The method includes frame format for setting up serial bus of system overhead. When receiving signal, the serial bus of system overhead transfers separated the system overhead to unit of treating the system overhead; and when sending out signal, the serial bus of system overhead transfers the system overhead needed to send to unit of treating signal of system. The disclosed unit is composed of the unit of treating signal and the unit of treating the system overhead. The unit of treating signal through serial data bus is coupled to the unit of treating the system overhead. The invention guarantees transfers overheads safely. Moreover, multiple overheads are guaranteed to transfer correctly through mode of inserting bytes. |