Mark and space are terms used in telecommunications to describe two different signal states of a communications signal, generally at the physical layer of a communications system. The terms derive from the early days of the electric telegraph system, where the marking state would cause a mark to be output on paper, and the spacing state would create no mark.
The terms would continue to be used in systems such as RS-232, with similar conventions, that "mark" would be encoded by a negative voltage (or current flow), and "space" by a positive voltage (or no current flow).[2] In such systems, the line is typically left in the "mark" state when idle.[3]
"Mark" is generally identified with the binary digit "1" and "space" with the binary digit "0".[2]
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