This document discloses a solution for configuring sub-frames in a cellular communication system. According to an aspect, a method comprises: allocating, in a network node of the cellular communication system, at least one set of available lengths for use in selection of a length of at least one of an uplink control part and a downlink control part of a sub-frame; selecting, in the network node, a length of at least one of the uplink control part and the downlink control part of the sub-frame, the length being selected amongst the at least one set of available lengths, the sub-frame comprising the uplink control part carrying at least one of uplink control information and an uplink reference signal and further comprising the downlink control part carrying at least one of downlink control information and a downlink reference signal; and causing transmission of a message comprising at least one information element indicating the selected length of the at least one of uplink control part and downlink control part of the sub-frame.