The analog telephone network is basically made up of four stages as shown in the following illustration. Exchanges at lower levels in the network hierarchy process calls over a shorter distance and those at higher levels process calls over a longer distance, improving facility efficiency.
Regional Center: RC
The area consisting of several district areas (DAs) is called a regional area (RA). The regional center is installed at the center of the regional area.
District Center: DC
The area consisting of toll areas (TAs) organized somewhat like a single prefecture unit is called a district area (DA). The district center is installed at the center of the district area.
Toll Center: TC
The area consisting of several local service areas (LAs) is called a toll area (TA). The toll center is installed at the center of the toll area.
End Office: EO
The end office is a switching office accommodating subscribers' lines, placed at the lowest level in the network hierarchy.
Regional Center: RC
