In a radio telecommunications system such as GSM in which a serving node operates the leaky bucket algorithm to control flow of packet data to a packet control unit and a virtual queue of data is formed in the control unit, upper and lower triggers BVC(u), BVC(l) are provided on the queue, and when the queue reaches the upper trigger, the packet control unit sends a request to the node to alter the leak rate R of the leaky bucket algorithm so as to reduce the data flow rate. When the lower trigger BVC(l) is reached, the control unit sends values of R and of Bmax, the maximum bucket size of the leaky bucket algorithm, to the node so that the length of the virtual queue is maintained close to the value of Bmax. Bmax is proportional to the number of allocated timeslots.