Parallel to Serial Conversion
The terms DTE and DCE are relative with respect to what part of a network you are observing. RS-232C is the recommended standard (RS) describing the physical interface and protocol for relatively low-speed, serial data communication between computers and related devices. The EIA originally defined RS-232C for teletypewriter devices. The DTE is the RS-232C interface that a computer uses to exchange data with a modem or other serial device. The DCE is the RS-232C interface that a modem or other serial device uses in exchanging data with the computer.
For instance, your PC typically uses an RS-232C interface to communicate and exchange data with connected serial devices such as a modem. Your PC also has a Universal Asynchronous Receiver/Transmitter (UART) chip on the motherboard. Since the data in your PC flows along parallel circuits, the UART chip converts the groups of bits in parallel to a serial stream of bits. To work faster, a UART chip has buffers so it can cache data coming from the system bus while it processes data going out the serial port. The UART is the DTE agent of your PC and communicates with the modem or other serial device, which, in accordance with the RS-232C standard, has a complementary interface called the DCE interface.