The switching equipment, common to all the subscribers of the area is located in
an exchange. To make possible for a subscriber to communicate with remaining
subscribers, telephone of each and every subscriber must be connected to the
exchange. The function of MDF is to provide a means for connecting side is
terminated at OCB where the switching take place. From OCB, through PCM
connected to various sections like WLL,TAX etc.
A line from the subscriber’s telephone set involves :
• Subscribers House wiring
• Overhead wires
• Cable Distribution Point
• Underground cables
• Exchange Main Distribution Frame
From the subscriber’s house wiring, the line is brought on overhead wires to a
point called distribution point(DP). From the DP, the pairs are extended to the
exchange through underground distribution cables, secondary cables and primary
cables. At the exchange are brought through underground cables to cable
chamber. In the cable chambers, they are jointed to PVC cables for terminating at
MDF. This frame incorporates protecting devices and provides for a flexible
arrangement for connecting subscriber’s lines to exchange equipments.
SUBSCRIBERS HOUSE WIRING :
PVC aluminium twin wire 1or1.12 mm is used for wiring at subscriber’s house.
Protective devices are not necessary at the subscriber’s premises as per present
standards.
UNDERGROUND CABLES : The underground cables laid at a depth to 2.5 feet
below the ground level connect the DP post to the exchange MDF.The cabinets and
pillars included in the cable network provide flexible arrangement of
interconnection between various sizes of cables.
FUNCTION OF MDF :
• A fixed means of terminating the external cables.
• A means for mounting the protective devices for incoming circuits.
• A convenient point of interception for locating of faults.
• A means for cross connecting the external circuits to the appropriate
internal circuits.
The MDF is properly earthed for the protection of the equipment. The external
pairs are area wise terminated on the line side of the frame, while connection from
the equipment is done on the exchange side in a numerical order. By
interconnections at this frame with the help of jumper wires, any subscriber in any
area can be given any exchange number. This MDF mounts Delay Fuses only.
PROTECTIVE DEVICES :
Comprehensive protection against effects of lightning and power line contacts, is
achieved in practice by fuses, arresters and heat coils.
They are not affected by normal speech and signaling voltage and current but
operates when the foreign voltage or current on the line is excessive. The line is
then disconnected automatically from the equipment or a connection to earth is
For safety precautions fuses are used. Every subscriber line has individual fuse.
These fuses are made of GD (gas discharge) tube, which are connected in parallel.
These fuses have two sides, one is exchange side and other is subscriber side.
Connections between the different tag blocks are made using the jumper wires of
red and white colours.
GD tubes are connected in parallel while electrical fuses are connected in series.
In electrical fuse, when high voltage appears across it, then it will break up the
connection and thus safeguard the system. If GD tube is connected in series, then
due to high voltage across any line, the whole exchange will be disconnected.
Thus when GD tubes are connected in parallel, it will provide the required
facility.
GASE DISCHARGE PROCTECTORS :
In case of heavy lightning discharges, gas discharge protectors are used and more consistent voltage is obtained. The gas discharge protector essentially
consists of two tungsten electrodes sealed in a special glass envelope containing
a mixture of inert gases, mainly neon. One of the electrodes are for connections
to the lines and the other is the earth electrode.
If the potential difference across the electrodes rises to a certain critical value(the
striking value) the gas is ionized and becomes conducting. This condition will
continue till the potential difference across the electrodes falls to the extinction
voltage value. For voltages less than striking value it will not conduct. For normal
operating voltages on the lines, it offers extremely high impedance and thus does
not introduce any transmission loss.
For equipment side:
MDF has many tag blocks of 100 and that numbered from 1 to 100.
In a tag block, there are 128 pair wire theoretically. Therefore, total number of
connecting wires are 1024 theoritically. But in practical, there are only 1000 pairs.
For equipment side:
From ground, a single pair of 400 wires originates, which is divided
in 4 pairs of 100 wires. Practically, each pair is provided with 102 wires. These
exact 2 wires are used in case of any manufacturing defect.
For broadband connections, different coloured tag blocks are provided.
Broadband is used to provide different facilities on land-line phones with high
speed to access them. A grey coloured tag block is used for line side while yellow
coloured tag block is used on equipment side. These tag blocks has 0 to 47 pairs.
In order to know the centre load point.:
Cabinet and pillar are provided with capacity as per requirement. The D.P. box is
provided with 10 or 20 or 5 pairs. Now a days, a D.P. box of 5 pairs is used which
is wall mount instead of being mounted on pole.
To identify any telephone, we require the following two addresses:-
1. Exchange/line address
2. Equipment address
Exchange address is written in given manner:
Vertical number-tag block number-pair number
For example 7-5-15
Here,vertical no.=7
Tag block no.=5
Pair no.=15
This gives the address of a telephone in the exchange.
Equipment address is given as:-
Rack number-tag block number-pair number
For example7-6-87
Here,rack no.=7
Tag block no.=6
Pair no.=87
This gives the information about the actual location of telephone equipment. This
address is provided to lineman for repairing, in case of any fault.
There is an another section of fault repairing located in MDF section. When
subscriber’s phone is not working, then subscriber call to exchange. For this he
dial,198. At other side in exchange anyone on computer feeds subscriber detail
and phone number. Further system is fully computerized. Computers do the
testing of line and gives fault at its output. Then fault is given to lineman of that
area. He checks out the lines of that area and removes the fault. After this,
he informs to the exchange that the line is OK and then the exchange informs the
subscriber at their phone number, that there telephone is working now.
