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Data Transmission: Data, Signals, and Transmission methods

Data Transmission: Data, Signals, and Transmission methods

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Data Transmission: Data, Signals, and Transmission methods

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  1. Data Transmission: Data, Signals, and Transmission methods Based on Chapter 3 of William Stallings, Data and Computer Communication, 8th Ed. Kevin BoldingElectrical EngineeringSeattle Pacific University

  2. Data Transmission • Three aspects of data transmission: • Data • Actual information being sent/received • Analog (continuous) or digital (discrete) • Signals • Electronic or electromagnetic representation of data • Analog or digital (independent of data type) • Transmission (signal boosting) • Communication of data by sending and processing signals • Processing depends on the data type

  3. Data • Data comes in thousands of flavors… • Audio • Speech and music are the most common • Video • Television, remote monitoring • Images • JPEG, GIF, etc. • Text • Files, email • Various computer formats • Word documents, Excel documents • Control information • Remote operation, commands

  4. 25dB 70dB Audio Data 0dB TelephoneChannel -20dB Speech Power ratio 30dB -40dB 3.1kHz -60dB Music 10Hz 100Hz 1kHz 10kHz 100kHz Frequency Source: Stallings, Fig. 3.9

  5. Portion of TV screen 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Horiz. Retrace Scan line (even) Scan line (odd) Vert. Flyback Source: http://www.ntsc-tv.com Analog Video (NTSC) Data NTSC Television:480 Lines x 450 pixels (more or less) Interlaced: Odd lines scanned first, then even lines Bandwidth lost to horizontal retrace and vertical flyback Try: http://www.ntsc-tv.com/images/tv/aa-raster-1.giffor an interesting animation.

  6. Text and Computer Data • Text data is human-readable • Transmitted in the International Reference Alphabet (IRA), known in the US as ASCII • Seven/eight bits per character • Computer data is not human-readable • May be in any one of thousands of formats (.doc, .xls, .wav, .mp3, .avi etc.) • Binary in nature – Interpretation is left to the computer

  7. Signals • Signals are the physical representation of data • Signal must have enough capacity (bandwidth) as the data being transmitted needs • Analog signals are continuous in nature • Contain an infinite number of possible signal levels • Limited by noise • Digital signals are discrete in nature • Finite number of signal levels • Still limited by noise, but easier to deal with it

  8. Transmitted Signal Received Signal Signal-to-Noise Ratio • The quality of a signal is judged by how well the original data can be extracted from it • Noise will corrupt the signal • The important measure is the power ratio: • Received Signal Power/Received Noise Power • In most cases, the ability to distinguish the signal is based on the log of the power ratio

  9. 5V 4V 3V 2V 1V 0V Received Signal Measuring Signal-to-Noise Ratio • SNR = Signal Power/Noise Power • Most signals are observed as a voltage waveform • Power = V2/R Typically use Peak Signaland Average Noise Received Noise= 1V Average Received Signal= 5V Peak Both signal and noisesee the same load, R, so it cancels out • SNR = (52/R) / (12/R)= 52 / 12 = 25 • In deciBels • SNRdB = 10 log10(PS/PN) =10 log10(25/1) = 13.97dB • Note: SNRdB = 10 log10(VS2/VN2) = 10 log10(VS/VN)2 = 20 log10(VS/VN) • SNR = 20 log10(5/1) = 13.97dB x10 if measuring Power,x20 if measuring Voltage

  10. Telephone Signals • Speech occupies a band between 100Hz and 7kHz • Almost all useful information is between 300Hz and 3.4kHz • Telephone signals (POTS) are electrical representations of the sound signals • Bandwidth of 3.1kHz (300 – 3400 Hz) • S/N ratio of 30dB (Maximum signal power is 1000x the average noise power) • S/N ratio (dB) = 10 log10(Signal power/Noise power)

  11. Video (NTSC) Signals • An analog signal giving a gray scale value for each pixel • Synchronizes to the TV’s scanning circuitry, then just blasted to the screen • Approximate Analysis: • Scanning frequency: 525 lines in 1/30 sec. 63.5ms/line, but 11ms used for retrace  52.5ms/line • Each line contains approx. 450 pixels • Highest frequency needed when displaying alternating black/white pattern • Two pixels per period (high/low portions of wave) • Requires 52.5ms/450 pixels/ 2 pixels/period = 233.3 ns/pixel  4.2MHz (high end) • Low end: All black or all white  DC (0 Hz) • Bandwidth needed is (4.2 – 0 MHz) = 4.2MHz

  12. Digital Signals See http://www.falstad.com/fourier/index.html for a demonstration of this • Digital signals are sent as pulses (square waves) • ‘1’ represented by a high voltage, ‘0’ by a low voltage • Other representations are possible as well A square wave: Requires Infinite bandwidth. Square wave using finite bandwidth:Using bandwidth of 6x base frequency Using bandwidth of 4x base frequency Source: Stallings, Fig. 3.7

  13. Analog Data Analog Signal Digital Data Modem Analog Signal Analog Data Digitizer Digital Signal Digital Data Transceiver Digital Signal Data and Signals Any combination of digital/analog data and digital/analog signalsis possible

  14. Transmission (Signal Boosting) • Once a signal is on a transmission medium, it may be modified or shaped • Analog transmission sends signals through amplifiers • Amplifiers do not distinguish between signal and noise • Digital transmission sends signals through repeaters • Restore signal to its original form • Filter out noise • May perform error correction • Digital data may be sent on an analog signal, but using digital transmission • Uses repeaters rather than amplifiers

  15. The Future: Digital or Analog? • Data is inherently digital or analog • Digital signals and digital transmission are taking over • Better data integrity • Possible to ensure 100% accurate transmission of a digital signal • Better utilization • Easier to multiplex digital signals • Security • Encryption is easy with digital data