A downstream external PHY interface (DEPI) interface apparatus for a modular-cable modem termination system (M-CMTS) cable system complying with a data over cable service interface specifications (DOCSIS) standard is provided. The DEPI interface apparatus for an M-CMTS cable system complying with the DOCSIS standard includes: an Ethernet interface transmitting Ethernet data packets to and receiving Ethernet data packets from a service providing device including at least one of an M-CMTS core, a video server, and a video encoder; an Ethernet packet analyzer analyzing Ethernet data packets, thereby extracting a control message and classifying the Ethernet data packets into corresponding sessions; a DEPI controller interpreting the extracted control message and completing control connection signaling and session signaling with the service providing apparatus; a transmission data processor extracting transmission data from the Ethernet data packets classified by session and outputting the transmission data as an MPEG-2 transport stream (TS) packet stream; and an output interface outputting the output MPEG-2 TS packet stream to a quadrature amplitude modulation (QAM) modulator of a channel corresponding to the stream.