Precise Tone Plan
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The Precise Tone Plan is a signaling specification for the public switched telephone network (PSTN) in North America. It defines the call-progress tones used for indicating the status and progress of telephone calls to subscribers and operators.
The tones are as follows:
- Dial tone is a continuous tone of a combination of the frequencies 350 and 440 Hz at a level of −13 dBm
- Ringback tone is defined as comprising frequencies of 440 and 480 Hz at a level of −19 dBm and a cadence of 2 seconds ON and 4 seconds OFF
- Busy tone is defined as having frequency components of 480 and 620 Hz at a level of −24 dBm and a cadence of half a second ON and half a second OFF
- Reorder tone, also often called fast busy tone, contains the same frequency components as busy tone, but with a cadence of 0.25 of a second ON and 0.25 of a second OFF; the original plan had two slightly different versions of the tone, with a cadence of 0.2 of a second ON and 0.3 of a second OFF to signal toll-circuit congestion and a cadence of 0.3 of second ON and 0.2 of a second OFF for local reorder.
Prior to the Precise Tone Plan, parts of the Bell System and various switching systems used slightly varying signal frequencies and levels. The standardization process began with the installation of the first electronic switching system, a Western Electric 1ESS at Succasunna, NJ in 1965.
- AT&T, Notes on Distance Dialing (1968).
- Bell Telephone Magazine, 41(2) p.63 (1968)