A communications apparatus includes an input for receiving a data stream being transmitted from a first network node to a second network node using a main channel. A processing resource of the communications apparatus identifies data signifying an idle period within the data stream and determines whether the idle period is at least a suitable minimum duration to support initiating transmission of sub-channel data in place of at least part of the data signifying the idle period. Further, the processing resource is arranged to identify when the idle period is not of the suitable minimum duration and a need arises to transmit the sub-channel data within a predetermined period of time. In such circumstances, the processing resource sends a flow control message upstream to the first network node to halt transmissions therefrom, thereby generating the idle period of at least the suitable minimum duration.