Main distribution frame
Encyclopedia
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 loop
s are connected by jumper wires at the MDF. All cable copper pairs supplying services through 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 terminate at the MDF. 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, termination
blocks are arranged horizontally at the front of rack shelves. Jumpers lie on the shelves and go through a 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 requires two workers, one on each side. 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 the termination block prevents the wires from covering each others' terminals. With disciplined administration the MDF can hold over a hundred thousand jumpers, changing dozens of them every day, for decades without tangling.
For the first half of the 20th Century, all MDF jumpers were solder
ed. This was reliable but slow and expensive. In the 1960s wire wrap
was introduced, and in the 1970s punch block
s.
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 the correct degree of brittleness.
Some urban central office MDFs are two stories high so they don't have to be more than a city block long. A few are three stories. 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.
The adoption of distributed switching
in the late 20th century diminished the need for large, active, central MDFs.
The MDF usually holds central office protective devices including heat coil
s and functions as a test point
between a line and the office.
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.
In order to automate the manual jumpering the Automated Main Distribution Frame
(AMDF)
becomes an important role.
Telephony
In telecommunications, telephony encompasses the general use of equipment to provide communication over distances, specifically by connecting telephones to each other....
, a main distribution frame (MDF or main frame) is a signal distribution frame
Distribution frame
In telecommunications, a distribution frame is a passive device which terminates cables, allowing arbitrary interconnections to be made.For example, the Main Distribution Frame located at a telephone central office terminates the cables leading to subscribers on the one hand, and cables leading to...
for connecting equipment (inside plant
Inside plant
In telecommunication, the term inside plant has the following meanings:*All the cabling and equipment installed in a telecommunications facility, including the main distribution frame and all the equipment extending inward therefrom, such as PABX or central office equipment, MDF heat coil...
) to cables and subscriber carrier
Pair gain
In telephony, pair gain is a method of transmitting multiple POTS signals over the twisted pairs traditionally used for a single traditional subscriber line in telephone systems. Pair gain has the effect of creating additional subscriber lines...
equipment (outside plant
Outside plant
In telecommunication, the term outside plant has the following meanings:*In civilian telecommunications, outside plant refers to all of the physical cabling and supporting infrastructure , and any associated hardware located between a demarcation point in a switching facility and a demarcation...
). The MDF is a termination point within the local telephone exchange
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
where exchange equipment and terminations of local loop
Local loop
In telephony, the local loop is the physical link or circuit that connects from the demarcation point of the customer premises to the edge of the carrier or telecommunications service provider's network...
s are connected by jumper wires at the MDF. All cable copper pairs supplying services through user
User (telecommunications)
In telecommunications, a user is a person, organization, or other entity that employs the services provided by a telecommunication system, or by an information processing system, for transfer of information....
telephone line
Telephone line
A telephone line or telephone circuit is a single-user circuit on a telephone communication system...
s are terminated at the MDF and distributed through the MDF to equipment within the local exchange e.g. repeater
Repeater
A repeater is an electronic device that receives asignal and retransmits it at a higher level and/or higher power, or onto the other side of an obstruction, so that the signal can cover longer distances.-Description:...
s and DSLAM. Cables to intermediate distribution frame
Intermediate distribution frame
Intermediate distribution frame : In a central office or customer premises, a frame that 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 or combined distribution frame to individual...
s terminate at the MDF. Trunk cables may terminate on the same MDF or on a separate trunk main distribution frame (TMDF).
Like other distribution frame
Distribution frame
In telecommunications, a distribution frame is a passive device which terminates cables, allowing arbitrary interconnections to be made.For example, the Main Distribution Frame located at a telephone central office terminates the cables leading to subscribers on the one hand, and cables leading to...
s the MDF provides flexibility in assigning facilities, at lower cost and higher capacity than a patch panel
Patch panel
A patch panel or patch bay is a panel, typically rackmounted, that houses cable connections. One typically shorter patch cable will plug into the front side, whereas the back holds the connection of a much longer and more permanent cable...
.
The most common kind of large MDF is a long steel rack accessible from both sides. On one side, termination
Electrical wiring
Electrical wiring in general refers to insulated conductors used to carry electricity, and associated devices. This article describes general aspects of electrical wiring as used to provide power in buildings and structures, commonly referred to as building wiring. This article is intended to...
blocks are arranged horizontally at the front of rack shelves. Jumpers lie on the shelves and go through a 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 requires two workers, one on each side. 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 the termination block prevents the wires from covering each others' terminals. With disciplined administration the MDF can hold over a hundred thousand jumpers, changing dozens of them every day, for decades without tangling.
For the first half of the 20th Century, all MDF jumpers were solder
Soldering
Soldering is a process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, the filler metal having a lower melting point than the workpiece...
ed. This was reliable but slow and expensive. In the 1960s wire wrap
Wire wrap
Wire wrap is a technology used to assemble electronics. It is a method to construct circuit boards without having to make a printed circuit board. Wires can be wrapped by hand or by machine, and can be hand-modified afterwards. It was popular for large-scale manufacturing in the 60s and early 70s,...
was introduced, and in the 1970s punch block
Punch block
A punch down block is a type of electrical connection often used in telephony. It is named because the solid copper wires are "punched down" into short open-ended slots which are a type of insulation-displacement connectors...
s.
Each jumper is a twisted pair
Twisted pair
Twisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference from external sources; for instance, electromagnetic radiation from unshielded twisted pair cables, and crosstalk between neighboring pairs...
. Middle 20th century jumper wires in the USA were 24 AWG single strand copper, with a soft polyethylene
Polyethylene
Polyethylene or polythene is the most widely used plastic, with an annual production of approximately 80 million metric tons...
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 the correct degree of brittleness.
Some urban central office MDFs are two stories high so they don't have to be more than a city block long. A few are three stories. 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
COSMOS (Telecommunications)
COSMOS was a record-keeping system for Main Distribution Frames in the Bell System. COSMOS was introduced in the 1970s after MDFs were found to be congested in large urban telephone exchanges. It assigns terminals so jumpers need not be so long, thus leaving more space on the shelves...
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.
The adoption of distributed switching
Distributed switching
Distributed switching is an architecture in which multiple processor-controlled switching units are distributed. There is often a hierarchy of switching elements, with a centralized host switch and with remote switches located close to concentrations of users....
in the late 20th century diminished the need for large, active, central MDFs.
The MDF usually holds central office protective devices including heat coil
Heat coil
Heat coils, also known as protectors, bugs or carbons serve as a surge protector between the telephone exchange and outside plant. They are commonly the last point of appearance for a telephone circuit before it leaves the office, for example on the outside plant side of the main distribution frame...
s and functions as a test point
Test point
A test point is a location within an electronic circuit that is used to either monitor the state of the circuitry or to inject test signals. Test points have two primary uses:...
between a line and the office.
Sometimes the MDF is combined with other kinds of distribution frame
Distribution frame
In telecommunications, a distribution frame is a passive device which terminates cables, allowing arbitrary interconnections to be made.For example, the Main Distribution Frame located at a telephone central office terminates the cables leading to subscribers on the one hand, and cables leading to...
in a CDF
Combined distribution frame
In telecommunication, a combined distribution frame is a distribution frame that combines the functions of main and intermediate distribution frames and contains both vertical and horizontal terminating blocks....
The MDF in a private branch exchange performs functions similar to those performed by the MDF in a central office.
In order to automate the manual jumpering the Automated Main Distribution Frame
Automated Main Distribution Frame
Automated Main Distribution Frame , , a technology to provide connectivity between subscriber - and office equipment lines in a telephone exchange main distribution frame .At the beginning of the 21st century the connections...
(AMDF)
becomes an important role.
See also
- Intermediate distribution frameIntermediate distribution frameIntermediate distribution frame : In a central office or customer premises, a frame that 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 or combined distribution frame to individual...
for more about vertical and horizontal main distribution frames. - Distribution frameDistribution frameIn telecommunications, a distribution frame is a passive device which terminates cables, allowing arbitrary interconnections to be made.For example, the Main Distribution Frame located at a telephone central office terminates the cables leading to subscribers on the one hand, and cables leading to...