CROSSBAR TANDEM AS A TOLL SWITCHING SYSTEM 



97 



digits as required, highly flexible code conversion (transmitting forward 



i different digits for the area or office code instead of the dialed digits), 



prefixing digits ahead of the called office code, and six-digit translation. 



ALTERNATE ROUTING 



The control switching points will be interconnected by a final or 

 "backbone" network of intertoll trunks engineered so that very few 

 calls will be delayed. In addition, direct circuits between individual 

 switching offices of all classes will be provided as warranted by the 

 traffic density. These are called "high-usage" groups and are not en- 

 gineered to handle all the traffic offered to them during the busy hour. 

 Traffic offered to a high-usage group which finds all trunks busy will be 

 automatically rerouted to alternate routes®-^ consisting of other high- 

 usage groups or to the final trunk group. The abi.ity of the crossbar 

 tandem equipment at the control switching point to select one of several 

 alternate routes automatically, when all choices in the first route are 

 busy, contributes to the economy of the plant and provides additional 

 protection against complete interruption of service when all circuits on 

 a particular route are out of service. 



Fig. 4 shows a hypothetical example of alternate routing when a 

 crossbar tandem office at South Bend, Indiana, receives a call destined 

 for ^Youngstown, Ohio. To select an idle path, using this plan, the 

 switching equipment at South Bend first tests the direct trunks to 

 Youngstown. If these are all busy, it tests the direct trunks to Cleveland 

 where the call would be completed over the final group to Youngstown. 

 If the group to Cleveland is also busy, South Bend would test the group 



CHICAGO 



SOUTH BEND 



CROSSBAR 



TANDEM 



CLEVELAND 



-YOUNGSTOWN 



ITT5BURGH 



Fig. 4 — Toll network — alternate routing. 



