Main distribution frame

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

With disciplined administration, the MDF can hold over a hundred thousand jumpers, with dozens changed every day, for decades without tangling.

Middle 20th century jumper wires in the USA were 24 AWG single strand copper, with a soft polyethylene inner jacket and a cotton wrapper, impregnated to make it slightly brittle and easy to remove neatly.

Late 20th century ones had a single, thicker coating of polyethylene cross-linked to provide a suitable degree of brittleness.

Access to the upper levels can be either by a traveling ladder attached to the MDF, or by mezzanine walkways at a suitable height.

Smaller MDFs, and some modern large ones, are single sided so one worker can install, remove or change a jumper.

COSMOS and other computerized Operations Support Systems help by assigning terminals close to one another, so most jumpers need not be long and shelves on either type of MDF do not become congested.

Small, single-sided MDF for a military base, 1940s
Modern main distribution frame
MDF at a central office with capacity for 67,000 users
MDF newly installed in a large urban exchange, early 20th century, no jumpers yet
Obsolete MDF in a cold-war era telephone exchange