Synchronous: All clocks are traceable to one Stratum 1 primary reference clock (PRC). All digital transitions in the signals occur at exactly the same rate.

Plesiochronous: Clocks are extremely accurate and almost exact, but a small difference exists between them. An example is a connection between two different carriers, each using different Stratum 1 clock sources.

Asynchronous: The clocks don't have to match or be equal.