Ethernet is gaining importance in fields such as automation, avionics and automotive. In these fields novel multimedia-based applications must coexist with traditional control systems, which leads to high diversity in size, intensity and timing requirements of the traffic traversing the channel. Multimedia traffic is characterised by having large size, low intensity and soft real-time requirements, while control traffic usually conveys small amounts of information with a high intensity and hard real-time requirements. Moreover, many modern applications must support on-line connection and disconnection of participants. Since Ethernet was designed as a general purpose data network protocol it lacks appropriate support for real-time communications and dynamic quality of service management. Several protocols were proposed to cope with these drawbacks, including Flexible Time-Triggered Switched Ethernet and, more recently, Audio Video Bridging. In this paper we discuss the importance of the admission control and make a comparison of the implementations carried out in the aforementioned protocols.