Data Transmission: Data and Signals

Based on Chapter 3 of William Stallings,

Computer Communication, 9 th Data and Ed.

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Seattle Pacific University Data Transmission Kevin Bolding Electrical Engineering Seattle Pacific University 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) 

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Data

•

Data comes in thousands of flavors…

• Audio • • Speech and music are the most common Video • Television, remote monitoring, videos • Images • • Text JPEG, GIF, etc.

• Files, email, text messages • Various computer formats • Word documents, Excel documents • Control information • Remote operation, commands 

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Audio Data

0dB -20dB -40dB 25dB Speech 30dB Telephone Channel 3.1kHz

-60dB 10Hz 100Hz 70dB 1kHz Frequency 10kHz Music 100kHz

Source: Stallings, Fig. 3.9

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Analog Video (NTSC) Data

Portion of TV screen 1 2 3 4 5 6 7 8 9 10 11 13 12 14 15 16 17 18 19 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

Scan line (even) Scan line (odd)

Source: http://www.ntsc-tv.com

Horiz. Retrace Vert. Flyback Try:

http://www.ntsc-tv.com/images/tv/aa-raster-1.gif

for an interesting animation.

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Text and Computer Data

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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 

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Signals

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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 

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Signal-to-Noise Ratio

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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 

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• •

Measuring Signal-to-Noise Ratio

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SNR = Signal Power/Noise Power

• • Most signals are observed as a voltage waveform Power = V 2 /R Received Noise = 1V Average Typically use Peak Signal and Average Noise Received Signal = 5V Peak 5V 4V 3V 2V 1V 0V Received Signal

SNR = (5 2 /R) / (1 2 /R) = 5 2 / 1 2 = 25

Both signal and noise see the same load, R,

In deciBels

• SNR dB = 10 log 10 (P S /P N ) • • so it cancels out =10 log 10 (25/1) = 13.97dB

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 ) = x10 if measuring Power, SNR = 20 log 10 (5/1) = 13.97dB

x20 if measuring Voltage 

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Telephone Signals

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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) 

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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

m s/line, but 11 m s used for retrace  52.5

m 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

m s/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

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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

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Data and Signals

Any combination of digital/analog data and digital/analog signals is possible Analog Data Digital Data Analog Data Digital Data Modem Digitizer Transceiver Analog Signal Analog Signal Digital Signal Digital Signal 

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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 

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