PURPOSE: To provide advanced priority having a recovery function onto a serial communication link and to obtain the method and device for an enhanced HDLCMAC layer protocol. CONSTITUTION: In a data/audio/multimedia mixture communication system, a protocol is defined to realize transmission reception of a high priority real time traffic on a low-speed digital communication link, by inserting a high priority traffic in a low priority/non-real time traffic. A high priority packet is transmitted, without causing a delay by prefetching a low priority packet. The low priority non-real time packet is buffered during a prefetch and its transmission is automatically recovered after the end of the high priority packet transmission. When a communication link is activated, a protocol is defined in the system, so that the communication system exchanges link capacity definition information with the communication link to process the high priority real time packet and the system permits the transmission method of the high priority traffic on the communication link. A packet, including a header 10B and data 10C, is identified by a flag. COPYRIGHT: (C)1994,JPO