MDF – Main Distribution Frame in an EPABX


Architecture diagram for MDF - Main Distribution Frame

A Main Distribution Frame (MDF) is a very important component in an EPABX / Enterprise Telephony setup. The above diagram represents the connectivity architecture diagram for an MDF.

A Main Distribution Frame (MDF) is a m x n matrix like box where all the cables in an EPABX set-up are terminated. So, the cables from the EPABX – Extension Cards, Trunk Cards, etc & the cables from the trunk lines (from telephony service providers) are terminated on the EPABX side MDF. The cables from the field (individual analog/ digital phones) are connected to the Field side MDF, through a Junction (DB) Box. A Junction box is like a Mini-MDF which is placed in individual departments where all the single pair cables from the analog phones terminate.

When you look at the set up in the above diagram, it should be evident that analog phones are not directly connected to the EPABX Analog/ Digital Cards, though they can be terminated that way). This is done for two main reasons – Consolidating individual telephone cables into bulk telephone cables (like 5-pair cables, 10-pair cables, etc) that can be carried over longer distances easily & To make the testing process easier.

In the above diagram, single pair cables from the individual telephones in each department are terminated on the Junction (DB) box which consolidates all the telephone connections from a single department (for example). From the Junction box, bulk cables like 5-Pair cables, 10-Pair cables, 20-Pair cables, 50-Pair cables, etc. are used to connect the field side MDF with the junction boxes. There are two advantages of doing this – The cables can be carried safely within an armored enclosure, either underground or through ducts (and) These specialty bulk cables are more compact as they save a lot of space, when compared to carrying loosely arranged individual RJ-11 single pair cables as a bunch.

In the above diagram, you would have also noticed that there are two MDF’s – The EPABX side MDF and the Field side MDF. One MDF could have been enough, but a two MDF set up makes it easier to test if the faults are on the EPABX side or on the field cabling side. One can check for the connections from the EPABX cables (on the EPABX side MDF) or one can also check for connections from the field cables (on the Field side MDF), to verify where exactly the fault lies.

Having two MDF’s is also important for identifying which connection goes where. For example, on the EPABX side MDF, the cables could be arranged horizontally and each point could be marked with the Analog/Digital card it is connected to. So, they can be marked as Analog Extension Card 1 – 1, 2, 3, etc. On the Field MDF, the cables could be arranged vertically and each point could be marked with the Department/ Phone it is connected to. So, they can be marked as Dept 1 – 1, 2, 3, etc. This enables an administrator to easily make adds, moves and changes in the telephone network in future and also to easily identify which cables connect where.

The MDF itself is just a frame and the cables connect to Krone Modules (on both sides). So, the cables from the EPABX are terminated behind the Krone modules in the EPABX side MDF and their front side is patched with corresponding Krone Modules in the Field side MDF. The bulk cables (10-pair cables, etc) are terminated on the Krone modules behind the field side MDF, which completes the whole set-up. The front/back sides mentioned here are interchangeable.

Generally, surge-protectors/ lightning arrestors are kept over each line coming from the field side cables to protect the EPABX cards from high-voltage or lightning that may be carried over to them. These are placed over individual Krone modules.

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