Main Distribution Frame

Main Distribution Frame or "Main Distributing Frame" - Telephony/Broadband equipment. Termination point within the local Telephone exchange. All exchange equipment and any terminations to the local loop appear at the MDF, i.e. all copper pairs supplying Telephony/Broadband services are terminated here and distributed to their subsequent piece of equipment within the local Exchange e.g. repeaters and DSLAM.

The most common kind of large MDF is a long steel rack accessible from both sides. On one side, termination blocks are arranged horizontally at the front of rack shelves. Jumpers lie on the shelves and go through a 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 requires two workers, one on each side. 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. With discipined administration the MDF can hold over a hundred thousand jumpers, changing dozens of them every day, for decades without tangling.

For the first half of the 20th Century, all MDF jumpers were soldered. This was reliable but slow and expensive. In the 1960s wire wrap ws introduced, and in the 1970s punch blocks.

Some urban MDFs are two stories high so they don't have to be more than a city block long. A few are three stories. By British custom the cables to the outside world are terminated on the horizontal side, and the indoors equipment on the vertical side. American usage is vice versa.

Smaller MDFs, and some modern large ones, are single sided so one worker can install, remove or change a jumper. With less shelf space for jumpers, they are administered so few jumpers need be long.

Sometimes the MDF is combined with other kinds of distribution frame in a CDF