Switching hierarchy and evolution until the telephone

Switching hierarchy

Exchanges are arranged in levels so that a call between any two subscribers passes through as few links as possible and traffic can overflow to higher levels when direct routes are busy.

          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)

• Class 5 – End office (local exchange): subscribers are connected through local loops.
• Class 4 – Toll centre / tandem: joins end offices and gives access to long-distance network.
• Class 3 – Primary centre, Class 2 – Sectional centre, Class 1 – Regional centre: higher levels of the long-distance network; regional centres are fully meshed.
• Routing rule: first try a direct high-usage trunk; if busy, overflow upward; the last choice is the final route along the hierarchy.

Evolution of telecommunication up to the telephone

1. Ancient signalling: drums, smoke and fire signals, messengers and carrier pigeons – line of sight or physical transport.
2. Optical (semaphore) telegraph, 1790s: Claude Chappe's towers with movable arms in France sent coded messages from tower to tower.
3. Electric telegraph, 1837–1844: Cooke and Wheatstone (UK) and Samuel Morse (USA) sent messages as electric pulses over wires using Morse code; the first Washington–Baltimore line (1844). Telegraph networks and undersea cables (1866 transatlantic) followed.
4. Telephone, 1876: Alexander Graham Bell patented the telephone, converting voice to a varying electric current with a transmitter and back to sound in a receiver. Edison's carbon microphone improved it. This led directly to telephone exchanges (1878) and the switching systems above.