Complex sentence: The RS-232 standard is asymmetrical as to the definitions of the two ends of the communications link so it assumes that one end is a DTE and the other is a DCE e.g. a modem.
Split into simpler sentences: The RS-232 standard is asymmetrical as to the definitions of the two ends of the communications link. It assumes that one end is a DTE and the other is a DCE, e.g. a modem.