Data Transmission No. 1  Seattle Pacific University Data Transmission: Data and Signals Based on Chapter 3 of William Stallings, Data and Computer Communication, 9 th Ed. Kevin Bolding Electrical Engineering Seattle Pacific University

Data Transmission No. 2  Seattle Pacific University 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)

Data Transmission No. 3  Seattle Pacific University Data Data comes in thousands of flavors… Audio Speech and music are the most common Video Television, remote monitoring, videos Text Files, , text messages Various computer formats Word documents, Excel documents Control information Remote operation, commands Images JPEG, GIF, etc.

Data Transmission No. 4  Seattle Pacific University Telephone Channel Audio Data 0dB -20dB -40dB -60dB 10Hz100Hz1kHz10kHz100kHz Power ratio Frequency Music Speech 3.1kHz 30dB Source: Stallings, Fig dB 70dB

Data Transmission No. 5  Seattle Pacific University Analog Video (NTSC) Data Try: for an interesting animation. 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 Horiz. Retrace Source: Scan line (odd) Scan line (even) Vert. Flyback Portion of TV screen

Data Transmission No. 6  Seattle Pacific University 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

Data Transmission No. 7  Seattle Pacific University 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

Data Transmission No. 8  Seattle Pacific University 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 SignalReceived 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

Data Transmission No. 9  Seattle Pacific University Measuring Signal-to-Noise Ratio SNR = Signal Power/Noise Power Most signals are observed as a voltage waveform Power = V 2 /R Received Signal = 5V Peak Received Signal 0V 1V 2V 3V 5V 4V Received Noise = 1V Average SNR = (5 2 /R) / (1 2 /R) = 5 2 / 1 2 = 25 In deciBels SNR dB = 10 log 10 (P S /P N ) =10 log 10 (25/1) = 13.97dB x10 if measuring Power, x20 if measuring Voltage Both signal and noise see the same load, R, so it cancels out Note: SNR dB = 10 log 10 (V S 2 /V N 2 ) = 10 log 10 (V S /V N ) 2 = 20 log 10 (V S /V N ) SNR = 20 log 10 (5/1) = 13.97dB Typically use Peak Signal and Average Noise

Data Transmission No. 10  Seattle Pacific University 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 log 10 (Signal power/Noise power)

Data Transmission No. 11  Seattle Pacific University 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.  63.5  s/line, but 11  s used for retrace  52.5  s/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 52.5  s/450 pixels/ 2 pixels/period = 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

Data Transmission No. 12  Seattle Pacific University Digital Signals 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 See for a demonstration of thishttp://

Data Transmission No. 13  Seattle Pacific University Data and Signals Digital Data Modem Analog Signal Analog Data Digitizer Digital Signal Digital Data Transceiver Digital Signal Analog Data Analog Signal Any combination of digital/analog data and digital/analog signals is possible

Data Transmission No. 14  Seattle Pacific University 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