The main distribution frame, referred to as MDF, is used to connect subscriber lines, trunk lines, and dedicated lines to the corresponding interface points of the telephone exchange for testing, wiring, and protection of internal equipment.

The structure of MDF is divided into two sides, in-line and in-line. In-line can be installed and guaranteed, and the test row or wiring is installed in horizontal. For in-line trunk lines or dedicated lines that do not need protection, test racks or wiring bars can also be installed. The main distribution frame of the same model can be spliced to meet the needs of expansion. The main distribution frame of the subscriber exchange adopts the box type because of its small capacity. In the early days, the total distribution frame was large, with a maximum capacity of 303 loops per line. The security guard adopts 20-wire or 21-wire carbon lightning arrester and uses a hot coil to limit the current. The test row is 20 lines and adopts a four-line shrapnel type, and the lead-in and lead-out lines can be separated by separating the shrapnel. The new type main distribution frame is matched with the program-controlled switchboard to greatly reduce the size and weight, and each in-line can accommodate 1,000 lines. Ensure to arrange 100 loops per block, and use metal discharge tubes to prevent high voltage and overcurrent. Each test row has 128 lines, and the contact reeds can be separated, so that the lines can be easily separated for testing. The wires connected to the terminals have their own independent wire troughs, which are easy to install and maintain. The guarantee arrangement adopts the card connection method to connect, and the insulation layer of the wire does not need to be stripped when the card is connected. The test row adopts winding method for wiring. Both of these two wiring methods are simple, reliable, fast, and non-polluting. According to different occasions, the new type of distribution frame has wall-mounted, floor-standing, single-sided and double-sided types.

The main functions of the main distribution frame are: ①Security function. It protects the subscriber line against high voltage or high current; ②wiring. Any user can choose the number in the office, and the trunk lines between different offices can choose to occupy the trunk modules in the office, and can connect the inter-office and related subscriber lines according to the needs of the dedicated line.

Intermediate distribution frame, referred to as IDF, is a distribution frame that connects the incoming and outgoing lines between the internal telephone exchanges. According to its installation location, it can be divided into two types: the main distribution frame and the intermediate distribution frame of the switching equipment and the inter-machine distribution frame.

The structure of IDF is also divided into two sides, in-line and horizontal, in-line connection of the equipment's outgoing line, and horizontal connection of the equipment's incoming line. Both in-line and horizontal lines are equipped with wiring strips. One side of the improved terminal block is winding. The other side is the wire bonding. The function of IDF is mainly to deploy the access lines between all levels of switching equipment in order to give full play to the role of all levels of switching equipment.