A data transmission system comprises a first device and a second device and a duplex data transmission connection therebetween. The first device sends to the second device user data on a data channel and control information on a control channel. The transmission is arranged in frames that comprise a user data field corresponding to a data channel and at least a first and a second control information field corresponding together to a control channel. In order to realise the control of the transmission power, there is determined a first transmission power level, a second transmission power level and a third transmission power level. From the second device to the first device there is transmitted a frame by applying said first transmission power level to the transmission of the user data field, said second transmission power level to the transmission of the first control information field and said third transmission power level to the transmission of the second control information field.