Telephone exchange and switching hierarchy

Telephone exchange

A telephone exchange (switching office or central office) is the system that connects the line of a calling subscriber to that of the called subscriber, or to a trunk leading to another exchange, for the duration of a call and releases it afterwards. It avoids wiring every telephone to every other ($n(n-1)/2$ links). Its functions include detecting a call, receiving the dialled number, finding a free path, ringing the called party, supervising the call, charging and clearing.

Switching hierarchy

Exchanges are arranged in a hierarchy so that any two subscribers can be connected with a small number of trunks. The classic (AT&T/North American) five-level hierarchy is:

          Class 1: Regional centre
          /                 \
   Class 2: Sectional    Class 2: Sectional
   centre                centre
       |                     |
   Class 3: Primary      Class 3: Primary
   centre                centre
       |                     |
   Class 4: Toll centre  Class 4: Toll centre
     /     \                 |
 Class 5   Class 5        Class 5
 end office end office    end office
   |  |       |              |
 subscribers subscribers  subscribers

 Final (hierarchical) route: ─── (vertical links)
 High-usage trunks: direct links between busy
 offices at any level (shortcuts)

Routing: a call first tries a high-usage trunk (direct route) between the two exchanges; if all are busy it overflows upward to the next level, and finally uses the final route through the hierarchy. ITU-T uses similar levels: local exchange, primary, secondary, tertiary and quaternary (international) centres.