Data transmission apparatus and data transmission method

A data transmission method for a network where a low-speed signal transmission frame is included in time slots of a high-speed signal transmission frame and a number of the time slots to include the low-speed signal transmission frame is variable, wherein the time slots have respective time slot numbers, the data transmission method comprising:

• storing signal data of the high-speed signal transmission frame corresponding to the number of the time slots based on a time slot number basis, wherein the signal data of the high-speed signal transmission frame are supplied on the time slot number basis;
• selecting and sequentially outputting the signal data on the time slot number basis, wherein the signal data have been stored in accordance with the number of the time slots;
• and
• being supplied with the signal data corresponding to the number of time slots, output from the selection and output unit on the time slot number basis, and detecting an overhead of the low-speed signal transmission frame on the time slot number basis.