There  are  a  number  of  basic  functions of  all  exchanges,  regardless  of
   whereabouts in the network they are situated. This system core comprises......

   Group Switch for switching functions
    Trunk and Signaling equipment for, e.g. signaling to other exchanges
   Operation and Maintenance functions
   Control function
   Power supply

   In addition, the following subsystems are found in local exchange :

   The subscriber stage, which among other things handles A/D conversion,
      concentration towards the group switch, signaling and power supply to the
      subscriber.

   Subscriber service function, where functions such as "follow me" , call-back,
      etc. , are controlled.

   The charging functions, which sets the right price on our calls.

   The subsystems in a local exchange are shown in Figure 29.
 

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