A method of controlling the volume of user-plane traffic on the Iub or Iub/Iur interface between a Radio Network Controller and a Node B of a UMTS Radio Access Network during periods of overload. The method comprises, for individual uplink or downlink connections established over the Iub or Iub/Iur interface, monitoring at the Radio Network Controller the late arrival of frames at the Radio Network Controller or at the Node B transmitted over the Iub or Iub/Iur interface, and based on the results of said monitoring, causing a reduction in the Iub or Iub/Iur load for a connection when appropriate.