Abstract: A medium access control (MAC) technique between a group of optical network units (ONU) and an optical line termination (OLT) on an asynchronous transfer mode passive optical network (ATM-PON) is disclosed. The downstream frame from the OLT to the group of ONUs includes PLOAM cells having data grant fields, PLOAM grant fields and divided-slot grant fields whose format, being allocated with different identification bit values, is distinguished one from another. The upstream frame from the group of ONUs to the OLT includes a divided-slot structure of 56-byte size, which is divided into four 14-byte minislots for transmitting cell information on the number of real-time cells and non-real-time cells that have arrived at a corresponding ONU. The data grant field format has a quality of service (QoS) class bit allocated for classifying real-time or non-real-time traffics, thus improving the efficiency of utilization as well as efficient transmission.