A Throwdown Machine for Telephone 

 Traffic Studies 



BY G. R. FROST, WILLIAM KEISTER AND 

 ALISTAIR E. RITCHIE 



(Manuscript received December 3, 1952) 



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 Jour operators. Its input is a statistically accurate representa- 

 tion 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. 



INTRODUCTION 



Existing analytical methods are inadequate for investigating many- 

 statistical problems in which a large number of variables and their inter- 

 actions must be considered. The problem of evaluating the performance 

 and traffic capacity of a large automatic telephone switching system is 

 one example. Others involve logistics, air and highway traffic control, 

 and certain phases of military and naval strategy. All these require the 

 assimilation of large quantities of data, processing the data according 

 to certain procedures which are often empirical, and producing final 

 information from which performance of the system or the excellence of 

 the procedures can be judged. 



These problems fall in the general category of "systems evaluation." 

 The types of systems considered are those that are capable of a large 

 number of variations depending on the nature of the input data, and 

 must be judged on a statistical basis. One method of study might be to 

 operate and observe an actual system. There are a number of objections 

 to this. Operation may be so slow that the accumulation of sufficient data 

 may require excessive time or, as in the case of a telephone switching 

 system, so rapid that it is impractical to make the necessary observations. 

 Operating the system under controlled conditions in these cases may be 



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