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Data Transmission: Data and Signals Based on Chapter 3 of William Stallings, Data and Computer Communication, 9th Ed. Kevin Bolding Electrical Engineering Seattle Pacific University Seattle Pacific University Data Transmission No. 1 Data Transmission • Two major aspects of data transmission: • Data – What you are trying to get to the receiver • Actual information being sent/received • Analog (continuous) or digital (discrete) • Signal – How the data is actually sent • Electronic or electromagnetic representation of data • Analog or digital (independent of data type) Seattle Pacific University Data Transmission No. 2 Data • Data comes in thousands of flavors… • Audio • Speech and music are the most common • Video • Television, remote monitoring, videos • Images • JPEG, GIF, etc. • Text • Files, email, text messages • Various computer formats • Word documents, Excel documents • Control information • Remote operation, commands Seattle Pacific University Data Transmission No. 3 Audio Data 0dB Power ratio 25dB -20dB Telephone Channel Speech 70dB 30dB -40dB 3.1kHz -60dB Music 10Hz 100Hz 1kHz 10kHz 100kHz Frequency Source: Stallings, Fig. 3.9 Seattle Pacific University Data Transmission No. 4 Analog Video (NTSC) Data 1 Portion of TV screen 2 3 4 5 NTSC Television: 480 Lines x 450 pixels (more or less) 6 7 8 9 10 11 12 13 Interlaced: Odd lines scanned first, then even lines 14 15 16 17 18 19 Scan line (even) Scan line (odd) Bandwidth lost to horizontal retrace and vertical flyback Horiz. Retrace Vert. Flyback Source: http://www.ntsc-tv.com Try: http://www.ntsc-tv.com/images/tv/aa-raster-1.gif for an interesting animation. Seattle Pacific University Data Transmission No. 5 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 Seattle Pacific University Data Transmission No. 6 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 Seattle Pacific University Data Transmission No. 7 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 Transmitted Signal Received 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 Seattle Pacific University Data Transmission No. 8 Measuring Signal-to-Noise Ratio • SNR = Signal Power/Noise Power • Most signals are observed as a voltage waveform Typically use Peak Signal • Power = V2/R Received Noise = 1V Average and Average Noise 5V 4V 3V 2V 1V 0V Received Signal = 5V Peak Received Signal Both signal and noise see 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 Seattle Pacific University x10 if measuring Power, x20 if measuring Voltage Data Transmission No. 9 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) Seattle Pacific University Data Transmission No. 10 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 Seattle Pacific University Data Transmission No. 11 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 Seattle Pacific University Data Transmission No. 12 Data and Signals Any combination of digital/analog data and digital/analog signals is possible Analog Data Analog Signal Digital Data Modem Analog Signal Analog Data Digitizer Digital Signal Digital Data Transceiver Digital Signal Seattle Pacific University Data Transmission No. 13 Is Digital or Analog “Better”? • 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 Seattle Pacific University Data Transmission No. 14