LINE CONTROL SYSTEM, REFERENCE STATION TO WHICH THE SYSTEM IS APPLIED AND SLAVE STATION

PROBLEM TO BE SOLVED: To provide a line control system for improving transmission efficiency by means of the reduction of line congestion and preferentially transmitting an urgent message. SOLUTION: The reference station 1 transmits signals showing slot timing and an up line preferential level showing the degree of the congestion of an up line 3 to plural slave stations 4, 5 and 6 through a down line 2. The slave stations 4, 5 and 6 compare the up line preferential level with a message preferential level allocated to the message to be transmitted. Only when the message preferential level is not less than the up line preferential level, the message is transmitted to the reference station 1 through the up line 3 in accordance with slot timing.