Main Distribution Frame MDF|Basics of Telephone Exchange- Main Distribution Frame MDF

 Main Distribution Frame MDF

Basics of Telephone Exchange:

Telephone Exchange is a place where switching between two subscribers is done through

either manually or electronically. 

In addition to switching, signaling and controlling are also done at the exchange.

It consists of the following functional blocks:

a) Main Distribution Frame with protective devices.

b) Card Frame.

c) Motherboard.

d) Power supply panel with protective devices.

Read Also : MDF and IDF with protection Devices

a) Main Distribution Frame (MDF): 

In a Telephone exchange different subscribers from

different places are terminated on a frame called “Main Distribution Frame” (MDF) in the

exchange and from there they are extended to subscriber’s line cards/Trunk cards kept in the

exchange rack. 

Protective devices are located in the MDF.


1 Rear of Media Gateway
2 Main Distribution Frame (MDF)
3 Port Distribution Field (Purple Field)
4 Station Distribution Field (White Field)
5 Trunk/Auxiliary Field
6 Purple Field
7 Yellow Field
8 Green Field
9 Satellite Closet
10 Auxiliary Cabinet (Yellow Field)
11 White Field
12 Blue Field
13 Cross-Connect Jumpers
14 103A or Modular Wall Jack
15 4-Pair Line Cord
16 To Line Circuit Pack
17 To Trunk Circuit Pack
18 To Network Interface

Purpose of MDF:

There are three purposes of MDF:

1) It is the place where both outdoor and indoor cables are terminated

2) The cross-connection between the two cables conductors is done on the MDF and this

is done by means of jumper wires (Red & White).

3) It carries all the protective devices used in the exchange.

 They are Fuses, Heat coils & Lightning protectors.

4) The MDF is the most suitable place for testing purposes.

b) Card Frame: It contains different slots in which the nominated cards are to be inserted. It is different in different types of exchanges.

c) Motherboard: It provides connectivity between different cards. It is a multilayer PCB.

d) Power supply panel:

It provides a power supply to different cards in the exchange at different low D.C. voltages. It also includes protective devices like fuses etc.

Main functional areas in Telephone Exchange:

a) Switching Function: The switching functions are carried out through the switching

network, which provides a temporary path for simultaneous, bi-directional speech between

the following:- 

 Two subscribers connected to the same exchange. This is called as “Local switching”

 Two subscribers connected to different exchanges. This is known as “Trunk


 Pairs of trunks towards different exchanges. This is known as “Transit switching”

b) Signalling function: The signaling function enables various equipment in a network to

communicate with each other in order to establish and supervise the call. 

It is of two types,

i) Subscriber line signaling: It enables the exchange to identify calling subscriber’s line,

extend dial tone, receive the dialed digits, extend the ringing voltage to the called

subscriber, extend the ring back tone to the calling subscriber to indicate the called

subscriber is being connected. In the event the called subscriber is busy, an engaged tone is

sent to the calling subscriber.

ii) Interexchange signaling: It enables a call to be set up, supervised, and cleared between


c) Controlling function: The controlling function performs the task of processing the

signaling information and controlling the operation of the switching network.

The control functions may be,

i) Wired logic control: In this pre-wiring is done between different speech path devices and

common control. 

If any changes are required in facilities of subscribers or introduction of

new services require wiring changes.

ii) Stored Program Control (SPC): After the introduction of the microprocessor, a stored program control system came into use. 

In this system, the establishment and supervision of the connections in the exchange is under the control of the “Microprocessor”, which is suitably programmed.

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