CPU Based Exchange:  In centralized control, all control equipment is replaced by a single processor which should be quite powerful. It should be capable of processing 10 to 100 calls per second, depending on load on the system and concurrently performing  many  other  ancillary  tasks.  A  typical  control configuration of an ESS using centralized SPC is demonstrated in Figure. A centralized SPC configuration can use more than one processor for redundancy purposes. Though there are a large number of services that can be offered by CPU based exchange, they can be grouped under four wide categories.

1.  Services associated with calling subscribers and designed to decrease time spent on dialling and number of dialling errors.

2.  Services associated with called subscriber and  designed to increase call completion rate.

3.  Services involving more than two parties.

4.  Miscellaneous services.