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Note for Data Communication and Computer Network - DCCN By Prakash Poudel

  • Data Communication and Computer Network - DCCN
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  • Anna university - ACEW
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DATA COMMUNICATION AND COMPUTER NETWORK Student Handbook What is data? Facts and statistics collected together for reference or analysis is called data. It refers to the quantities, characters, or symbols on which operations are performed by a computer, which may be stored and transmitted in the form of electrical signals and recorded on magnetic, optical, or mechanical recording media. Data can exist in a variety of forms; as numbers or text on pieces of paper, as bits and bytes stored in electronic memory, or as facts stored in a person's mind. Strictly speaking, data is the plural of datum, a single piece of information. In practice, however, people use data as both the singular and plural form of the word. Information is organized or classified data which has some meaningful values for the receiver. Information is the processed data on which decisions and actions are based. For e.g. the history of temperature readings all over the world for the past 100 years is data. If this data is organized and analyzed to find that global temperature is rising, then that is information. Data processing is the restructuring or re-ordering of data by people or machine to increase their usefulness and add values for particular purpose. Data processing consists of basic steps input, processing and output. Differences between data and information The major differences between data and information are: 1. Data is the input language for a computer and information is the output language for human 2. Data is unprocessed facts or mere figures but information is processed data which has been made sense of 3. Data does not depend on information but information depends on data and without it, information cannot be processed 4. Data is not specific but information is specific enough to generate meaning 5. Data is the raw material that is collected but information is a detailed meaning generated from the data. Data Representation Information today comes in different forms such as text, numbers, images, audio, and video. Text In data communications, text is represented as a bit pattern, a sequence of bits (O’s or1’s). Different sets of bit patterns have been designed to represent text symbols. Each set is called a code, and the process of representing symbols is called coding. Today, the prevalent coding system is called Unicode, which uses 32 bits to represent a symbol or character used in any language in the world. The American Standard Code for Information Interchange (ASCII), developed some decades ago in the United States, now constitutes the first 127 characters in Unicode and is also referred to as Basic Latin. Numbers Numbers are also represented by bit patterns. However, a code such as ASCII is not used to represent numbers; the number is directly converted to a binary number to simplify mathematical operations. © Er. PRAKASH POUDEL JIGYASU Page 1

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DATA COMMUNICATION AND COMPUTER NETWORK Student Handbook There are 10 digits i.e. 1, 2, 3, 4, 5, 6, 7, 8, 9 available in decimal number system. It is known as Base 10 system. The value of a digit in a number depends upon its position in the number e.g. the number 546 in this system is represented as (546)10 546 = (4*102) + (8*101) + (6*100) Images Images are also represented by bit patterns. In its simplest form, an image is composed of a matrix of pixels (picture elements), where each pixel is a small dot. The size of the pixel depends on the resolution. For example, an image can be divided into 1000 pixels or 10,000 pixels. In the second case, there is a better representation of the image (better resolution), but more memory is needed to store the image. After an image is divided into pixels, each pixel is assigned a bit pattern. The size and the value of the pattern depend on the image. For an image made of only black-and-white dots (e.g., a chessboard), a I-bit pattern is enough to represent a pixel. If an image is not made of pure white and pure black pixels, you can increase the size of the bit pattern to include gray scale. For example, to show four levels of gray scale, you can use 2-bit patterns. A black pixel can be represented by 00, a dark gray pixel by 01, a light gray pixel by 10, and a white pixel by 11. There are several methods to represent color images. One method is called RGB, so called because each color is made of a combination of three primary colors: red, green, and blue. The intensity of each color is measured, and a bit pattern is assigned to it. Another method is called YCM, in which a color is made of a combination of three other primary colors: yellow, cyan, and magenta. Audio Audio refers to the recording or broadcasting of sound or music. Audio is by nature different from text, numbers, or images. It is continuous, not discrete. Even when we use a microphone to change voice or music to an electric signal, we create a continuous signal. In Chapters 4 and 5, we learn how to change sound or music to a digital or an analog signal. Video Video refers to the recording or broadcasting of a picture or movie. Video can either be produced as a continuous entity (e.g., by a TV camera), or it can be a combination of images, each a discrete entity, arranged to convey the idea of motion. Again we can change video to a digital or an analog signal, as we will see in Chapters 4 and 5. What does Data Communications (DC) mean? Data communication refers to the exchange of data between a source and a receiver. Data communication is said to be local if communicating devices are in the same building or a similarly restricted geographical area. Some devices/technologies used in data communications are known as data communication equipment (DCE) and data terminal equipment (DTE). DCE is used at the sending node, and DTE is used at the receiving node. The meanings of source and receiver are very simple. The device that transmits the data is known as source and the device that receives the transmitted data is known as receiver. Data communication aims at the transfer of data and maintenance of the data during the process but not the actual generation of the information at the source and receiver. © Er. PRAKASH POUDEL JIGYASU Page 2

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DATA COMMUNICATION AND COMPUTER NETWORK Student Handbook Data communications incorporates several techniques and technologies with the primary objective of enabling any form of electronic communication. These technologies include telecommunications, computer networking and radio/satellite communication. Data communication usually requires existence of a transportation or communication medium between the nodes wanting to communicate with each other, such as copper wire, fiber optic cables or wireless signals. For example, a common example of data communications is a computer connected to the Internet via a Wi-Fi connection, which uses a wireless medium to send and receive data from one or more remote servers. Datum mean the facts information statistics or the like derived by calculation or experimentation. The facts and information so gathered are processed in accordance with defined systems of procedure. Data can exist in a variety of forms such as numbers, text, bits and bytes. The Figure is an illustration of a simple data communication system. The effectiveness of a data communications system depends on four fundamental characteristics: delivery, accuracy, timeliness, and jitter. 1. Delivery. The system must deliver data to the correct destination. Data must be received by the intended device or user and only by that device or user. 2. Accuracy. The system must deliver the data accurately. Data that have been altered in transmission and left uncorrected are unusable. 3. Timeliness. The system must deliver data in a timely manner. Data delivered late are useless. In the case of video and audio, timely delivery means delivering data as they are produced, in the same order that they are produced, and without significant delay. This kind of delivery is called real-time transmission. 4. Jitter. Jitter refers to the variation in the packet arrival time. It is the uneven delay in the delivery of audio or video packets. For example, let us assume that video packets are sent every 30-ms. if some of the packets arrive with 3D-ms delay and others with 40-ms delay, an uneven quality in the video is the result. Communication System Communication is the process of establishing connection or link between two points for information exchange. OR Communication is simply the basic process of exchanging information. The electronics equipment which are used for communication purpose, are called communication equipment. Different communication equipment when assembled together forms a communication system. Typical example of communication system are line telephony and line telegraphy, radio telephony and radio telegraphy, radio broadcasting, point-to-point communication and mobile communication, computer communication, radar communication, television broadcasting, radio telemetry, radio aids to navigation, radio aids to aircraft landing etc. The Communication Process In the most fundamental sense, communication involves the transmission of information from one point to another through a succession of process as listed below: 1. The generation of a thought pattern or image in the mind of an originator. 2. The description of that image, with a certain measure of precision, by a set of oral visual symbols. © Er. PRAKASH POUDEL JIGYASU Page 3

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DATA COMMUNICATION AND COMPUTER NETWORK Student Handbook 3. The encoding of these symbols in a form that is suitable for transmission over a physical medium of interest. 4. The transmission of the encoded symbols to the desired destination. 5. The decoding and reproduction of the original symbols. 6. The recreation of the original thought pattern or image, with a definable degradation in quality, in the mind of recipient. Block Diagram of Digital Communication System The basics components or elements of data communication system are as follows: 1. Message 2. Sender 3. Receiver 4. Medium or Communication Channel 5. Encoder and Decoder 6. Protocol Sender Encoder Message Decoder Receiver Communication Channel 1. Message: - The message is the information or data that is to be communicated. It may consist of text, numbers, pictures, sounds, videos or any combination of these. 2. Sender: - A device that is used for sending messages (or data) is called sender. It is also called transmitter or source. The sender can be a computer, telephone, or a video camera etc. Usually, a computer is used as sender in data communication system. 3. Receiver: - A device that is used for receiving messages is called receiver. It is also known as sink. The receiver can be a computer, telephone set, printer, or a fax machine etc. Usually, a computer is also used as receiver in data communication system. 4. Medium: - The path through which data is transmitted (or sent) from one location to another is called transmission medium. It is also called communication channel. It may be a wire, or fiber optic cable, or telephone line etc. If the sender and receiver are within a building, a wire is used as the medium. If they are located at different locations, the medium may be telephone line, fiber optics, and microwave or satellite system. 5. Encoder and Decoder: - In communication systems, computers are used for senders and receivers. A computer works with digital signals. The communication channels usually use analog signals. The encoder and decoder are used in communication systems to convert signals from one from to another. Encoder: - The encoder is an electronic device. It receives data from sender in the form of digital signals. It converts digital signals into a form that can be transmitted through transmission medium. Decoder: - The decoder is an electronic device. It receives data from transmission medium. It converts encoded signals (i.e. analog signals) into digital form. © Er. PRAKASH POUDEL JIGYASU Page 4

Lecture Notes