Data Transmission No. 1Seattle Pacific University

Data Transmission: Data and Signals

Based on Chapter 3 of William Stallings, Data and Computer Communication, 9th Ed.

Kevin BoldingElectrical Engineering

Seattle Pacific University

Data Transmission No. 2Seattle 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. 3Seattle Pacific University

Data

• Data comes in thousands of flavors…• Audio

• Speech and music are the most common• Video

• Television, remote monitoring, videos

• Text

• Files, email, text messages

• Various computer formats

• Word documents, Excel documents

• Control information

• Remote operation, commands

• Images

• JPEG, GIF, etc.

Data Transmission No. 4Seattle Pacific University

TelephoneChannel

Audio Data

0dB

-20dB

-40dB

-60dB

10Hz 100Hz 1kHz 10kHz 100kHz

Po

we

r ra

tio

Frequency

Music

Speech

3.1kHz30dB

Source: Stallings, Fig. 3.9

25dB

70dB

Data Transmission No. 5Seattle Pacific University

Analog Video (NTSC) Data

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

10

12

14

16

18

Horiz. Retrace

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

1

3

5

7

9

11

13

15

17

19

2

4

6

8

Scan line (odd)Scan line (even)

Vert. Flyback

Portion of TV screen

Data Transmission No. 6Seattle 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. 7Seattle 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. 8Seattle 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 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

Data Transmission No. 9Seattle Pacific University

Measuring Signal-to-Noise Ratio

• SNR = Signal Power/Noise Power

• Most signals are observed as a voltage waveform

• Power = V2/R

Received Signal= 5V Peak

Received Signal

0V1V2V3V

5V4V

Received Noise= 1V Average

• SNR = (52/R) / (12/R) = 52 / 12 = 25• In deciBels

• SNRdB = 10 log10(PS/PN) =10 log10(25/1) = 13.97dB

x10 if measuring Power,x20 if measuring Voltage

Both signal and noisesee the same load, R,so it cancels out

• Note: SNRdB = 10 log10(VS2/VN

2) = 10 log10(VS/VN)2 = 20 log10(VS/VN)

• SNR = 20 log10(5/1) = 13.97dB

Typically use Peak Signaland Average Noise

Data Transmission No. 10Seattle 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 log10(Signal power/Noise power)

Data Transmission No. 11Seattle 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. 63.5s/line, but 11s used for retrace 52.5s/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 52.5s/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

Data Transmission No. 12Seattle 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 frequencyUsing bandwidth of 4x base frequency

Source: Stallings, Fig. 3.7

See http://www.falstad.com/fourier/index.html for a demonstration of this

Data Transmission No. 13Seattle 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 signalsis possible

Data Transmission No. 14Seattle 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