Merits and Demerits of Fiber Optic Communication System

Merits and Demerits of Fiber Optic Communication System Points : merits and demerits of fiber optic communication system Merits 1. Very low attenuation: When high frequency signals are propagated through conventional coaxial cables, it loses half of its power only after few hundred meters where as the fiber optics loses the same amounts of power in 15 Km or more. Thus repeaters will be required at very large distances.

2. Large bandwidth: The transmission rate exceeding 10 G-bits/s are possible as compared to 1 G-bits/s for coaxial cables. This capacity can be further increased by using carrier waves of different wavelengths.

Uses of Fiber Optics in Telecommunication System

Uses of Fiber Optics in Telecommunication System Points : uses of fiber optics in telecommunication system The fiber optics is superior to metallic conductors as transmission medium for signals because of its high bandwidth, low attenuation, interference immunity, low cost, and light weight.

Due to these advantages the fiber optics is extensively used in the field of telecommunication. Some of the applications of fiber optics in the telecommunication system are:

Because of its very low attenuation, the fiber optics is used as junction cables between exchanges. where channel capacity is in excess of 240 channels.

LASER (Light Amplification by Stimulated Emission of Radiation)

LASER (Light Amplification by Stimulated Emission of Radiation) Points : laser (light amplification by stimulated emission of radiation) The Laser is like the LED but emit light in a different way, resulting in a higher output powers and more directional beams. The beam is produced by a cavity resonator that reflects light again and again through the excited medium. The cavity consists of a pair of mirrors, one at each end of the region occupied by excited atoms. The light emitted toward one mirror is reflected again and again between the two mirrors thus stimulating emission from electrons.

Light Emitting Diode (LED)

Light Emitting Diode (LED) Points : light emitting diode (led), Definition The basic operation of the LED is shown in the figure. The LED is a PN-junction diode and work when forward biased. When a small forward biased voltage is applied, it causes a current to flow across the junction. Electrons in the N-region and holes in the P-region flow toward the junction where they recombine and release some energy. The recombination continues as long as the voltage is applied. In Silicon and Germanium semiconductors, the released energy is dissipated as heat. While in the LEDs, the released energy appears as photon of light. The semiconductor materials used for LEDs in fiber optic systems arc gallium aluminum arsenide or gallium arsenide. Other semiconductor compound used to make LEDs is gallium-arsenide-phosphide and indium-gallium-arsenide- phosphide. GaAs LEDs emit near 930 nm. While GaAIAs emit light at wavelengths of 750 to 900 nm. GaAsP emits visible red light at 665 nm.

Physics of Optical Fibers

Physics of Optical Fibers Points : Physics of Optical Fibers, Electromagnetic Spectrum, Reflection of Light, Refraction of Light, Total Internal Reflection, Numerical Aperture, The working of optical fibers depends on the basic principles of optics and the interaction of light with matter. From physical point of view, light can be considered either electromagnetic waves or as photons, quanta of electromagnetic energy. Both points of views are valid and valuable. But the most useful point of view is that light is rays and travel in straight lines, between or within optical elements, which can reflect or refract (bend) light rays at their surfaces.

Block Diagram of Fax-Machine

Block Diagram of Fax-Machine Points : block diagram of fax-machine, components of fax-machine, Central Microprocessor, Fax-Modem, Control Panel, Image Buffer and Control, CCD (Charge Coupled Device) Image Sensor, Thermal Printer, Power Supply In spite of the fact, that Fax-machine is small in appearance yet it incorporates a large amount of electronics and mechanical components into its operation. A simplified block diagram of the Fax-machine is shown in the figure. Central Microprocessor It is the heart of all Fax-Machines. The microprocessor manages all the operations of the machine and coordinates the flow of data into and out of the system. The microprocessor consists of the following memories:
a. ROM: It stores the program used to run the machine.
b. RAM: It stores results of calculations, variables, system status flags, or any other information that will change regularly during normal operations.

Facsimile or Fax Machine

Facsimile or Fax Machine Points : facsimile or fax machine, Definition The Fax Machine digitizes printed images and transmits the corresponding, Data over the telephone network. Similarly, it receives digital image information from the telephone network and reproduces an exact copy of the image on the paper.

To transmit a document through Fax machine, the document is positioned in the machine. Then a telephone call is made by dialling the number of the person who is to receive this document. By doing so, an electrical connection is established between the calling and called machines. The “Send” key is then pressed at the transmitting end, which switches the telephone circuits to the Fax machine. When the circuit between the two machines is complete, a process called handshaking takes place which means that both machines are in synchronization.

Application of Computer in Digital Exchange

Application of Computer in Digital Exchange Points : application of computer in digital exchange A computer is programmable electronic device that can store, retrieve, and process data. The use of computer and, more recently, microprocessor has brought a revolution in the design and operation of switching systems. In modern exchanges, particularly those using S.P.C., the use of a powerful and highly reliable computer control all the functions of an exchange.

All the information used in the translation processes are stored in the computer, thus it is easier to update the system when required. The computer has a complete overview of the system, so is able to analyse traffic patterns and diagnose faults more easily. Details of traffic and faults along with information for the preparation of user’s bills can also be recorded in the computer.

Stored Program Control (SPC)

Stored Program Control (SPC) Points : Stored Program Control (SPC), digital exchange The switching procedures in SPC exchanges are controlled by means of stored programs (a program is a sequence of instructions stored in the computer memory). The SPC uses a computer as the time-shared control. The computer is defined by the program stored in its memory. The computer is programmed to test the conditions of the inputs and old states and decide on new outputs and states. Stored program control of switching systems is based on this principle. The computer controllers provide the time-shared decision maker, expressed as programs, which can be used to change or extend the functions of a control system.

Basic Principle of Store

Basic Principle of Store Points : Basic Principle of Store In the digital system, a large amount of data is required to be stored in the memory and retrieved when desired. The data stored in the memory consists of many words. Each word consists f a sequence of 0’s and 1’s and is stored in a memory location called its address. When a word is entered in the memory, it is said that it has been written into the memory, and when it is located, it has been read. The word that is being written in or read from the memory is called addressing. The switching principles are provided with two types of memories. One is called data memory or slot-content memory and the other is control memory (slot interchange information).

Digital Switching

Digital Switching Points : Digital Switching, Time Switching, Space Switching, Space Time Switching In the digital exchanges, the connection between two subscribers is established with the help of digital switches. With the introduction of these switches, the bulky expensive mechanical contacts points operated by an equally bulky, expensive and power consuming, electromagnets have been replaced by digital gates. A connection between two subscribers is made, simply by enabling the gate connecting the two lines. Also, logic gates are unidirectional; two paths through a switching matrix must be established to accommodate a two-way communication.

The digital techniques are equally advantageous for both signal transmission and switching. These advantages can be fully enjoyed if both the transmission systems and exchanges used digital techniques with TDM.

Block Diagram of Digital Telephone Exchange

Block Diagram of Digital Telephone Exchange Points : block diagram of digital telephone exchange The block diagram of a digital telephone exchange is shown.
The digital exchange includes the following hardware subsystems and components:

DIGITAL LINE UNIT (DLU) The DLU is connected to the LTG and is responsible for terminating subscriber lines and concentrating subscriber line traffic. The DLU can be installed locally or remotely. It has the following four primary functions:

1. Concentration of subscriber lines
2. Conversion of analog lines into digital format
3. Flexibility to accommodate variable traffic loads
4. Facilitation of ISDN (Integrated Services Digital Network) introduction

Advantages of Digital Systems over Analog Systems

Advantages of Digital Systems over Analog Systems Points : advantages of digital systems over analog systems The digital telephone systems offer the following advantages over the analog systems:

1. Digital technology used throughout the system has high noise immunity
2. Multiple use of lines and exchange equipment by means of TDM
3. Each speech direction has a separate channel