robin which have the lowest rate and the second lowest rate as seen from fig4 now operation of the arbiter circuit of fig3 after addition or deletion of a connection and updating of information of the round robin 1 2 1 to 2 4 and 3 1 to 3 3 are described with reference to fig3 to 8 when a connection of the rate of 5 mbps is to be added step s 1 of fig7 that one of the round robins subordinate to the round robin 1 of the highest order stage which has the lowest rate is searched for step s 2 of fig7 then it is discriminated whether or not the round robin searched out by the search has robins subordinate thereto step s 3 of fig7 if robins subordinate to the searched out round robin are detected then the processing advances to step s 4 of fig7 but in another other case the processing advances to step s 6 of fig7 in the case described above the round robin 2 1 is searched out and the thus searched out round robin 2 1 has the round robins 3 1 to 3 3 subordinate thereto as seen in fig4 therefore that one of the round robins subordinate to the round robin 2 1 which has the lowest rate is searched for step s 4 of fig7 then it is discriminated whether or not the round robin searched out by the search has round robins subordinate thereto step s 5 of fig7 if round robins subordinate to the searched out round robin are detected then the processing advances to step s 4 of fig7 but in.