Editing Main distribution frame (section)

==Overview==
The MDF is a termination point within the local [[telephone exchange]] where exchange equipment and terminations of [[local loop]]s are connected by jumper wires at the MDF.  All cable copper pairs supplying services through [[User (telecommunications)|user]] [[telephone line]]s are terminated at the MDF and distributed through the MDF to equipment within the local exchange e.g. [[repeater]]s and [[DSLAM]].  Cables to [[intermediate distribution frame]]s (IDF) terminate at the MDF.  [[trunk (telecommunications)|Trunk]] cables may terminate on the same MDF or on a separate trunk main distribution frame (TMDF).

Like other [[distribution frame]]s the MDF provides flexibility in assigning facilities, at lower cost and higher capacity than a [[patch panel]].

The most common kind of large MDF is a long steel rack accessible from both sides. On one side, [[Electrical wiring|termination]] blocks are arranged horizontally at the front of rack shelves.  Jumpers lie on the shelves and go through an insulated steel hoop to run vertically to other termination blocks that are arranged vertically.  There is a hoop or ring at the intersection of each level and each vertical.  Installing a jumper historically required two workers, one on either side of the MDF. The shelves are shallow enough to allow the rings to be within arm's reach, but the workers prefer to hang the jumper on a hook on a pole so their partner can pull it through the ring.  A fanning strip at the back of each termination block prevents the wires from covering each other's terminals.  With disciplined administration, the MDF can hold over a hundred thousand jumpers, with dozens changed every day, for decades without tangling.

The MDF usually holds [[telephone exchange]] protective devices including [[heat coil]]s, and functions as a [[test point]] between a line and the exchange equipment.