Data circuit-terminating equipment (DCE) is a device that
connects to the leased line provided by the telco. The term
circuit-terminating refers to the fact that a leased line is
sometimes called a data circuit, or simply a circuit, and the
CSU/DSU sits at the end of the circuit. The DCE also provides
clocking to the data terminal equipment (DTE). The DTE is a device
that needs to send data over a circuit. Routers act as DTEs because
they need to send IP packets, encapsulated in data link frames,
over WAN links.