AUTOMATIC TOLL SWITCHING SYSTEMS 8()3 



in the T^iiitod States or Canada - and for a particular destination the 

 same set of digits will be used wherever the call may originate. All CSP's 

 must, therefore, be able to advance a call toward the same place when 

 the same set of digits is re('(M\-ed. 



To make use of destination codes possible, each C'SP must store the 

 digits as received and pass along to the next point whate\er digits may 

 1)(^ HMiuired there for advancing the call. If the next point is a C'SP not 

 in the home numbering area of the called place, the complete ten-digit 

 number will be sent forward. If it is a CSP in the home numbering area 

 of the called point the area code will be dropped and the remaining seven 

 digits will be sent forward. That CSP may in turn complete to a local 

 office directly, dropping the office code, or through a step-by-step T( ) 

 (Tandem Outlet) or TC (Ordinary Toll Center), substituting arbitrary 

 digits for the area or office code, thereby exercising the third of the listed 

 CSP features. 



2. Automatic AJternote Routin(/ 



The system is arranged to ot^er the maximum number of alternate 

 routes possible under the switching plan. As explained by Mr. Pilliod, 

 a maximum of five alternates will actually be used. This number is possi- 

 ble, of course, only at PO's since higher ranking CSP's have fewer CSP's 

 above them in the final chain. 



3. Code Conversion 



This refers to the ability to substitute one, two or three arbitrary 

 digits for the area code, the office code or both. It is economically im- 

 portant to be able to do this because it makes it possible to work with 

 the step-by-step e(iuipment extensively used in local offices and in toll 

 offices in TC's or TO's without the changes in local numbering plans or 

 learrangements - and in some cases extra selectors - required for the 

 step-by-step TC's or TO's to use ABX codes for routing purposes. Even 

 though eventually all customers are listed as ABX-XXXX and TC's 

 are arranged to use the listed number for routing the calls, this will not 

 l)e accomplished for some time. Moreover, after such arrangements are 

 in effect there will still be need for code conversion, particulai'ly for 

 routing calls through TO's. Many combinations of digit dropping and 

 sulistitution are reciuired to cover all possible cases. 



4. Six-Digit Translation 



When a CSP receives a ten-digit iuiml)er it is sometimes sufficient 

 to translate only the area code digits and sometimes necessary to trans- 



