In order to study the traffic‐carrying characteristics of the No. 5 crossbar switching system, a machine has been built to simulate the operation of the system. This machine, known as a throwdown machine, is controlled by a team of four operators. Its input is a statistically accurate representation of telephone traffic and its output is a detailed record of the course of each call through the system. This paper discusses the design principles of the throwdown machine, its operation, and the type of results obtained.

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