Summer Training at BSNL
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From the ancient days it has been deepest human desire to stay closed with those they need. Many different ways have been evolved and implemented to fulfill this; Telephony is also such a mechanism that uses Telephone, electronic equipment that converts sound into electrical signals that can be transmitted over distances and then converts received signals back into sounds.
Electronic exchanges play major role in this system. It’s a workplace that serves telecommunication facilities where lines from telephones can be connected together to permit communication. The switching center that houses the terminating and switching equipment is called TELEPHONE EXCHANGE. Following are the types of exchanges
Digital switching has become a synonym for time division multiplex. In digital switching system, digital signals are switched in two nodes, time switching and space switching. Time switching basically involves rearrangement of channel sequence in the same PCM from whereas space switching involves interconnection of same channels of a different PCM high way. In a practical digital switching both time and space switching stage is employed to increase the traffic handling capacity of the switch at minimum cost. Among the various possible combinations TST structure is most commonly used.
Synchronization of various exchange clocks in a digital network is essential to provide required services. Causing a slip due to mismatch between the clocks will result in multination of information and description of services. Hence, slip has to be controlled using plesiochronous and synchronous approaches.
Often the message signal is not suitable for transmission over the medium in its original forms. Therefore, the message signal is located in a carrier wave, which is more suited for transmission over the medium. This process is called modulation of the carrier wave. At the receiving end station the message signal received back from the incoming modulated carrier wave by a process is called demodulation which is inverse of modulation.
Modulation enables multiple message signals to be simultaneously transmitted over the same medium without interfering each other. Transmission of multiple message signals simultaneously over a medium is called multiplexing.
Modulation is essentially a process of relocation of a message signal in a carrier wave, which is more suitable for transmission for transmission over the medium than the message signal itself.
Three types of modulation can occur-
1. Amplitude modulation
2. Frequency modulation
3. Pulse modulation
One of the primary requirements of any computer system is that service will be available to the subscriber at any time. In the vast majority of exchanges the electrical energy required for signaling, switching, speed transmission mains. The exchange power system is designed to provide continuous and uninterrupted telephone service even when the public electric fails. We can divide power plant in three major blocks such as
• FLOAT RECTIFIER
• BATTERY CHARGER
• SWITCHING CUBICAL
A microprocessor is a complex electronic processor on a single silicon chip (Large-scale integrated circuit). It is capable of performing arithmetic and logical functions under the control of program.
Functional units of microprocessor-
1. Arithmetic and logic unit
2. Registers (Temporary memory)
3. Timing and control unit
Various switching peripherals are connected to central processor by means of a common bus system. A bus is a group of wires on which data and command pulses are transmitted between the various sub units of a switching processor and switching peripherals. The device to be activated is addressed by sending its address on address bus. The common bus system avoids costly interconnection among various devices (Fig-6).
I.S.D.N (Integrated service digital network)-
Presently, there is a multiplicity of services such as telephoning, telex, data cable television etc. that are provided by separate dedicated network for each of them. These non-integrated services require separate switching system. Special subscribers access plants, individual trunk networks and separate operations and maintenance staff. These services are costly to the subscriber as well as to department. In I.S.D.N, these services are provided by one integrated network, which can be accessed by different customer premises equipment.
In I.S.D.N, subscriber loop from the local exchange will be terminated at the network terminal (N.T). From N.T the signal will be distributed to the information sockets. These information sockets will be distributed in the premises just like power sockets are distributed for providing on the information sockets, different terminals for voice and nonvoice services can be connected.
These are entirely computerized and electronic devices, which work on store programme control. Electronic logic component can do ten thousand times operation during signal in the comparison of electromechanical exchange.
In an electronic exchange a single control device simultaneously process a number of calls on time-sharing basis. Such centralization is more economical, but it is disadvantageous in making switching system more vulnerable to system failure. This, however, can be overcome a standby control devices.
These electronic exchanges have powerful processing unit. Part of control equipment capacity utilize for function other than for processing. There are two types of electronic exchange.