Jabarkan Pre-digital automatic exchanges dari Sentral telepon dalam bahasa sunda

Automatic exchanges, or dial service, came into existence in the éarly 1900s. Their purpose was to eliminate the need for human telephone operators.  Before the exchanges became automated, operators had to complete the connections required for a telephone call.  Almost everywhere, operators have been replaced by computerized exchanges. A telephone switch is the brains of an automatic exchange.  It is a device for routing calls from one telephone to another, generally as part of the public switched telephone network.

The local exchange automatically senses an off hook (tip) telephone condition, provides dial tone to that phone, receives the pulses or DTMF tones generated by the phone, and then completes a connection to the called phone within the same exchange or to another distant exchange.

The exchange then maintains the connection until a party hangs up, and the connection is disconnected. This tracking of a connection's status is called supervision. Additional féatures, such as billing equipment, may also be incorporated into the exchange.

In Bell System dial service, a féature called automatic number identification (ANI) was implemented. ANI allowed services like automated billing, toll-free 800-numbers, and 9-1-1 service. In manual service, the operator knows where a call is originating by the light on the switchboard's jack field. In éarly dial service, ANI did not exist. Long distance calls would go to an operator queue and the operator would ask the calling party's number, then write it on a paper toll ticket.   See also Automatic Message Accounting.

éarly exchanges used motors, shaft drives, rotating switches and relays. In a sense, switches were relay-logic computers. Some types of automatic exchanges were Strowger (also known as Step-By-Step), All Relay, X-Y, Panel and crossbar. These are referred to collectively as electromechanical switches.