332 



THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1953 



in the course of the run the call is discarded. The other important factor 

 is a ''feedback" effect due to the calls meeting a situation which prevents 

 successful completion and the probability that these calls are originated 

 later. 



A simple example is where the called line is found busy. As previously 

 mentioned, there is then a possibility that subsequent attempts will 

 be made at a later time. All attempts make use of circuits and control 

 equipment and should be considered in determining the load capacity 

 of the office. Other situations which produce this effect are illustrated 

 in Fig. 15. Lines entering the figure at the left represent classes of 

 calls which enter the system. A certain number of calls will be partial 

 dials, no dials and false starts regardless of the performance of the system 

 at the time the call is originated. The partial dials represent cases where 

 the subscriber makes an error in dialing or does not wait for dial tone 

 and dials a digit before he is connected to a register. These may be aban- 

 doned before the register obtains a marker or may "time out" in the 

 register and be connected to an overflow tone trunk. In either case the 

 subscriber may re-originate the call later. The probability of re-origina- 

 tion has been estimated from field data and the dotted lines in the figure 

 represent these reoriginated calls. 



In the throwdown machine the random circuit is used to determine 

 which calls will re-originate and the elapsed time before the second at- 



GOOD CALL 



STARTS 

 X 



GOOD CALLS 

 COMPLETED 



REORIGINATED 

 CALLS 



r,*r,-r,*, ABANDONED 

 PARTIAL ^ ^_. 



DIAL^^' 



~^ ME OUT 



{. 



NO 

 DIAL 



ABANDONED 



►- 



{ 



TIME OUT 



DON'T ANSWER 



K 



BUSY i 



^-K 



PARTIAL DIAL 



STARTS 

 X 



ABANDONED j 

 ^ — I 



I 



"\TIM 



E OUT 



^-- 



NO DIAL 

 STARTS 

 X 



FALSE 

 STARTS 

 X 



ABANDONED 



► 



{ 



TIME OUT 



► 



Fig. 15 — Composition of the load on a crossbar office. 



