Main distribution frame

In telecommunication, a main distribution frame (MDF) is 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.

The MDF is used to cross-connect any outside line with any desired terminal of the multiple cabling or any other outside line.

The MDF usually holds central office protective devices and functions as a test point between a line and the office.

The MDF in a private exchange performs functions similar to those performed by the MDF in a central office. Synonym (in telephony) main frame.

Source: from Federal Standard 1037C and from MIL-STD-188