Unit2_DataCom - Data Communications 2-1 Unit 2 Data...

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Unformatted text preview: Data Communications 2-1 Unit 2 Data Communications After studying this unit, you will be able to • explain how data bits can be sent in signals via local and long-distance communication interfaces and links. Data Communications 2-2 Outline of Unit 2 ¡ 2.1 Digital Representation of Information ¡ 2.2 Transmission Mode ¡ 2.3 Transmission Media ¡ 2.4 Error Correction and Detection Data Communications 2-3 Unit 2.1 Digital Representation of Information Data Communications 2-4 Information Types ¡ E-mail: text ¡ Web: text, image, music, video,… ¡ Internet phone: audio ¡ Video conferencing: audio, video ¡ How to classify them? Data Communications 2-5 Classification of Information Types ¡ Block-Oriented Information ¢ e.g. text, image ¢ How many bits are needed to represent a block? ¡ Stream-Oriented Information ¢ e.g. audio, video ¢ Bit Rate : How many bits are required to represent the information in each second? Data Communications 2-6 Character Encoding ¡ How to convert characters into bits? ¡ Common encoding methods: ¢ Morse Code ¢ ASCII ¢ Unicode • Represent and manipulate text expressed in most of the world’s writing system. • You are encouraged to learn more about it on your own. Data Communications 2-7 Morse Code ¡ Introduced in 1840s ¡ Encode each alphabet and numeral as a series of short and long elements ¡ Used for telegraph ¡ Try http://morsecode.scphillips.com/jtranslator.html Data Communications 2-8 ASCII ¡ ASCII - The American Standard Code for Information Interchange ¢ pronounced as “ask-key” ¢ a standard seven-bit code that was proposed by ANSI in 1963, and finalized in 1968. ¢ The Extended ASCII Character Set also consists of 128 decimal numbers and ranges from 128 through 255 representing additional special, mathematical, graphic, and foreign characters. Data Communications 2-9 Data Communications 2-10 Image Encoding ¡ BMP: bitmap ¢ Uncompressed format ¢ Consider a 24-bit color BMP image with dimension 1024x768 pixels. • 24-bit means that 8 bits for Red, 8 bits for Green, and 8 bits for Blue. ¢ What is the file size of the image? ¡ GIF: Graphics Interchange Format ¢ Lossless compression • the original image can be exactly reproduced. ¢ Low compression ratio. ¢ Mainly used for simple images. ¡ JPEG: Joint Photographic Experts Group ¢ Lossy compression • acceptable as long as there is little visible degradation in image quality. ¢ High compression ratio • typically, about 15 Data Communications 2-11 Digitization of Analog Signals ¡ Two steps: ¢ Sampling ¢ Quantization Data Communications 2-12 Sampling ¡ Obtain sample values of the analog signal every T seconds. ¡ The value of T depends on how fast the signal varies ....
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Unit2_DataCom - Data Communications 2-1 Unit 2 Data...

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