In data communication by packets of data, transmission not only commands but also data that cannot be accommodated in one packet is to be achieved by a packet similar to one used in exchanging-commands. To this end, a field in which to store the information specifying the format of a frame in a packet is provided in the packet. As one of the formats that can be discriminated by this field, there is provided a format which provides so that the frame has a packet label field which to store the information specifying the relation between plural packets divided from the transmission data and a data field in which to store the command and/or optional data for an equipment of destination of transmission. IMAGE
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