A method of allocating data transmission resources in a packet-switched telecommunications system which comprises a terminal and a fixed network to which an operational entity is defined for defining resources for a radio bearer. Radio bearer resources are defined for the terminal on the basis of an application used on the radio bearer, and the used compression method of header fields in data packets is selected before the radio bearer resources are defined, the resources being defined in such a manner that they also comprise the capacity required by the selected compression method of header fields in data packets. The compression methods of header fields in data packets, which are supported by the terminal, are transmitted to the operational entity in the fixed network for defining the compression method to be used. Capacity is defined for the radio bearer in two directions, if the selected compression method of header fields in data packets requires a bi-directional connection.