a fault occurs in the communication systems shown in fig5 fig5 shows a construction of another communication system in which the third embodiment is applied in fig5 40 are relay equipments 41 42 are communication equipments the communication equipment and the control information insertion and extraction circuit of the third embodiment are the same as those of the first embodiment of fig2 43 is a terminal equipment connected to the communication equipment 41 44 a terminal equipment connected to the communication equipment 42 fig6 shows the format construction of the service channel construction which is set when a fault occurs in the communication systems in fig6 is a frame construction having 192 bits 46 shows a head channel ch 1 47 shows second channel ch 2 48 is a time slot in the channel ch 1 49 is a remote loop back setting area the remote loop back setting area 49 is used in order to send the test pattern from the communication equipment 41 or 42 to the relay equipment 39 and in order to execute the return test at the relay equipment 39 when the relay equipment 39 fails during the terminal equipment 43 and the terminal equipment 44 communicate through the communication equipment 41 the relay equipment 39 and the communication equipment 42 50 is a program down load information transmission area in which the communication equipment 41 and 42 alternatively download the control information in order to make another route which bypasses the failed route the remote back area or program download information transmission area is allocated for the time slot in the channel in this case the remote loop back setting area 49 and program download information transmission area 50 correspond to the service channel the remote loop back setting area 49 and.