Field devices in factory automation and process control systems generate time-critical, periodic, and time-available data. These data share the bandwidth of one fieldbus network medium. This paper reviews a bandwidth allocation scheme (BAS) that is applicable to many different kinds of fieldbuses. The BAS satisfies the real-time transmission requirements of time-critical and periodic data, and fully utilizes the bandwidth resources of the fieldbus network. The basic concept of the BAS is introduced and its implementation in Foundation Fieldbus, Profibus, and CAN protocols is discussed.