A method and apparatus for determining when to switch between buffer estimation states for a fixed frame rate services session at a terminal. In the event that the state is passive, a network node determines a first time between two received Radio Link Control Service Data Units (RLC SDU). If the first time is lower than a first time threshold, then the state is changed to proactive. When the state is proactive, the network node determines that two consecutive padding transport blocks have been received and further determines that a second time between two padding transport blocks is greater than a second time threshold. If so then the state is changed to passive.