Single-frequency signaling
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Single-frequency signaling (SF) is signaling (in telephony) in which dial pulses or supervisory signals are conveyed by a single voice-frequency tone in each direction.
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.
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.
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.