The distinct audible tones two fax devices exchange to connect and negotiate, separate from the beeps a phone keypad makes.
- Fax tones are the audible signals two fax devices exchange over a phone line: a calling tone, an answering tone, and the negotiation tones of the handshake.
- The calling tone (CNG) tells the receiving line a fax machine is calling; the answering tone (CED) confirms a fax device picked up.
- Fax tones are distinct from DTMF, the touch-tone signals used to dial the call in the first place.
- The rapid, layered beeping heard during a fax call is the handshake negotiating speed and settings before any page data moves.
What Are Fax Tones?
Fax tones are the audible signals two fax devices exchange over a phone line to identify each other and prepare for a fax transmission. The calling tone (CNG) tells the receiving line that a fax machine is on the other end, and the answering tone (CED) confirms a fax device picked up and is ready to proceed.
The two tones play different roles in that exchange. The calling tone is sent by the device placing the call, repeating at intervals while it dials out so the receiving line has more than one chance to recognize it. The answering tone is the receiving device's reply, sent back once the call connects to confirm that a fax device, not a person or voicemail system, picked up.
Fax Tones vs. Touch-Tone Dialing
It's easy to confuse fax tones with the beeps heard when dialing, but they're separate signals entirely. Touch-tone dialing only gets the call connected over the phone network; fax tones only begin once that connection already exists.
Fax Tones and the Handshake
After CNG and CED, the layered, screeching tones commonly associated with fax calls are the handshake negotiation that follows, agreeing on speed and settings before any page data moves. Any error-correction capability, including ECM, the error-correction mechanism that follows, is negotiated during this same exchange.
A Common Case: Recognizing a Fax Call by Ear
An office worker hears the distinctive screech through a speakerphone and immediately knows it's an incoming fax rather than a stalled call, because that sound is fax tones negotiating a connection, not silence or a dropped line.
FAQs
Why does a fax machine make beeping sounds?
Those sounds are fax tones: a calling tone and answering tone that identify the two devices, followed by the handshake's rapid negotiation tones that agree on speed and settings before any page data moves.
Are fax tones the same as touch-tone dialing sounds?
No. Touch-tone dialing (DTMF) only connects the call itself. Fax tones, like the calling and answering tones, begin only after that connection is already established.
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