Single-frequency signaling

Note 1: An SF signaling unit converts E & M signaling to a format (characterized by the presence or absence of a single voice-frequency tone), which is suitable for transmission over an ac path, e.g. , a carrier system. The SF tone is present in the idle state[?] and absent during the seized state. In the seized state, dial pulses are conveyed by bursts of SF tone, corresponding to the interruptions in dc continuity created by a rotary dial or other dc dialing mechanism.

Note 2: The SF tone may occupy a small portion of the user data channel spectrum, e.g., 1600 Hz or 2600 Hz ("in-band" SF signaling), usually with a notch[?] filter at the precise SF frequency, to prevent the user from inadvertently disconnecting a call if user data has a sufficiently strong spectral content at the SF frequency. The SF tone may also be just outside the user voice band, e.g., 3600 Hz.

Note 3: The Defense Data Network[?] (DDN) transmits dc signaling pulses or supervisory signals[?], or both, over carrier channels or cable pairs on a 4-wire basis using a 2600-Hz signal tone. The conversion into tones, or vice versa, is done by SF signal units.

Source: from Federal Standard 1037C and from MIL-STD-188