476 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1956 



Table VII — Sum of Direct Group Overflow Loads, 



Offered to OST's 



Average. 

 Variance 



Theory 



86.06 

 129.5 



Observed 



87.12 

 127.4 



local multi -alternate route systems, a throwdown was made in 1941 on a 

 proposed trunk plan for the Murray Hill-6 office. The arrangement of : 

 trunks is shown on Fig. 32. Three successive alternate routes, Office 

 Selector Tandems (OST), Crossbar Tandem (XBT), and Suburban: 

 Tandem (ST), are available to the large majority of the 123 direct trunk 

 groups leading outward to 169 distant offices. (The remaining 46 parcels 

 of traffic did not have direct trunks to distant offices but, as indicated 

 on the diagram, offered their loads directly to a tandem group.) A total 

 of 726 trunks is involved, carrying 475 erlangs of traffic. 



A throwdown of 34,001 offered calls corresponding to 2.7 hours of 

 traffic was run. Calls had approximate exponential holding times, averag- 

 ing 135 seconds. Records were kept of numbers of calls and the load from 

 the traffic parcels offered to each direct group, as they were carried or 

 passed beyond the groups of paths to which they had access. Loads car- 

 ried by each trunk in the system were also observed by means of a 36- 

 second "switch-count." (The results on the 17 OST groups reported in 

 Section 7.3.1 were part of this study.) 



Comparisons of observation and theory which are of interest include 

 the combined loads to and overflowing the 17 OST's. Observed versus 

 calculated parameters (starting with theory from the original direct 

 group submitted loads) are given in Table VII. The agreement is seen 

 to be very good. 



The corresponding comparison of total load from all the OST's is 

 given in Table VIII. Again the agreement is highly satisfactory. 



Not all of the overflow from the OST's was offered to the 22 crossbar 

 tandem trunks; for economic reasons certain parcels by-passed XBT andf 

 were sent directly to Suburban Tandem.* This posed the problem of 

 breaking off certain portions of the overflow from the OST's, to be added"' 

 again to the overflow from XBT. An estimate was needed of the contri 

 bution made by each parcel of direct group traffic to any OST's over 

 flow. These were taken as proportional to the loads offered the OST by 

 each direct group (this assumes that each parcel suffers the same over- 



* In the toll alternate route system by -passing of this sort will not occur. 



