According to an aspect, there is provided a method of operating a terminal device in a communication network, the network defining an extended period that comprises a plurality of predefined periods, each predefined period corresponding to the time taken to transmit a predefined plurality of frames, the method comprising receiving information blocks, IBs, broadcast by a network node in the communication network, each IB indicating a value for a relative shift indication for the predefined period in which the IB is broadcast; comparing the value for the relative shift indication for a current predefined period to a value for the relative shift indication for a predefined period preceding the current predefined period; and determining whether the current predefined period is a particular one of the predefined periods in the extended period based on the result of the comparison.