the network port unit 180 is the same as that of the line setting unit 190 in step s301 the line setting unit 190 performs a parameter recognition operation more particularly the external interface unit 228 of the mux 108 shown in fig5 receives an input signal control signal via the external interface unit 228 and transfers the received input signal to the recognition unit 229 the recognition unit 229 recognizes parameters to be used from the received input signal more particularly as shown in fig1 each of the muxs 101 and 108 includes a master clock generator 241 and a frequency divider 242 connected to the control bus 240 fig5 the master clock generator 241 generates a master clock signal and the frequency divider 242 generates clock signals by dividing the frequency of the master clock signal the recognition unit 229 identifies the clock signal from the input signal further the recognition unit 229 identifies the encoding method from the received input signal the external interface unit 228 is capable of performing a plurality of encoding methods furthermore the recognition unit 229 identifies the frame structure from the received input signal information concerning the identified parameters is transferred as parameter information to the memory 230 in step s302 shown in fig6 the memory 230 stores the parameter information sent from the recognition unit 229 under the control of the controller 227 in step s303 the comparator 231 compares the parameter information obtained at time t n with that of time t n 1 when the parameter information obtained at time t n does not match the parameter information obtained at time.