Single-frequency signaling

In telephony, single-frequency signaling (SF) is line signaling in which dial pulses or supervisory signals are conveyed by a single voice-frequency tone in each direction. SF and similar systems were used in 20th-century carrier systems. The Defense Data Network (DDN) transmitted DC line signaling pulses or supervisory signals, or both, over carrier channels or cable pairs on a four-wire circuit basis using a 2600 Hz signal tone. The conversion into tones, or vice versa, is done by SF signal units.

Single-frequency signaling

In telephony, single-frequency signaling (SF) is line signaling in which dial pulses or supervisory signals are conveyed by a single voice-frequency tone in each direction. SF and similar systems were used in 20th-century carrier systems. The Defense Data Network (DDN) transmitted DC line signaling pulses or supervisory signals, or both, over carrier channels or cable pairs on a four-wire circuit basis using a 2600 Hz signal tone. The conversion into tones, or vice versa, is done by SF signal units.