02 THE BELL SYSTEM TECHXIf AL JOURNAL, JANUARY 1956 



traffic to and from the local offices into larger amounts for routing over 

 common triuik groups to gain increased efficiency resulting in fewer over- 

 all trunks. 



A second important function is to permit handling calls economically 

 between different types of local offices which are not compatible from the 

 standpoint of intercommunication by direct pulsing. Crossbar tandem 

 offices serve to connect these offices and to supply the conversion from 

 one type of pulsing to another where such incompatibilities exist. 



The third major function is that of centralization of equipment or 

 services. For example, centralization of expensive charging equipment at 

 a crossbar tandem office results in efficient use of such equipment and 

 over-all lower cost as compared with furnishing this equipment at each 

 local office requiring it. Examples of such equipment are remote control 

 of zone registration and centralized automatic message accounting.^ Cen- 

 tralization of other services such as weather bureau, time-of-day and 

 similar services can be furnished. 



The first crossbar tandem offices were installed in 1941 in New York, 

 Detroit and San Francisco. These offices were equipped to interconnect 

 local panel and No. 1 crossbar central offices in the metropolitan areas, 

 and to complete calls to manual central offices in the same areas. The war 

 years slowed both development and production and it was not until the 

 late 40's that many features now in use were placed in service. These 

 later features enable customers in step-by-step local central offices on the 

 fringes of the metropolitan areas to interconnect on a direct dialing basis 

 with metropolitan area customers in panel, crossbar, manual and step- 

 by-step central offices. This same development also permitted central 

 offices in strictly step-by-step areas to be interconnected by a crossbar 

 tandem office where direct interconnecting was not economical. Facilities 

 were also made available in the crossbar tandem system for completing 

 calls from switchboards where operators use dials or multifrequency key 

 pulsing sets. 



Since a crossbar tandem office usually has access to all of the local 

 offices in the area in which it is installed, it is attractive for handling 

 short and long haul terminating traffic. The addition of toll terminal 

 equipment at Gotham Tandem in New York City in 1947 permitted 

 operators in New York State and northern New Jersey as well as distant 

 operators to dial or key pulse directly into the tandem equipment for 

 completion of calls to approximately 350 central offices in the New York 

 metropolitan area. This method of completing these calls without the 

 aid of the inward operators was a major advance in using tandem switch- 

 ing ecjuipment for speeding completion of out-of-town calls. 



