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Ch. 8: Data Communications Fundamentals

Ch. 8: Data Communications Fundamentals. Outline. Representation Timing Encoding Data Flow Connecting Devices. Digital Encoding of Analog Data. Primarily used in retransmission devices Uses pulse-code modulation (PCM) Sample analog signal

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Ch. 8: Data Communications Fundamentals

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  1. Ch. 8: Data Communications Fundamentals

  2. Outline • Representation • Timing • Encoding • Data Flow • Connecting Devices

  3. Digital Encoding of Analog Data • Primarily used in retransmission devices • Uses pulse-code modulation (PCM) • Sample analog signal • Represent sample as a digital value (Ex: 8 bit -> 256 values) • Send digital value as a “pulse code” – a wave form that represents the digital values via Amplitude Shift Keying. • The sampling theorem: If a signal is sampled at regular intervals of time and at a rate higher than twice the significant signal frequency, the samples contain all the information of the original signal. • 8000 samples/sec sufficient for 4000hz

  4. Converting Samples to Bits • Quantizing • Similar concept to pixelization • Breaks wave into pieces, assigns a value in a particular range • 8-bit range allows for 256 possible sample levels • More bits means greater detail, fewer bits means less detail

  5. Example with 3-bit digitization Max amplitude = 3.0 Sample 1 1.4; sample as 1.0, digitize as 100.Sample 2 2.6; sample as 3.0, digitize as 110. Sample 3 3.0; sample as 3.0, digitize as 110. Sample 4 1.7; sample as 3.0, digitize as 101. Sample 10 0.4; sample as 0.0, digitize as 011. Sample 11  -2.0; sample as -2.0, digitize as 001. 3 110 3.0 2 101 2.0 4 1 100 1.0 10 011 0.0 010 -1.0 11 001 -2.0 Sampling points 000 -3.0 Bit values Discrete amplitude values

  6. Voice Example • 4000 Hz frequencies • 8-bit digitization = 256 values • 8000 Hz sampling rate (2 x frequencies) • 8000 samples/sec x 8 bits/sample = 64,000 bps/ • Thus 64 kbps needed for a single voice signal. • With other techniques, this can be reduced to 8 kbps.

  7. Data Flow • Data can flow between devices in various ways • One direction only • Two direction, one at a time • Two directions, simultaneously

  8. Simplex Illustration

  9. Half Duplex Illustration

  10. Full Duplex Illustration

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