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 



Table IX — Load Offered to Crossbar Tandem 



I Average. 

 Variance 



Theory 



25.18 

 47.67 



Observed 



25.51 

 56.10 



0.10 r 



-RANDOM TRAFFIC 



-THROWDOWN 



,--NEGATIVE BINOMIAL 



to 20 30 40 50 



n = NUMBER OF SIMULTANEOUS CALLS 



P^n 



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. 



