Serial Ports

Ports which communicate the information in a serial basis; i.e. one bit at a time. Examples include:

One-bit-at-a-time (transmits either "0 bit" or "1 bit" signal at any instant):

Only one or 2 wires (definitely not parallel, but not one-bit-at-a-time either):

Constellation diagrams show all the possible symbols that may be transmitted at any instant, using phase-amplitude modulation. The first group has exactly 2 symbols in their constellation diagram ("2-level modulation"), so they transmit one bit at a time. The second group has more than 2 symbols, so at any instant, they are transmitting one "symbol" that conveys more than one bit of information.
Doesn't 100 Base-T Ethernet also combine bits in a more complex manner? (using phase and amplitude changes, just as are used by modern modems)

Yes, but not quite as complicated.
See also:

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