Note for Computer Network - CN by Technical Bhai

Topics ( 9 )

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www.chennaiuniversity.net 1.1 DATA COMMUNICATIONS Need for Communication A communication service enables the exchange of information between users at different locations.Communication services & applications are everywhere. Some examples are given below Need for Computer Communication over Distances Computer communication has become essential for the following reasons: (a) Computers can send data at a very fast speed over long distances using satellite and microwave links. Therefore, the cost of transfer of documents using computers is cheaper than other conventional means like telegrams. (b) Computers can have fax system integrated with them. This allows them to send pictures along with the text. Thus the newspaper reports can be prepared and sent all over the world at a very high speed by composing and publishing them from different centers. -1-

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www.chennaiuniversity.net (c) The time taken to transmit the messages is very small. Hence different computers can be connected together and the users can work together as a group. Software packages have been developed for group working in Data Base Management (DBMS) and graphic works. (d) Different departments of an organization may be separated physically being at distant places but their data could be stored on a central computer. This data is accessed by computers located in different departments. The data at the central computer may be updated from time to time and accessed by all users. This prevents any bottlenecks in the smooth functioning of the organization. The latest data (say for inventory) will be easily available at all times to all the users. (e) Fluctuations of prices in foreign exchange and shares/equities can be communicated instantaneously using the medium of computer communications only. The transfer can be accelerated and verified at any instant of time. Data Communication Data Communication is defined as the exchange of data between two devices via some form of transmission medium such as a wire cable. The communicating devices must be a part of a communication system made up of a combination of hardware (physical equipment) and software (programs). Characteristics of data Communication: The effectiveness of a data communication depends on three characteristics 1. Delivery 2. Accuracy 3. Timeliness Delivery: The system must deliver data to correct destination. Accuracy: The system must deliver data accurately. Timeliness: The system must deliver data in a timely manner. Data delivered late are useless. 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. Components The components of a data communication are • Message • Sender • Receiver • Medium • Protocol -2-

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www.chennaiuniversity.net Five Components of data communication 1. Message: The message is the information to be communicated. It can consist of text, pictures, numbers, sound, video or audio. 2. Sender: The sender is the device that sends the data message. It can be a computer, workstation, telephone handset, video camera and so on. 3. Receiver: The receiver is the device that receives the message. It can be a computer, workstation, telephone handset, video camera and so on. 4. Medium: The transmission medium is the physical path by which a message travels from sender to receiver. It could be a twisted pair wire, coaxial cable, fiber optic cable, or radio waves. 5. Protocol: A protocol is a set of rules that governs data communications. It represents an agreement between the communicating devices. Data Representation Information 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. The number of bits in a pattern depends on the number of symbols in the language. Different sets of bit patterns have been designed to represent text symbols. Each set is called a code. The process of representing the symbols is called coding. ASCII The American National Standards Institute developed a code called the American Standard code for Information Interchange .This code uses 7 bits for each symbol. 128 (27) different symbols can be defined by this code. Extended ASCII To make the size of each pattern 1 byte (8 bits), the ASCII bit patterns are augmented with an extra 0 at the left. Each pattern is exactly 1 byte of memory. -3-

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www.chennaiuniversity.net Unicode To represent symbols belonging to languages other than English, a code with much greater capacity is needed. Unicode uses 16 bits and can represent up to 65,536 (216) symbols. ISO The international organization for standardization known as ISO has designed a code using a 32 – bit pattern. This code can represent up to 4,294,967,296 (232) symbols. Numbers Numbers are also represented by using bit patterns. ASCII is not used to represent numbers. The number is directly converted to a binary number. Images Images are also represented by bit patterns. An image is divided into a matrix of pixels, where each pixel is a small dot. Each pixel is assigned a bit pattern. The size and value of the pattern depends on the image. The size of the pixel depends on resolution. Audio Audio is a representation of sound. Audio is by nature different from text, numbers or images. It is continuous not discrete Video Video can be produced either a continuous entity or it can be a combination of images. Direction of Data Flow Communication between two devices can be simplex, half-duplex or full-duplex Simplex In simplex mode, the communication is unidirectional. Only one of the devices on a link can transmit; the other can only receive. Simplex Keyboards and monitors are examples of simplex devices. The keyboard can only introduce input. The monitor can only accept output. -4-