A first communications device 102 and a method therein for transmitting data to a second communications device 104 on a sub-channel. The first device performs carrier sensing on a first sub-channel assigned an unknown occupancy state. When the first sub-channel is determined as being vacant, the first device transmits, to the second device, the data on the first sub-channel. When the first sub-channel is determined as being occupied, the first device assigns the first sub-channel an occupied state, starts a first time period associated with the first sub-channel, and performs carrier sensing on a second sub-channel assigned with the unknown occupancy state. When the second sub-channel is determined as being vacant, the first device transmits, to the second device, the data on the second sub-channel. In absence of the second sub-channel assigned an unknown occupancy state, the first device enters an inactive mode until the first time period has expired.