Office classification

Office classification
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It has been suggested that this article or section be merged into Public Switched Telephone Network. (Discuss)
AT&T PSTN Office Classification HierarchyOffice classification numbers were classifications assigned to telephone company offices before the breakup of AT&T in 1984. They were applied in the 1950s as part of organizing the system of Direct Distance Dialing (DDD). The numbers indicated an office's hierarchical function in the U.S. public switched telephone network (PSTN).

The following class numbers were used:

Class 1: Regional Center (RC) Originally to be several around the United States, then one in Missouri, but none was ever designated and the class was later used for international gateway exchanges.
Class 2: Sectional Center (SC) More than a dozen. All had plentiful trunks to all other Class 2, forming, in the main, the last possible route of a call when all more direct trunks were in use.
Class 3: Primary Center (PC) Approximately a hundred, serving local in and out traffic, and some interregional traffic. Trunk groups among Class 3 varied in size according to expected traffic; some pairs had no direct trunks between them but used an intermediate Class 3 or Class 2 office.
Class 4: Toll Center (TC) (Only if operators are present; otherwise Toll Point (TP)). These did not handle interregional traffic; only traffic in and out of their local area. Some were owned by the local telco. Late in the 20th Century this class was replaced by Access Tandems (AT) to comply with Bell System Divestiture.
Class 5: End Office (EO) (Local telephone exchange).
Any one center handles traffic from one or more centers lower in the hierarchy, especially in its region, and from higher class centers. For each call the route of first resort went by the most direct trunks to low class offices, and overflowed to higher class offices when those were occupied. Since the breakup of AT&T, these designations have become less firm.