In an integrated communications system, HDLC variable-length packets and non-HDLC fixed-length packets are decomposed into cells and a cell identifier is generated for each of the cells for identifying its type. A frame sync code is transmitted from one end of a transmission channel, and the cell identifier and each of the cells are assembled into a field and a plurality of such fields are assembled into a frame for transmission. The frame sync code is detected at the other end of the transmission channel as a timing reference and the frame is deassembled into fields in response to the timing reference and each field is deassembled into a cell identifier and a cell. According to each deassembled cell identifier, the cells of each field are composed into the original HDLC variable-length packet of non-HDLC fixed-length packet.