Editing Main distribution frame (section)

== History ==
[[File:American telephone practice (1905) (14733380746).jpg|thumb|MDF newly installed in a large urban exchange, early 20th century, no jumpers yet]]
Before 1960, MDF jumpers were generally [[soldering|solder]]ed.  This was reliable but slow and expensive.  [[Wire wrap]] was introduced in the 1960s, and [[punch block]]s in the 1970s. In the early 21st century most exchanges in the UK, still used soldered blocks but were being slowly phased out.

Each jumper is a [[twisted pair]].  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 [[Cross-linked polyethylene|polyethylene cross-linked]] to provide a suitable degree of brittleness.

Some urban telephone exchange MDFs are two stories high so they do not have to be more than a city block long.  A few are three stories. Access to the upper levels can be either by a traveling ladder attached to the MDF, or by mezzanine walkways at a suitable height. 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 the opposite.
[[File:Kingsway tunnels -JPP 7820 (14436555228).jpg|thumb|Obsolete MDF in a cold-war era telephone exchange]]
Smaller MDFs, and some modern large ones, are single sided so one worker can install, remove or change a jumper.  [[COSMOS (Telecommunications)|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.  This database keeps track of all terminals and jumpers.  In the early and middle 20th century these records were kept as pencil entries in ledger books.  The later database method saves much labor by permitting old jumpers to be reused for new lines.<ref>{{cite web|title=MDF Jumpering|url=https://mrtelco.com/}}</ref>

The adoption of [[distributed switching]] in the late 20th century diminished the need for large, active, central MDFs.

Sometimes the MDF is combined with other kinds of [[distribution frame]] in a [[Combined distribution frame|CDF]].

The MDF in a [[private branch exchange]] performs functions similar to those performed by the MDF in a central office.

[[Automated Main Distribution Frame]] (AMDF) has been a subject of experiments.