WEEK 6 DAY 1 : INTRODUCTION TO TELEPHONE EXCHANGE

6.1 INTRODUCTION TO TELEPHONE EXCHANGE

        Exchange is basically switched telephone connection between different office and phones usually used in large offices to facilitate communication between various employees.

6.1.1 THE BASICS

        Let's get a general overview of "How exchange works??". Given Below is the general overview of an office with exchange.

Overview of office with exchange

        Let us assume we have 8 offices which are assigned exchange codes as shown above and an MTNL/BSNL number. All the phone are connected to a single control unit called exchange. The codes assigned to the offices are programmed in it. Now let's assume you are in Office 1 and want to ask your subordinate at Office 8 for bringing some file. You will simply take your phone and dial 24 (Code for office 8). Now the exchange will look up in its database code 24 and immediately know that you want to talk to office 8 and will connect the line.

        Again for example you want to call your home and want to inform your mother that you are going to be late today. You will simply dial 9 in your phone and and the exchange will connect you to MTNL/BSNL exchange and you can dial the number to your home and proceed to talk to her (Subject to condition she picks up the phone)

6.1.2 THE TELEPHONE EXCHANGE

        There are three types of telephone exchanges that we were there at the exchange

1) Omni 230 16 circuit

    It is the ancient of them all. It was procured in 1980s and the primary instrument we were taught on. It was about the size of an full size cupboard/almirah and can handle 5000 telephone lines at once.

2) Coral ISBF 24 circuit

    It was procured in 2003 and was about the size of 1/4th of an almirah. It can also handle about 5000 line at once but the size was much less. They were stacked one over the another. That means for the same space on Omni 230 16 circuit it can handle 20000 lines!

3) Nec 32 circuit     
     It was procured in 2007 and was about the size of 1/8th of an almirah. It can also handle about 5000 line at once. That means for the same space on Omni 230 16 circuit it can handle 40000 lines.

Basic working of Omni 230 16 circuit

Basic Block Diagram of OMNI 230 16 circuit

        The whole exchange consists of 8 compartments, These compartments can be divided into two parts

1) CEM (Control Equipment Module) :- They are two in number. They are the brain of the exchange and consists of all programming

2) PEM (Peripheral Equipment Module) :- It consists of different kinds of chips called cards from where the external devices like phine are connected to it.

The exchange works on PCM/TDM i.e. Pulse Code Modulation/Time Division Multiplexing. 

Input Power Supply

        It can run on 230 V AC supply. Besides it can also run on 48 volt battery. The curious thing here is that the polarity of battery is reversed i.e ground is connected to +ve of battery and +ve is connected to -ve side of battery. There are two reasons for this arrangement.

1) It increases the life of battery as electrons are excessive in -ve side of battery.

2) It protects the equipment in case of lightening.

EVERY TELEPHONE EQUIPMENT USES THE SAME ARRANGEMENT FOR SAFETY.

    The exchange uses Switched Mode power supply (SMPS) which divides the voltage according to need of individual equipments in the exchange. It basically changes AC to DC then again to AC step it down and then again to DC. It then uses voltage divider to get required voltage.

6.1.3 CONTROL EQUIPMENT MODULE (CEM)

        Control Equipment Module (CEM) is analogous to brain of the exchange. It has to be programmed to work . It stores as well as executes the programming to ensure that the telephone connection works properly.

        There are two CEM in OMNI 230. One acts as a standby in case of failure of primary one. Each consists of all basic things a computer has like RAM, ROM, processor and even hard disk separately. The programming is generally done on the primary one but automatically gets copied to the backup unit

6.1.3 PERIPHERAL EQUIPMENT MODULE (PEM)

        The rest of the slots are basically peripheries attached to the CEM which helps it to interact with outside world. These peripheries are basically huge chips called cards which are used for different applications like

1) Connecting the exchange to telephone.

2) Connecting exchange to another exchange etc.

       Peripheral card are programmed to work desirably for each Telephone line. They are classified into different categories which we will talk later

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