chap1-notes - 1 1 Digital Data Communication and Storage...

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1 Digital Data Communication and Storage Digital communication systems include cell phones, dig- ital television via satellite or cable, digital radio, Wi-Fi and WiMax, cable modem, and so on. Additional examples include digital data storage de- vices, including magnetic (“hard") disk drives, magnetic tape drives, optical disk drives, and fl ash drives. In the case of data storage, information is communi- cated from one point in time to another rather than one point in space to another. Each of these examples fi t into the digital communica- tion framework of C. Shannon’s A Mathematical Theory of Communication (1948) 1
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This framework is depicted in the fi gure above whose various components are described as follows. source and user (or sink): The information source may be originally in analog form (e.g., speech or music) and then later digitized, or it may be originally in digital form (e.g., computer fi les). The user of the information may be a person, a com- puter, or some other electronic device. 2
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source encoder and source decoder: The encoder is a processor which converts the information source bit sequence into an alternative bit sequence with a more e cient representation of the information, i.e., with fewer bits. This operation is often called compression , which can be lossless (e.g., for computer data fi les) or lossy (e.g., for video, still images, or music, where the loss can be made to be imperceptible or acceptable). The source decoder is the encoder’s counterpart which recovers the source sequence exactly (lossless) or approx- imately (lossy). channel encoder and channel decoder: The chan- nel encoder converts its input into an alternate sequence (e.g. of codewords) which possesses redundancy whose role is to provide immunity from the various channel im- pairments. 3
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