This chapter discusses the use of prototype hardware test beds as a tool for ascertaining actual measures, for performing controlled experiments to determine the operational characteristics of different parts of a network, and for the validation of software simulation models. It describes the implementation of a hardware test bed, defines the measurable quantities, derives operational relationships for nonmeasured quantities, and gives some results. The chapter discusses the network that is situated in a prototype test bed that is instrumented for data collection and can generate network traffic. In constructing a prototype test bed, it is desirable to keep hardware development costs at a minimum, to provide a flexible system so that changes in network design can be accommodated, and to provide the general purpose driver capabilities. The chapter examines network that uses a token access protocol. It illustrates the use of time tags, derives the performance evaluation parameters of interest, and shows some experimental results that exemplify the techniques.