A Main Distribution Frame (MDF) is a signal distribution frame in a central office or customer premises that connects external telecommunications lines to internal lines through a series of interchangeable components. Typically, it. The main distribution frame MDF and intermediate distribution frame IDF serve as critical junctions in network infrastructure, each fulfilling distinct yet complementary roles within an organization's connectivity architecture. ) (H by W by D) and are capable of holding 648 fibers. Think of it as the main “junction box” for your structured cabling system.