Functions of analog telephone signaling
Signaling carries the information required to establish, supervise, and release a call. On a local subscriber loop it uses DC current changes, ringing voltage, and audible tones. These signals control the connection and are separate from subscriber speech.
- Line supervision detects seizure, answer, and release.
- Address signaling carries the digits of the called number.
- Call progress signals report readiness, alerting, busy status, or failure.
DC and AC signaling
The exchange identifies loop state from DC current. Dialing and call-progress tones lie in the audible range and can be heard through the receiver. Ringing voltage uses a frequency below the speech band to operate the telephone ringer.
In-band and out-of-band describe signal frequency relative to the speech channel. They do not identify the physical source of the signal or its transmission direction.
Line seizure and dial tone
- In the state, the telephone does not draw normal loop current.
- Lifting the handset closes the circuit. The exchange detects and assigns a digit receiver.
- Dial tone tells the subscriber that the exchange is ready to accept a number. Frequency and cadence depend on the national tone plan.
Pulse dialing and DTMF
Pulse dialing represents a digit by a number of brief loop-current breaks. Ten pulses represent zero. Correct decoding depends on pulse rate and on the proportion between the break and make intervals.
DTMF represents a key with two simultaneous tones. One belongs to a low frequency group and the other to a high frequency group. ITU-T Q.23 specifies 697, 770, 852, and 941 Hz for rows and 1209, 1336, 1477, and 1633 Hz for columns.
A DTMF receiver checks both components, their levels, and their duration. This prevents speech from being accepted as a digit. Four rows and four columns encode 16 symbols, although a normal subscriber set exposes 12.
Alerting and speech-path connection
- The exchange analyzes the received digits and selects a route to the destination.
- If the destination line is idle, ringing voltage is applied to it. The calling subscriber receives ringback tone.
- The called subscriber going off-hook causes current to flow in that loop. The exchange removes ringing and records the answer condition.
- The connected path carries speech. In a digital exchange, the analog local-loop signal is converted into a PCM channel first.
Call-progress tones
- Ringback tone confirms that the network is alerting the destination. It is not audio returned from the called telephone ringer.
- Busy tone indicates that the line or required resource is occupied.
- Congestion tone reports that the network cannot allocate a required resource.
- Recorded announcements can provide a more specific failure reason.
Tone frequencies and cadences differ among national plans. Values used by one network should not be presented as universal POTS parameters.
Call release
Returning a handset to its cradle opens the loop and removes current. The exchange detects the transition, stops charging, and starts releasing the path. Behavior after only one party hangs up depends on the switching system and its supervision timers.
No single release delay applies to every exchange. Timing values must be tied to a particular system or operator specification. The stable local-loop event is the transition from off-hook to on-hook.