In-band vs. out-of-band, DTMF and ANI

The telephone network is a widely distributed system of intelligent switches. These switches, to establish and tear down calls, must communicate with each other and with the end users equipment. The communication process is called signaling.

The original signaling was the human voice, when a caller spoke to the operator. The first automatic terminal equipment used special switch-generated call progress tones to give call status information to callers.

Rotary signaling turns current on and off corresponding to the number dialed. As the dial is released after each digit of a phone number, the current is turned off and on from one to 10 times, depending on the digit. This coded signal goes to the Central (telephone switching) Office and used to make the correct connection to the destination telephone number.

An analog input device (like a phone) takes the acoustic signal (which is a natural analog signal) and converts it into an electrical equivalent in terms of volume (signal amplitude) and pitch (frequency of wave change).

Removing the handset at the ringing telephone results in a loop current flow. When a called party removes the handset in response to a ring, the loop to that phone is completed by its closed switchook and loop current flows through the called telephone. The central office then removes the ringing signal and ringback tone from the circuit.

See the PBX (Private Branch Exchange) chapter for more about ring generators and progress tones.

Dial Tone Multi Frequency (DTMF)