a padding pad field 143 1 an lng field 143 2 and a cyclic redundancy check crc 32 field 143 3 thereby generating and issuing a cpcs protocol data unit pdu 143 the cpcs pdu 143 is sliced or u201c segmented u201d into a plurality of 48 byte data items 144 2 to be referred to as u201c payloads u201d hereinafter each payload 144 2 is then added with a cell header 144 1 by the atm layer processing to thereby provide cells 144 the resultant cells are subject to physical phy layer processing for transmission to a corresponding one of the transfer paths 7 of fig2 on the other hand those cells received are reconstructed or u201c reassembled u201d into an llc frame or frames in a procedure with steps in the order reverse to the above in the illustrative embodiment the llc frame s may be transported between the terminal 1 and atm control device 2 in a way such that every data block 145 of fixed data length acts as a minimal or u201c unit u201d transmit receive segment as shown in fig4 the cell header 144 1 may consist of several fields including a generic flow control gfc field 144 1 a virtual path identifier vpi field 144 1.