )), and makes grants output to output schedulers 22 - 1 , 22 - 3 , 22 - 4 invalid , i . e ., sets them to “ 0 ” ( at the 2nd row and the 1st column , the 2nd row and the 3rd column , and the 2nd row and the 4th column in fig1 ( d )). controller 27 - 3 of input scheduler 21 - 3 searches for output lines for which the number of request counted by the request counters is 1 or more , from output line # 3 indicated by grant pointer g 3 ( 24 - 3 ). since the request counter from output line # 3 has a count of 2 , input scheduler 21 - 3 selects output line # 3 as a candidate that can be connected to input line # 3 . controller 27 - 3 updates the value of the grant pointer to output line # 4 according to the round - robin rules , and decrements by 1 the value of request counter 23 - 1 - 3 to selected output line # 3 , thereby updating it to “ 1 ” ( at the 2nd row and the 2nd column in fig1 ( c )). controller 27 - 3 makes grants output to output scheduler 22 - 3 valid , i . e ., sets them to “ 1 ” ( at the 3rd row and the 3rd column in fig1 ( d )), and makes grants output to output schedulers 22 - 1 , 22 - 2 , 22 - 4 invalid , i . e ., sets them to “ 0 ” ( at the 3rd row and the 1st column , the 3rd row and the 2nd column , and the 3rd row and the 4th column in fig1 ( d )). controller 27 - 4 of input scheduler 21 - 4 searches for output lines for which the value of requests counted by the request counters is 1 or more , from output line # 4 indicated by grant pointer g 4 ( 24 - 4 ). since the request counter from output line # 4 has a count of 0 , output line # 4 cannot be selected as a candidate to which packets can be output . since the number of requests counted by the request counter from output line # 1 , as it is referred to , according to the round - robin rules is 0 , output line # 1 cannot be selected as a candidate to which packets can be output . since the number of requests counted by the request counter from output line # 2 , as it is referred to , is 1 , controller 27 - 4 thus selects output line # 2 as a candidate that can be connected to input line # 4 . controller 27 - 4 updates the value of the grant pointer to a value indicative of output line # 3 according to the round - robin rules , and decrements by 1 the value of request counter 23 - 4 - 2 to selected output line # 2 , thereby updating it to “ 0 ” ( at the 4th row and the 2nd column in fig1 ( c )). controller 27 - 4 makes grants output to output scheduler 22 - 2 valid , i . e ., sets them to “ 1 ” ( at the 4th row and the 2nd column in fig1 ( c )), and makes grants output to output schedulers 22 - 1 , 22 - 3 , 22 - 4 invalid , i . e ., sets them to “ 0 ” ( at the 4th row and the 1st column , the 4th row and the 3rd column , and the 4th row and the 4th column in fig1 ( c )). in step 3 , output schedulers 22 - 1 through 22 - 4 select a grant for each input line from valid grants input from input schedulers 21 - 1 through 21 - 4 according to the round - robin rules ( fig1 ( a ) through 11 ( d )). fig1 ( a ) shows valid grants from the input lines to the output lines and accept pointers before step 3 is carried out , and fig1 ( b ) shows selected grants and accept pointers after step 3 is carried out . fig1 ( c ) shows scheduling results after step 3 is carried out , and fig1 ( d ) shows canceling information output from output schedulers 22 - 1 through 22 - 4 after step 3 is - carried out . controller 28 - 1 of output scheduler 22 - 1 receives grants input from respective input schedulers 21 - 1 through 21 - 4 by grant registers 25 - 1 - 1 through 25 - 1 - 4 . controller 28 - 1 refers to grant register 25 - 1 - 2 for input line # 1 indicated by accept pointer a 1 ( 16 - 1 ), and it is valid , i . e ., it is set to “ 1 ”. therefore , controller 28 - 1 selects input line # 1 as a line that can be connected to output line # 1 . controller 28 - 1 updates the value of accept pointer a 1 to input line # 2 according to the round - robin rules . controller 28 - 1 makes a scheduling result for output line # 1 of input buffer 1 - 1 valid , i . e ., sets it to “ 1 ”, ( at the 1st row and 1st column in fig1 ( c )), and makes scheduling results for output line # 1 of input buffers 1 - 2 , 1 - 3 , 1 - 4 invalid , i . e ., sets them to “ 0 ” ( at the 1st row and the 2nd column , the 1st row and the 3rd column , and the 1st row and the 4th column in fig1 ( c )). controller 28 - 2 of output scheduler 22 - 2 receives grants input from respective input schedulers 21 - 1 through 21 - 4 using grant registers 25 - 2 - 1 through 25 - 2 - 4 . controller 28 - 2 refers to grant register 25 - 2 - 3 for input line # 3 indicated by accept pointer a 2 ( 16 - 2 ), and it is invalid , i . e ., it is set to “ 0 ”. therefore , controller 28 - 2 cannot select input line # 3 . controller 28 - 2 refers to grant register 25 - 2 - 4 for input line # 3 according to the round robin rules , and it is valid , i . e ., it is set to “ 1 ”. controller 28 - 2 selects input line # 4 as a line that can be connected to output line # 2 . controller 28 - 2 updates the value of accept pointer a 2 to a value of input line # 1 according to the round - robin rules . controller 28 - 2 makes a scheduling result for output line # 2 of input buffer 14 valid , i . e ., sets it to “ 1 ”, ( at the 4th row and 2nd column in fig1 ( c )), and makes scheduling results for output line # 2 of input buffers 1 - 1 , 1 - 2 , 1 - 3 invalid , i . e ., sets them to “ 0 ” ( at the 1st row and the 2nd column , the 2nd row and the 2nd column , and the 3rd row and the 2nd column in fig1 ( c )). furthermore , controller 28 - 2 outputs information indicating that the grant for input line # 2 that is not selected though the grant register is valid , has been canceled to input scheduler 21 - 2 ( at the 2nd row and 2nd column in fig1 ( c )). output scheduler 22 - 3 receives grants input from respective input schedulers 21 - 1 through 21 - 4 by grant registers 25 - 3 - 1 through 25 - 3 - 4 . controller 28 - 3 of output scheduler 22 - 3 refers to grant register 25 - 3 - 3 for input line # 3 indicated by accept pointer a 3 ( 16 - 3 ), and it is valid , i . e ., it is set to “ 1 ”. therefore , controller 28 - 3 selects input line # 3 as a line that can be connected to output line # 3 . controller 28 - 3 updates the value of accept pointer a 3 to a value indicative of input line # 4 according to the round - robin rules . controller 28 - 3 makes a scheduling result for output line # 3 of input buffer 1 - 3 valid , i . e ., sets it to “ 1 ”, ( at the 3rd row and 3rd column in fig1 ( c )), and makes scheduling results for output line # 3 of input buffers 1 - 1 , 1 - 2 , 1 - 4 invalid , i . e ., sets them to “ 0 ” ( at the 1st row and the 3rd column , the 2nd row and the 3rd column , and the 4th row and the 3rd column in fig1 ( c )). controller 284 of output scheduler 22 - 4 receives grants input from respective input schedulers 21 - 1 through 21 - 4 by grant registers 25 - 4 - 1 through 25 - 4 - 4 . controller 284 refers to grant register 25 - 4 - 1 for input line # 1 indicated by accept pointer a 4 ( 26 - 4 ), and it is invalid , i . e ., it is set to “ 0 ”. therefore , controller 28 - 4 successively refers to grant registers 25 - 3 - 2 , 25 - 3 - 3 , 25 - 3 - 4 for input lines # 2 , # 3 , # 4 according to the round robin rules , and all of them are invalid , i . e ., they are set to “ 0 ”. therefore , controller 28 - 4 judges that there is no line outputting packets to output line # 4 , and controls accept pointer a 4 to hold the preceding value . controller 28 - 4 makes scheduling results for output line # 4 of input buffers 1 - 1 , 1 - 2 , 1 - 3 , 1 - 4 invalid , i . e ., sets them to “ 0 ” ( at the 1st rod and 4th column , the 2nd row and the 4th column , and the 3rd row and the 4th column in fig1 ( c )). the scheduling results are also used as connection settings of crossbar switch 2 . according to the second embodiment , the scheduling process is first performed on the input lines , and if a plurality of output lines are overlappingly selected according to the scheduling results , then one of the selected output lines is selected . therefore , the second embodiment offers the same advantages as the first embodiment . while preferred embodiments of the present invention have been described using specific terms , such description is for illustrative purposes only , and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims .