CROSSBAR TANDEM AS A TOLL SWITCHING SYSTEM 



105 



translated in the marker and the second three digits in a foreign area 

 translator which is associated with the marker. Fig. 11 shows, in simpli- 

 fied form, how this translation is accomplished. 



The first three digits, corresponding to the area code, are received by 

 a relay code tree in the marker which translates it into one of a thousand 

 code points. This code point is cross-connected to the particular relay of 

 the five area relays A(3-A4 which has been assigned to the called area. 

 A foreign area translator is now connected to the marker and a corre- 

 sponding area relay is operated in it. The translator also receives the 

 called office code from the sender via the marker and by means of a 

 relay code tree similar to that in the marker translates the office code 

 to one of a thousand code points. This code point plus the area relay is 

 sufficient to determine the actual route to be used. As shown on the 

 sketch, wires from each of the code points are threaded through trans- 

 formers, two for each area. When the marker is ready to receive the 

 route information, a surge of current is sent through one of these threaded 

 wires which produces a voltage in the output winding to ionize the 

 T- and U- tubes. Only the tubes associated with the area involved in 

 the translation pass current to operate one each of the eight T- and U- 

 relays. This information is passed to the marker and registered on 

 corresponding tens and units relays. These operate a route relay which 



WISCONSIN 



MICH. 



J 



ILLINOIS 



CHICAGO = 

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1 AREA IplaINeJ 

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^- — 1 



I N D. 



ROUTE WITHOUT 6 DIGIT TRANSLATION 



ROUTE WITH 6 DIGIT TRANSLATION 



Fig. 10 — Six-digit translation. 



