The present invention relates to a method and system for performing a random access operation between a mobile station and a network element such as a base transceiver station (10), wherein allowed access slots for random access channels are defined by the network and are signaled to the mobile station (20). The definition of the allowed access slots is performed on the basis of a parameter set by the base transceiver station (10) and transmitted to the mobile station (20) which determines the allowed access slots based on the parameter. Thus, the number of allowed access slots can be defined by the network and can be changed dynamically based on the random access messaging load and the hardware requirements.