SF (Single Frequency) signaling systems are designed for transport over all 4-wire analog systems, and where metallic interconnection is not possible (e.g. Wideband FDM microwave systems). SF signaling is a type of 'Inband' signaling scheme, where all information is transmitted in the voice band.
In these SF systems, a frequency of 2600 Hz or 2280 Hz is used for signaling. When in an on-hook state, a 2600 Hz (US) or 2280 Hz (UK) tone is transmitted to the remote site. When in an off-hook state, the 2600/2280 Hz tone is dropped. The 2280 Hz SF signaling is used in British Telecom's SSAC15 signaling specification.
These tones are transmitted at either HIGH or LOW ranges, although the Low Level tones (-20 dBM0) are typically implemented:
Note that there are some 2-wire SF systems that typically operate using different transmit and receive frequencies. Also, SF units are available for FXS (Ground/Loop Start), FXO, and E&M signaling systems.
SF units can pass dial pulse address signaling at speeds from 8 to 12 PPS with 56 to 69 percent break.
The receiver can detect the SF tone in the range of -1 to -31 dBm0.