THEORIES FOR TOLL TRAFFIC ENGINEERING IN THE U. S. A. 477 



flow probability). The variance of this overflow portion by-passing XBT 

 was estimated by assigning to it the same variance-to-average ratio as 

 was found for the total load overflowing the OST. Subtracting the means 

 ' and variances so estimated for all items by-passing XBT, left an approxi- 

 mate load for XBT from each OST. Combining these corrected overflows 

 gave mean and variance values for offered load to XBT, Observed values 



0.10 r 



^-.--RANDOM TRAFFIC 

 -THROWDOWN 



--NEGATIVE BINOMIAL 



10 20 30 40 50 



n = NUMBER OF SIMULTANEOUS CALLS 



THEORY OBSD 



( ) ( ) 



AVG 25.18 25.51 



VAR 47.67 56.10 



Pin 



10 20 30 40 50 



n = NUMBER OF SIMULTANEOUS CALLS 



Fig. 33 — Distribution of load offered to crossbar tandem trunks; negative bi- 

 nomial theory versus throwdown observations. 



