Abstract: A data communications system comprising first (B), second (C) and third (A) remote units arranged for communication over a common communications channel. A first message to be sent to the second unit (C) is generated at the first unit (B). The first unit (B) waits for the channel to become available for independent transmission of the message. If (t.sub.0), before the channel becomes available for independent transmission, a new message is received from the third unit (A) requiring an acknowledgement, the new message is received from the third unit (A), an acknowledgement of receipt of the message is transmitted (t.sub.1) and in a substantially continuous transmission (t.sub.2), the first message to the second unit is transmitted in a "piggy-back" manner. In a preferred arrangement, multiple messages can be piggy-backed to different units. Priorities can be assigned to these messages for acknowledgements.