There is provided a mechanism for controlling communications conducted via different frequency bands or communication spectrums. When a communication via a first communication path, for example an unlicensed or white space communication spectrum, is to be established, e.g. for offloading traffic from a second communication path, for example a licensed cellular communication spectrum, a preamble transmission command is generated and transmitted from a communication network control element, such as an eNB, via the second communication path to a communication element, such as a UE. By means of the preamble transmission command, the transmission of a preamble signal by the receiving communication element on the first communication path with predetermined communication resources is triggered. Based on a receipt or non-receipt of the preamble signal via the first communication path, the communication network control element is able to determine that the communication element can establish a connection on the first communication path, for example the unlicensed or white space communication spectrum.