sar pdu header is formed by the sixth and seventh octets of the atm cell a sar pdu payload is formed by the eighth through the fifty first octets of the atm cell a sar pdu trailer is formed by the last two octets of the atm cell the sar pdu trailer includes a length indicator li and a cyclic redundancy code crc fig6 shows the signal format of the ais cell which is used in this embodiment this ais cell may be regarded as a special kind of atm cell an appropriate value is given to the vpi and 34 0008 34 in hexadecimal is fixedly given to the vci a value 34 11 34 is given to the pt to indicate the use of the cell for the network operation the remaining parts of the ais cell are all 34 1 34 except for the hec fig7 shows the signal format of a stop ais sais cell compared to the ais cell shown in fig6 this sais cell differs in that a value 34 0009 34 in hexadecimal is given to vci fig8 shows the signal format of a send cell compared to the ais cell shown in fig6 this send cell differs in that a value 34 008 34 in hexadecimal is given to the vpi this send cell is used when a one way circuit failure occurs as will be described later of course a predetermined value may be given to the vpi and the vci of the send cell fig9 is a diagram for explaining the operation of this embodiment when recovering a circuit failure in fig9 those parts which are the same.