THE ROUTING SYSTEM
The phone system consists of three main components: toll switching offices, tandems, and trunks. Each switching office contains thousands of tandems, which are lines with relays capable of communicating with other tandems in any other location on the continent. Trunks are the long-distance lines that connect tandems and switching offices to each other.
When an operator activates a long-distance call, it may be routed through any combination of tandems to get from its origin to its destination. It does not always take the shortest path; if traffic is heavy on the trunks directly between two cities, the call may be routed through a more indirect route, possibly passing through tandems all over the country before reaching its final destination.
A tandem can be thought of as having two sides. One side faces toward
the residential lines in that particular exchange, and the other side faces
into other trunk lines, which in turn lead to other tandems. When a tandem
is not in the process of connecting a call, it whistles a constant tone
of 2600 Hz (cycles per second) in both directions. This signal indicates
to the rest of the system that the tandem is idle, and is ready to be used
to route a call. When a long-distance number is dialed, the sending side
of the tandem stops whistling 2600Hz over the trunk line. Now, the trunk
line is said to be seized. The sending tandem then begins to transmit the
dialed number to the tandem at the destination, which then interprets the
number and figures out which phone to ring in that local exchange. The process,
simplified, is this: Customer dials number -- number goes to local CO (central
office) -- CO transmits number to the toll office for that area -- that
toll office sends it to the toll office in the destination city -- the toll
office sends it to that city's CO -- the CO rings the local phone. Again,
this is simplified; the actual route taken by the call may include more
than two toll offices.