1090 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1952 



number of Western Electric Company 100-line and 20-line automatics 

 were in commercial service ; a small amount of semi-automatic equipment 

 was also in operation with the equipment under direct control of the "A" 

 operator's dial; and planning and development work were under way to 

 remove some of the limitations and extend the field of use of the auto- 

 matic and semi-automatic systems. 



The rotary dial was developed in 1896. However, many of the early 

 systems did not use this type of dial. Various calling devices were used 

 for a number of years. Among these were lever operated pre-set devices, 

 keysets of several types, and dials with holes (in one case as many as 100) 

 in which a peg could be inserted to act as a stop for an arm which was 

 pulled around and allowed to restore. In all the early systems, regardless 

 of the device used, the signals generated at the calling station directly 

 controlled the selections. 



RECOGNITION OF NEED FOR ACCESS TO LARGER TRUNK GROUPS 



While mechanisms and circuits were being developed for direct dial 

 control switching, work of a theoretical nature was going on which was 

 to have an important effect on future designs. This work consisted of 

 traffic probability studies and observations the outcome of which was 

 the development of formulae and curves on the efficiency of trunk 

 groups which influenced strongly the views of engineers as to the eco- 

 nomical sizes of switches. G. T. Blood of the American Telephone and 

 Telegraph Company in 1898 found that the binomial distribution closely 

 fitted the observed data on the distribution of calls. The first compre- 

 hensive paper on the matter was one by M. C. Rorty in 1903, Application 

 of the Theory of Probability to Traffic Problems. Curves accompanying his 

 paper indicated that trunking efficiency improved with group size. Subse- 

 quent work by E. C. Mohna in postulating that the grade of service 

 experienced by a particular call applied to every call in the office and 

 in developing the Poisson approximation to the binomial expansion 

 formed the basis for trunking theory as used in the Bell System. Fig. 1 

 is a reproduction of three curves produced by Molina on July 6, 1908, 

 showing the average load carried by various numbers of trunks for three 

 probability conditions namely P.Ol, P.OOl and P.OOOl corresponding to 

 an all trunks busy condition encountered by calls once in a hundred, 

 once in a thousand, and one in ten thousand times respectively. From 

 these curves it can be seen, for example, that ten trunks can carry a load 

 averaging shghtly over four calls with a probability of loss of P.Ol. 

 Twenty trunks can carry an average of over eleven simultaneous calls 



