A method synchronizes frame counters for protecting data transmissions between a first end-device and a second end-device. The data, in particular data frames, are transferred between the first end-device and the second end-device. The data frames are provided with frame counters to protect the data transfer between the first end-device and the second end-device. The second end-device sends a first data frame to the first end-device. The first data frame contains a marker in its payload data. The first end-device sends back a second data frame as an answer to the second end-device. The second data frame contains a frame counter in the header data, and the second data frame contains the frame counter and the marker in its payload data.