Editing Main distribution frame

{{Short description|Distribution frame where cables are cross-connected in telephony}}
{{more footnotes|date=June 2015}}
[[File:"Marine Cpl. Robert L. Hardin...checks the main distributing frame in Montford Point's headquarters for line difficultie - NARA - 532515.jpg|thumb|Small, single-sided MDF for a military base, 1940s]]
[[File:Jarretiere telephonie.jpg|thumb|Modern main distribution frame]]
[[File:Gigantic main distribution frame MDF at a central office.jpg|thumb|MDF at a central office with capacity for 67,000 users]]
In [[telephony]], a '''main distribution frame''' ('''MDF''' or '''main frame''') is a signal [[distribution frame]] for connecting equipment ([[inside plant]]) to cables and [[pair gain|subscriber carrier]] equipment ([[outside plant]]).

==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.

== 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.

==See also==
*[[Distribution frame]]
*[[Intermediate distribution frame]] for more about vertical and horizontal main distribution frames
*[[Mainframe computer]]

==References==
{{Reflist}}
* {{FS1037C MS188}}

[[Category:Telephony equipment]]
[[Category:Telephone exchanges]]