Main distribution frame

A main distribution frame (MDF ), English: Main Distribution Frame in short: MDF, is the central distributor of a communications cabling a building or property. In the area of ​​public telephone network is located in each local exchange of the main distribution of the associated " local access network ".

Main distribution of the telephone network in Germany

Main distribution frame at the center of a connection area (ASB ) of a local network. From the main distribution trunk cables are routed to the individual cable distributors, of which it is the branching cables are routed to the individual households.

At the main distributor of a local exchange, the local loops that lead out to the participants, and linked if necessary with the technical equipment of a network operator (configured). At the MDF, inter alia, the analog (POTS) and digital ports (ISDN) a switch and connecting cable are connected to other network operators.

At the MDF also xDSL connections and leased lines are switched out of the phone lines. In Germany there are about 8000 central offices. Surrounded by a main distribution area is supplied, the connection area.

Each main distribution helps to distinguish the identification number of the connection area (eg 35 for MDF Main Distribution No. 35). In earlier times had all telephone lines that were connected through a main distribution, identical part numbers. For example, all connected terminals of HVt35 had a phone number that started with 35. This was due to the electro-mechanical equipment. With the introduction of digital switching equipment and the associated possibility to port a number across different terminal regions, accounted for this fixed port range allocation on the basis of the number.


The main distribution of a switch consists of an approximately 3 m high free-standing steel frame, are attached to the individual interconnection elements ( " Contact Strip " and " disconnection modules "). He has a " vertical side " and a " horizontal page ". On the vertical side of the splitter bars are mounted vertically, then run cable through the basement division of the exchange. From there pneumatically monitored main cable leading to the street cabinets. On the horizontal side of the contact strips are mounted horizontally, will include technical facilities have been completed. These are the port circuit packs of the exchange, as well as the port of the DSLAM and the connection cables to other system operators.

There are also main distribution frames with wall installation, in which the horizontal side up and the vertical side are arranged below. But these are only very small network nodes that do not require a large amount of space.

In some exchanges also contact strip on the side of the " vertical " strips have been installed. These are mostly fed by network nodes that are outside the exchange. Because they are strictly fed via underground cables, they are consequently to be arranged on the vertical side.

In the 70 years of the 20th century, there were also attempts in Mannheim to construct main distribution frames with vertical sides, this should encourage the use of switching robots, but could not ultimately prevail.

Currently, there are two different forms of the main distribution: MDF MDF 55 and 71 The numbers indicate roughly the formative years (1955 and 1971 ) of the respective components. Both types can also occur on the same MDF as the mechanical construction of the " skeleton " is almost identical and only the switching strip differ. The main differences are:

  • The MDF 71 made ​​possible by a higher packing density of the contacts much more connection possibilities of technology ( horizontal) and main cables ( vertically).
  • While had to be soldered to the circuit strip the wires at the main distribution frame 55, they are connected at the MDF 71 with solder, screw - cutting and insulation displacement terminals ( LSA ) technique.

In addition to the types already mentioned (the Series 55 is not rebuilt ) many other series are offered, for example:

  • Series 5000 ( / HVT 95)
  • LSA ( and type variations)
  • 1000RT Series.

These series are still available on the German market. On the world market there are other products, their distribution in Germany, however, has almost no meaning. The product currently in Germany probably the most frequently used is the MDF 71 In addition to the task of establishing contact between the participants wiring and electrical cables, the main distributor protects often optional pluggable protective tapes to overvoltages and overcurrents (eg by lightning or so-called PowerContact ). The data necessary for the operation of DSL high-speed turnouts can also be accommodated in the main distribution frame (MDF - integrated DSL Soft, English MDF integrated splitter).

Switching connections

To connect any line on the horizontal side with any line on the vertical side is between the two sides with a jumper wire (or jumper wire ) ranks. Each switched line is documented in a database., There are, both on the horizontal and on the vertical side of "Coordinates".

The coordinates are referred to the vertical side of the row, Header, Male Pins, on the horizontal side of the bay, line, switching element and pen.

For example, means an indication of a port (horizontal side) of 501-07-16-3, that this ( the names of the horizontal side are mostly 501 numbers ) about the first bay is, the altitude is 7 (of counted top to bottom), in dash strip is about the 16 switching element, and within this element of the pin 3 is meant. A horizontal strip is in the Rule 5 bays (eg, 501-505 ), but there are also smaller control strips, which assume either only one or one and a half bays. If a strip is a multiple bays, the coordinates refer generally to the starting point, in strips, which each fit into a bay alone, each bay would be specified (eg 503 instead of 501).

There are currently working on solutions designed to automate the manual maneuvering with the help of remote controlled switching matrices. See also: Automatic switching matrix.

Other Features

At the MDF and test equipment ( in Germany System External Testing ( SEPT )) are applied to make both the local loop (ULL ) can be measured and examined, as well as the port of the subscriber on the switch ( for example, power and ring voltage ). In some countries, surge arresters are installed to protect the switching device from overvoltage, which can be supplied via the subscriber line, the main header.