It has been conventional to divide telephone exchanges into two main categories : space division (or analog), and time division (or digital).
- Space Division (or Analog) – in which direct physical paths are established right through the exchange from one subscriber's line to another. Connections may be metallic contacts or by solid-state analog devices.
- Time Division (or Digital) – in which some or all of the switching stages in the exchange operate by shifting signals in time. Basically a connection is made between incoming and outgoing channels by transferring each PCM word from the time-slot of the incoming channel to that of the outgoing channel. This time-shifting is carried out by means of stores. Information is written ...