A method for operating system to transmit a plurality of frames from a first communication device to a second communication device is disclosed. The method incudes generating a security-sequence using a security-sequence-counter-value, changing the security-sequence-counter-value to a new security-sequence-counter-value; encrypting, at the first communication device, the security-sequence using a first-derived-session-key to provide an encrypted-security-sequence-counter-value; providing, at the first communication device, a frame comprising the security-sequence and encrypted-security-sequence-counter-value, transmitting a first-type frame from the first communication device to the second communication device, decrypting the encrypted-security-sequence-counter-value using the first-derived-session-key to provide the security-sequence-counter-value, configuring a security-sequence detector to detect a security-sequence using the security-sequence-counter-value, setting a key-validity-counter-value in accordance with the number of first-type frames transmitted since the first-derived-session-key was updated; and updating the first-derived-session-key based on a master-session-key in response to the key-validity-counter-value reaching an update value.