Main Distribution Frame (MDF)

An MDF can provide flexibility in assigning telecommunications facilities at a lower cost and higher capacity than a patch panel.

The most common kind of MDF is a long steel rack that is accessible from both sides. Termination blocks are arranged horizontally on one side at the front of the rack shelves. The jumpers lie on the shelf and move through a steel hoop in order to run through vertically arranged termination blocks.

A typical MDF can hold hundreds of thousands of jumpers and dozens of them can be changed every day for decades without tangling when administered by experienced professionals. Jumpers are twisted pairs of cable, each one corresponding to an individual telephone line.

MDFs are single-sided so that the workers can install, remove or change jumpers. However,
old manual jumpering systems are now mostly automated using automated main distribution frames. Computer systems that control MDF operations assign terminals close to one another so that jumpers need not be long and shelves are not congested with wires as the jumpers are shorter.

MDFs in private branch exchanges perform functions that are similar to those performed by those in central offices but on a smaller scale.