A distribution frame on one part of which the external trunk cables entering a facility terminate, and on another part of which the internal user subscriber lines and trunk cabling to any intermediate distribution frames terminate. Note 1: The MDF is used to cross-connect any outside line with any desired terminal of the multiple cabling or any other outside line. Note 2: The MDF usually holds central office protective devices and functions as a test point between a line and the office. Note 3: The MDF in a private exchange performs functions similar to those performed by the MDF in a central office. Synonym (in telephony) main frame.