THROWDOWN MACHINE FOR TRAFFIC STUDIES 303 



be developed by a study of the course of events in setting up calls through 

 the system. As has been intimated earlier, the establishment of calls is 

 largely a matter of marker operation. In order to illustrate the marker 

 functions, Figs. 3 and 4 show two charts indicating the order of events in 

 establishing a connection, first, between a caUing or originating sub- 

 scriber line and a register, and second, after dialing, between two sub- 

 scriber lines within the same office. These two types of connection are 

 known as a dial tone call and an intraoffice call, respectively. 



Figs. 3 and 4 are drawn as sequence charts with time flowing do^\^l- 

 ward. There is no attempt to maintain an accurate time scale; the x 

 marks on the vertical line merely represent the relative order in which 

 important control functions take place. In actuality, of course, the time 

 between x marks is known mth fair precision. Brief descriptions of the 

 control functions are listed to the left of the vertical lines. The call il- 

 lustrated is presumed to encounter no difficulties in completion. How- 

 ever, points at which blocking might occur are marked with an asterisk 

 to the right of the lines. If any of the difficulties noted were to develop, 

 the marker would have to take alternative action which will be illus- 

 trated later. Also shown to the right of the lines are potential points of 

 delay, where a call may have to wait until a connector, a marker, or a 

 desired frame becomes idle. It must be remembered that during mod- 

 erate or heavy traffic, several or all of the markers are working simul- 

 taneously and tending to interfere with each other. 



In a Avell-balanced and soundly engineered central office, the aggrega- 

 tion of parts are nicely adjusted to give on the average no more than 

 certain preassigned values of delay and blocking at some average busy 

 hour traffic level chosen as a base. A typical example of permissible delay 

 is no more than 1 per cent of calls having a dial tone delay greater than 

 three seconds. When traffic is heavier than the engineering base, the 

 percentages of delay or blocking will increase. 



A summation of all the possible alternative sequences which a marker 

 may have to take when trunk busy, line busy and channel busy con- 

 ditions are encountered becomes extremely complex. Although no at- 

 tempt will be made to discuss this in detail, a chart shomng the opera- 

 tional variations of a marker on an intraoffice call is presented on Fig. 5. 

 Even this figure does not include all possible variations since, for ex- 

 ample, the contingency of all tone trunks being busy is not shown on 

 the diagram. This chart, similar in form to Fig. 4, will later be found 

 useful in discussing the throwdo^\Ti machine. In order to simplify the 

 presentation, some of the control events are combined in time with the 

 frame seizure which precedes the event. The normal course of a call 



