A method is performed by a wireless transmitting device for a communication network, the communication network implementing a standard in which transmitting devices contend for access to a wireless transmission medium, a transmitting device transmitting using the wireless transmission medium during a transmission opportunity (TXOP) upon winning contention. The method comprises: initiating a first, initial transmission to one or more wireless receiving devices in the TXOP, the first transmission comprising a first preamble and user data; and receiving at least one acknowledgement message from the one or more wireless receiving devices, wherein the at least one acknowledgement message identifies that user data was not correctly received. The method further comprises, subsequent to the first transmission, initiating a second transmission to the one or more wireless receiving devices in the TXOP, the second transmission comprising a second preamble which is shorter than the first preamble, or comprising no preamble. The second transmission further comprises a re-transmission of user data that was not correctly received.