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In telephony, a main distribution frame (MDF or main frame) is a signal distribution frame for connecting equipment (inside plant) to cables and subscriber carrier equipment (outside plant).
The MDF is a termination point within the local telephone exchange where exchange equipment and terminations of local loops are connected by jumper wires at the MDF. All cable copper pairs supplying services through user telephone lines are terminated at the MDF and distributed through the MDF to equipment within the local exchange e.g. repeaters and DSLAM. Cables to intermediate distribution frames (IDF) terminate at the MDF. Trunk cables may terminate on the same MDF or on a separate trunk main distribution frame (TMDF).
Like other distribution frames 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, 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 coils, and functions as a test point between a line and the exchange equipment.
Before 1960, MDF jumpers were generally soldered. This was reliable but slow and expensive. Wire wrap was introduced in the 1960s, and punch blocks in the 1970s. In most exchanges in the UK, soldered blocks are still in use, but are 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 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.
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. 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. [1]
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 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.
In telecommunication, a combined distribution frame (CDF) is a distribution frame that combines the functions of main and intermediate distribution frames and contains both vertical and horizontal terminating blocks.
An intermediate distribution frame (IDF) is a distribution frame in a central office or customer premises, which cross connects the user cable media to individual user line circuits and may serve as a distribution point for multipair cables from the main distribution frame (MDF) or combined distribution frame (CDF) to individual cables connected to equipment in areas remote from these frames.
In telecommunication, the term outside plant has the following meanings:
Components of an electrical circuit are electrically connected if an electric current can run between them through an electrical conductor. An electrical connector is an electromechanical device used to create an electrical connection between parts of an electrical circuit, or between different electrical circuits, thereby joining them into a larger circuit. Most electrical connectors have a gender – i.e. the male component, called a plug, connects to the female component, or socket. The connection may be removable, require a tool for assembly and removal, or serve as a permanent electrical joint between two points. An adapter can be used to join dissimilar connectors.
A digital subscriber line access multiplexer is a network device, often located in telephone exchanges, that connects multiple customer digital subscriber line (DSL) interfaces to a high-speed digital communications channel using multiplexing techniques. Its cable internet (DOCSIS) counterpart is the cable modem termination system.
A registered jack (RJ) is a standardized telecommunication network interface for connecting voice and data equipment to a service provided by a local exchange carrier or long distance carrier. Registered interfaces were first defined in the Universal Service Ordering Code (USOC) system of the Bell System in the United States for complying with the registration program for customer-supplied telephone equipment mandated by the Federal Communications Commission (FCC) in the 1970s. They were subsequently codified in title 47 of the Code of Federal Regulations Part 68. Registered jack connections began to see use after their invention in 1973 by Bell Labs. The specification includes physical construction, wiring, and signal semantics. Accordingly, registered jacks are primarily named by the letters RJ, followed by two digits that express the type. Additional letter suffixes indicate minor variations. For example, RJ11, RJ14, and RJ25 are the most commonly used interfaces for telephone connections for one-, two-, and three-line service, respectively. Although these standards are legal definitions in the United States, some interfaces are used worldwide.
In telecommunications, a distribution frame is a passive device which terminates cables, allowing arbitrary interconnections to be made.
An access network is a type of telecommunications network which connects subscribers to their immediate service provider. It is contrasted with the core network, which connects local providers to one another. The access network may be further divided between feeder plant or distribution network, and drop plant or edge network.
In telecommunications, structured cabling is building or campus cabling infrastructure that consists of a number of standardized smaller elements called subsystems. Structured cabling components include twisted pair and optical cabling, patch panels and patch cables.
A 66 block is a type of punch-down block used to connect sets of wires in a telephone system. They have been manufactured in four common configurations, A, B, E and M. A and B styles have the clip rows on 0.25" centers while E and M have the clip rows on 0.20" centers. The A blocks have 25 slotted holes on the left side for position the incoming building cable with a 50 slot fanning strip on the right side for distribution cables. They have been obsolete for many years. The B & M styles have 50 slot fanning strip on both sides. The B style is used mainly in distribution panels where several destinations need to connect to the same source. The M blocks are often used to connect a single instrument to such a distribution block. The E style has 5 columns of 10 2 clips rows and are used for transitioning from the 25 pair distribution cable to a 25 pair RJ21 style female ribbon connector.
Verizon New York, Inc., formerly The New York Telephone Company (NYTel), was organized in 1896, taking over the New York City operations of the American Bell Telephone Company.
The serving area interface or service area interface (SAI) is an outdoor enclosure or metal box that allows access to telecommunications wiring.
COSMOS was a record-keeping system for main distribution frames (MDFs) in the Bell System, the American Bell Telephone Company and then, subsequently, AT&T–led system which provided telephone services to much of the United States and Canada from 1977 to 1984.
The Number Five Crossbar Switching System is a telephone switch for telephone exchanges designed by Bell Labs and manufactured by Western Electric starting in 1947. It was used in the Bell System principally as a Class 5 telephone switch in the public switched telephone network (PSTN) until the early 1990s, when it was replaced with electronic switching systems. Variants were used as combined Class 4 and Class 5 systems in rural areas, and as a TWX switch.
The Number One Crossbar Switching System (1XB), was the primary technology for urban telephone exchanges served by the Bell System in the mid-20th century. Its switch fabric used the electromechanical crossbar switch to implement the topology of the panel switching system of the 1920s. The first No. 1 Crossbar was installed in the PResident-2 central office at Troy Avenue in Brooklyn, New York which became operational in February 1938.
Automated main distribution frame (AMDF),, a technology to provide connectivity between subscriber – and office equipment lines in a telephone exchange main distribution frame (MDF).
Tip and ring are the two conductors or sides of a telephone line. Their names are derived from the telephone plugs used for connecting telephone calls in manual switchboards. One side of the line is connected to the metal tip of the plug, and the second is connected to a metal ring behind the tip, separated and insulated from the tip by a non-conducting material. When inserted into a jack, the plug's tip conductor connects first, followed by the ring conductor. In many European countries, tip and ring are referred to as the A and B wires.
A telephone exchange, telephone switch, or central office is a telecommunications system used in the public switched telephone network (PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital systems to establish telephone calls between subscribers.
ANSI/TIA-568 is a technical standard for commercial building cabling for telecommunications products and services. The title of the standard is Commercial Building Telecommunications Cabling Standard and is published by the Telecommunications Industry Association (TIA), a body accredited by the American National Standards Institute (ANSI).
The Panel Machine Switching System is a type of automatic telephone exchange for urban service that was used in the Bell System in the United States for seven decades. The first semi-mechanical types of this design were installed in 1915 in Newark, New Jersey, and the last were retired in the same city in 1983.
This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 2022-01-22. (in support of MIL-STD-188).