A facsimile (shortened to "fax") transmission is direct, machine-to-machine communication between two fax lines (physical or virtual) for the purpose of delivering a document. The communication itself is, in essence, a "conversation" composed of high-frequency noise, made over the medium of a phone call.
Each page of the document is converted into a set of pixels that can either be full (black) or empty (the printed page's colour, typically white).
The transmission is one way; one machine sends the document while the other receives it. If the receiver wishes to send a document back to the sender, this cannot be done during the same transmission; the receiver must establish a new transmission to the sending fax machine (in which the roles of "sending fax machine" and "receiving fax machine" are reversed).
The sending and receiving fax messages manage a fax transmission in five phases:
Phase A: Establishing the call.
Phase B: Negotiating the connection.
Phase C: Sending the pages.
Phase D: Post-send confirmations.
Phase E: End of call.
In a single page sending with no errors, the fax call will flow through each phase once. In multi-page sending, the sending fax machine will repeat phases C and D for each page sent; if the receiving fax machine identifies an error in a received page, the fax machines will go back to phase B to eliminate the error and potentially repeat Phases C and D if the errors on a particular page are deemed to be too severe.