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Data Transmission: Data, Signals, and Transmission methods. Based on Chapter 3 of William Stallings, Data and Computer Communication, 8 th Ed. Kevin Bolding Electrical Engineering Seattle Pacific University. Data Transmission. Three aspects of data transmission: Data

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data transmission data signals and transmission methods

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

data transmission
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
slide3
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
audio data

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

analog video ntsc data

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.

text and computer data
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
signals
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
signal to noise ratio

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
measuring signal to noise ratio

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

telephone signals
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)
video ntsc signals
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
digital signals
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

data and signals

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

transmission signal boosting
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
the future digital or analog
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
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