The T.30 negotiation two fax devices complete at call setup, before a single page of data moves.
- A fax handshake is the T.30 negotiation two fax devices complete right after a call connects, before any page data moves.
- During the handshake, both sides identify themselves, agree on the fastest shared speed and resolution, and confirm they're ready to exchange a document.
- The handshake is a different signal entirely from DTMF tone dialing, which only handles getting the call connected in the first place.
- A handshake failure shows up as a distinct fax negotiation error, not the same thing as a busy line or an unanswered call.
What Is a Fax Handshake?
A handshake is the negotiation two fax devices complete immediately after a call connects and before a single page of data moves. Defined by the T.30 recommendation itself, it's how a sending and receiving device identify each other, exchange each device's CSID and TSID, and agree on the fastest transmission speed, resolution, and error-handling method both sides actually support. The specific tones exchanged during this negotiation are covered in more detail on their own.
Handshake vs. Dialing
It's easy to conflate the handshake with the tones used to dial the call in the first place, but they're entirely separate signals. DTMF tone dialing only gets the call connected over the phone network; the handshake begins only once that connection already exists, and uses a completely different set of tones to negotiate the transmission itself.
What the Handshake Decides
The outcome of the handshake determines which fax group and speed both sides support, and it's why two devices from different manufacturers and different eras can still exchange a fax successfully: as long as both complete the same handshake procedure, the rest of the transmission follows automatically. A handshake that fails to complete shows up as a distinct negotiation error, not the same thing as a busy line or an unanswered call.
Training and Falling Back to a Slower Speed
Agreeing on a speed during the handshake doesn't guarantee the line can actually sustain it once data starts moving. Fax devices run a short training sequence to test whether the connection holds up at the speed they just negotiated, and if it doesn't, they can retrain to a slower, more conservative speed instead of failing the call outright. On a line that stays unstable, this fallback can happen more than once during the same transmission, trading speed for a completed fax rather than no fax at all.
A Common Case: A Fax Connecting to an Older, Slower Machine
A logistics coordinator sending an urgent shipping document to a client's decade-old fax machine notices the transmission takes a bit longer than usual. That's the handshake in action: the two devices negotiate down to the older machine's maximum supported speed before a single page transfers, rather than failing to connect at all.
FAQs
How long does a fax handshake take?
Typically just a few seconds. It happens automatically right after the call connects and finishes before the first page of data is sent.
Is the fax handshake the same as the tones used to dial the call?
No. Dialing tones only connect the call over the phone network. The handshake begins only after that connection exists and uses a separate set of tones to negotiate the transmission itself.
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