single-frequency signaling

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Last updated

July 13, 2026

Quick Summary

Single-frequency signaling is a method used in telecommunications where a single frequency tone is employed to convey information. This technique was historically used in telephone networks to signal the availability of a trunk line between switches.1

In digital communications, a related concept is frequency-shift keying (FSK), where digital data is transmitted by varying the frequency of a carrier wave. In its simplest form, one frequency represents a binary '1' and another frequency represents a binary '0'. This was notably used in early modems to transmit data over telephone lines.2

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telephone
In-band signaling
Single-frequency tones were used in the switching network to signal availability of a trunk. Once a trunk line became available, multiple-frequency tones were used to pass the address information between switches. Multiple-frequency signaling employed pairs of six tones, similar to the signaling used in Touch-Tone…
Read More
Single-frequency tones were used in the switching network to signal availability of a trunk. Once a trunk line became available, multiple-frequency tones were used to pass the address information between switches. Multiple-frequency signaling employed pairs of six tones, similar to the signaling used in Touch-Tone…
Read More