A congestion control method of a cellular communication system is disclosed. A first protocol data unit having a first sequence number is transmitted from a radio network controller to a base station via a transport network. The base station detects a congestion situation of the transport network by detecting loss of the first protocol data unit during transmission over the transport network. In response to detecting the congestion situation, a notification message indicative of the detected congestion situation is transmitted from the base station to the radio network controller via the interface of the transport network.