A communications system comprises a first network node coupled to a second network node so as to support a first main channel and a second main channel. A communications apparatus is coupled in-line between the first and second network nodes to support a first sub-channel in the first main channel and second sub-channel in the second main channel. The communications apparatus is arranged to determine whether an alternative sub-channel to the first sub-channel is required. If required, the communications apparatus selects the second sub-channel as the alternative sub-channel for communicating the sub-channel data.