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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 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

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DATA COMMUNICATION AND COMPUTER NETWORK Student Handbook 6. Protocol: - It is a set of rules that governs the data communications. It represents an agreement between the communicating devices. Without a protocol, two devices may be connected but not communicating. Protocol performs the following functions: 1. Data sequencing. It refers to breaking a long message into smaller packets of fixed size. Data sequencing rules define the method of numbering packets to detect loss or duplication of packets, and to correctly identify packets, which belong to same message. 2. Data routing. Data routing defines the most efficient path between the source and destination. 3. Data formatting. Data formatting rules define which group of bits or characters within packet constitute data, control, addressing, or other information. 4. Flow control. A communication protocol also prevents a fast sender from overwhelming a slow receiver. It ensures resource sharing and protection against traffic congestion by regulating the flow of data on communication lines. 5. Error control. These rules are designed to detect errors in messages and to ensure transmission of correct messages. The most common method is to retransmit erroneous message block. In such a case, a block having error is discarded by the receiver and is retransmitted by the sender. 6. Precedence and order of transmission. These rules ensure that all the nodes get a chance to use the communication lines and other resources of the network based on the priorities assigned to them. 7. Connection establishment and termination. These rules define how connections are established, maintained and terminated when two nodes of a network want to communicate with each other. 8. Data security. Providing data security and privacy is also built into most communication software packages. It prevents access of data by unauthorized users. 9. Log information. Several communications Software are designed to develop log information, which consists of all jobs and data communications tasks that have taken place. Such information may be used for charging the users of the network based on their usage of the network resources. What does Computer Network mean? The generic term "network" refers to a group of entities (objects, people, etc.) which are connected to one another. A network, therefore, allows material or immaterial elements to be circulated among all of these entities, based on well-defined rules. © Er. PRAKASH POUDEL JIGYASU Page 5

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