Telecommunications Fundamentals, Chapter 2A: The Local Loop

MDF Functions

Main Distributing Frames perform 7 functions.

1. Termination of Facilities and Equipment.

2. Cross-Connection.

3. Flexibility.

4. Technological Transition.

5. Separation.

6. Test Access.

7. Administration.

One More Function of MDF

In addition to the seven basic functions, Main Distributing Frames perform one more function:

Electrical Protection

Electrical protection devices on the MDF prevent surges of voltage on OSP cable from entering the central office and endangering equipment and per sonnel. These devices are referred to as heat coils. MDF Physical Characteristics (Figure 2.8) The conventional MDF is a double-sided steel structure composed of a vertical and horizontal side. Height Length

• Because of their heights, lengths, and shapes, MDFs can be unwieldy to work on and tend to be labor intensive.
• As mentioned earlier, outside plant cable terminates on protector blocks mounted on the vertical side where electrical protection is provided.
• Cable from central office equipment (switch) is terminated on equipment blocks usually mounted on the horizontal side.

The blocks on this side of the frame also terminate 100 lines that are sepa rately identified by office equipment location. Thus, the cable pair from the local loop is permanently terminated to the vertical side of the conventional MDF. Cable from the horizontal side of the MDF is permanently connected to the office equipment/switch. Only the jumper wire which connects the vertical side to the horizontal side of the MDF is connected when the customer's line is put in service.

Modular Frames

As our network grew, so did the problem of jumper congestion on conven tional frames. Increased capacity of new switches required more termina tions on the frame. As frames exhausted their capacity, they were augmented, and sometimes in awkward ways because adequate floor space was not available. In some cases, room additions were built to accommodate frame growth. Modular frames were an important turning point in the evolution of cross connect systems because they provided:

1. Front facing one side operations

2. Higher denisity per foot of frame space

Modular Frame Physical Characteristics

(Figure 2.9) Modular frames are usually single-sided and about 7 feet high. The frames have horizontal bars for mounting connector blocks that terminate office equipment and outside plant cable pairs. Terminations are normally restricted to subscriber cable, line equipment,, and tie cable. Nonswitched special services such as trunk cable, T-carrier, and Metallic Facility Terminals (MFT) are terminated on a separate frame. A separate protector frame is required for electrical protection.

Note: The COSMOS program keeps track jumpers and the inventory of COSMOS is known as COSMIC.

Protector Frame

Because heat coil protectors take a lot of space on the frame and defeat the purpose of high-density termination, a separate protector frame evolved. Note: When space is limited, single-sided protector frames are available for wall mounting. Riser cable is:

1. First terminated on a protector block;

2. Then coupled to the modular frame.

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