In a general aspect, a network transmission interface can include, within an egress data path, a physical coding sublayer (PCS) operating in a constant bitrate domain for transmitting data frames on a network link; a timestamp unit configured to insert timestamps in payloads of the frames; a transmission media access control (MAC) unit located at a boundary between the constant bitrate domain and a variable bitrate domain, configured to receive the frames at a variable bitrate, encapsulate the frames, and provide the encapsulated frames at a constant bitrate; a MAC layer security unit located downstream from the timestamp unit, configured to sign and optionally encrypt the payloads and expand each frame with a security tag and an integrity check value (ICV). The timestamp unit and the MAC layer security unit (26b) can both operate in the constant bitrate domain.