Switched Ethernet is a dominating technology in local area networks,and a strong contender in system area networks. This thesis describes measurements on switched Ethernets, in thecontext of high speed system area networks with real timerequirements. One such system is the ATLAS level 2 trigger, whichformed the basis of the measurements. Further on the thesisdescribes the design, implementation and results from a simulationmodel of the measured devices. In general the results indicate that switched Ethernetprovides a service close to the ideal for this switch architecture.The devices were found to be near non-blocking and it was possibleto prevent packet loss with Ethernet flow control. Some performanceanomalies were found, and the thesis emphasises that validation testsare needed in this application since it is beyond normal usagepatterns for these commodity products. A simulation model of the network was developed. It was possible toobtain a quite accurate model of the latency distributions. Packetloss proved to be difficult to model accurately. The lack of a headof line blocking prevention mechanism in the model is thought to bethe reason for the packet loss inaccuracies.