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1 CHP: 1 INTRODUCTION Data Communication: Data communication is a vital part of the information society because it provides the infrastructure allowing the computers to communicate with one another. An airline data communication system uses data communications to link reservation offices to the compute r. The space flight use data communications systems to send data to and from the rockets and command centers on Earth. The purpose of data communication system is transport of user data between and among user machines. Evolution of Data Co mmunication: It came to existence shortly after the computer were widely used in the organizations. The 1970s and 1980s saw a merger of the fields of computer science and data communications that profoundly changed the technology, products, and companies of the now-combined computer- communications industry. Although the consequences of this revolutionary merger are still being worked out, it is safe to say that the revolution has occurred, and any investigation of the field of data communications must be made within this new context. In order to obtain the services of the computer user's simply walk to the room where the computer is located and submitted a request for the computer to per form a service. This request was called a job. The computer accepted the user’s job, performed its operations, and returns the results in hard copy formats. As the computers grew, it became inefficient for all users to walk to the computer room, submit their job and return to get the results. Consequently, computer based terminals were built and were placed in user work spaces within a building. This approach a llowed users to submit their job from each office. As organizations, grew and the need for the computer grew it became necessary to share the computer with other users in different buildings. The solution was to utilize the widely used telephone system to transport these traffic. Even though the telephone system was designed for voice traffic, various techniques were employed to send data through the telephone system. This became known a s remote time sharing a nd is still a prevalent form of data communications. Analog and Digital Communication: Analog data take on continuous values in some intervals. For example voice and video are continuously varying patterns of intensity. Similarly, digital data can be text or character strings. Character form cannot be easily stored or transmitted by data processing and c ommunication system. Hence, such systems are designed for binary data. In a communication system, data are propagated from one point to a nother by me ans of electromagnetic signals. An analog signal is a continuously varying electromagnetic wave that may be propagated ov er a va riety of media. A digital signa l is a sequence of voltage pulses that may be transmitted over a wired medium. For example, a constant +ve v oltage level ma y represent binary zero and a const ant negative voltage level may represent binary 1. Analog transmission is a means of transmitting analog signals without regard to their content. The signals may represent analog data (eg. voice) or digital data( eg. binary data that p ass through a modem) . The analog signal will become weaker a fter a certain distance. Amplifiers are use d to

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