There is provided mechanisms for data transmission on a contention based physical data channel (PDCH). The method is performed by a wireless device. The method comprises sending a first data transmission on the contention based PDCH to an access node. The method comprises receiving a response to the first data transmission from the access node. The method comprises determining a signal strength of the response. The method comprises acting as the first data transmission either was collision-free or collided with another data transmission of another wireless depending on a comparison between the signal strength and a threshold defined by an expected signal strength of the response.