Data is transmitted from a sending processor over a network to one or more receiving processor in a forward direction during an allocated slot, and acknowledge signals are sent in a reverse direction during the same allocated slot, to indicate whether the receiving processor is able to receive data If one or more of the receiving processors indicates that it is unable to receive the data, the data is retransmitted during the next allocated slot. This means that the sending processor is able to determine within the slot period whether a retransmission is necessary, but that the slot period only needs to be long enough for one-way communication.