A wireless telecommunications system with mobile terminals, MSs and a Base Station, BS, through which traffic to and from the MSs in a cell, is routed, so that there can be Down Link, DL, traffic from the BS of a cell to the MSs in that cell, and Up Link, UL, traffic from those MSs to the BS. At least part of the UL traffic is contention based, and DL data to an MS must be acknowledged by the MS. The method comprises letting the BS detect MSs in the cell whose DL traffic rate exceeds a certain limit, and also comprises a mechanism for allocating to said detected MSs an UL bandwidth which enables them to acknowledge DL traffic within a certain time period from its reception by the MS.