868 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1952 



both codes are again translated since Boston also has a choice of routes 

 to Maine, and the route to Bangor is selected. The translating equipment 

 in Boston knows that Bangor is in the Maine area and that the area 

 code will, therefore, not be needed. However, since Bangor is a TO 

 having no senders, the Boston sender must pulse forward all of the 

 digits needed to complete the call through switches in Bangor, Houlton 

 and Monticello. It is assumed that Houlton is arranged to route the call 

 to Monticello on receipt of the digits AC4. Numerical digits 2345 will 

 route the call through the Monticello switches to the called customer's 

 line. These digits are all registered in the Boston sender but the digits 

 required to switch the call through Bangor are not and must be supplied. 

 An arbitrary set of digits beginning with "1" can be used for this purpose 

 since no office code begins with "1" and there will, therefore, be no 

 conflict. 



The decoder in Boston, therefore, gives the sender the proper set of 

 arbitrary digits, say 16, to be placed ahead of the office code AC4. The 

 sender sends forward by the DP method 16-AC4-2345 driving switches 

 in Bangor, Houlton and JMonticello to the called subscriber's line, and 

 ringing starts automatically. The talking connection is now established 

 and the common control equipment at all intermediate points is free. 



When the called subscriber answers, the Atlanta operator's cord lamp 

 is extinguished. When he hangs up the lamp lights to denote end of 

 conversation. The removal of the operator's cord automatically releases 

 the entire connection, the release of each link causing the next in line to 

 release. 



In setting up this call all of the characteristic CSP features were em- 

 ployed, automatic alternate routing in Atlanta, six-digit translation in 

 New York and Boston, digit storing and variable spilling at all CSP's 

 with substitution of arbitrary digits for the area code at Boston. 



TRANSMISSION 



All talking connections through the CSP system are made on a four- 

 wire basis, that is, separate pairs of conductors are provided for trans- 

 mission in the two directions. This is done in order to simplify the 

 problem of maintaining satisfactory balance so that the loss introduced 

 by extra links in a connection can be held to a minimum value. The 

 importance of this feature is emphasized hy the fact that the switching 

 plan permits as many as eight intertoll trunks to be connected in tandem 

 for the completion of a call. 



The advantages of four-wire switching were fully explained in the 

 paper on the toll crossbar system now in service. 



