Plain old telephone system

Manual Operator

Jan 14, 1878

Manual operation.
Human control.
Space/analog switching method.
Plug/cord/jack network.

Step-by-step

Apr 20, 1892

Electromechanical operation.
Space/analog switching method.
Control distributed stage by stage.
stepping switch train network.

Crossbar

Jul 22, 1918

Electromechanical operation.
Space/analog switching method.
Common control.
X-bar switching network.

Lucent ESS - First Generation

May 26, 1960

Semielectronic operation.
Space/analog switching method.
Common control.
Reed switch network.

Lucent ESS - Second Generation

Oct 7, 1972

Semielectronic operation.
Space/analog switching method.
Stored program control.
Reed switch network.

Lucent ESS - Third Generation

Feb 11, 1976

Electronic operation.
Time/digital switching method.
Stored program common control.
Pulse code modulation network.