Download Chapter 6: Data Transmission

Chapter 6:
Data Transmission
Business Data Communications,
4e
Information Transmission
Electromagnetic Signals
Function of time
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Analog (varies smoothly over time)
Digital (constant level over time, followed
by a change to another level)
Function of frequency (more important)
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Spectrum (range of frequencies)
Bandwidth (width of the spectrum)
Periodic Signal Characteristics

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Amplitude (A): signal value, measured in
volts
Frequency (f): repetition rate, cycles per
second or Hertz
Period (T): amount of time it takes for one
repetition, T=1/f
Phase (f): relative position in time,
measured in degrees
s (t)=A sin(2ft+f)
Bandwidth
Width of the spectrum of frequencies
that can be transmitted

if spectrum=300 to 3400Hz,
bandwidth=3100Hz
Greater bandwidth leads to greater
costs
Limited bandwidth leads to distortion
sin(2ft)
1/3 sin(23ft)
sin(2ft)+ 1/3 sin(23ft)
Why Study Analog in a Data
Comm Class?
Much of our data begins in analog form; must
understand it in order to properly convert it
Telephone system is primarily analog rather
than digital (designed to carry voice signals)
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Low-cost, ubiquitous transmission medium
If we can convert digital information (1s and 0s)
to analog form (audible tone), it can be
transmitted inexpensively
Data vs Signals
Analog data

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Voice
Images
Digital data
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Text
Digitized voice or images
amplitude (volts)
Analog Signaling
represented by sine waves
1 cycle
phase
difference
time
(sec)
frequency (hertz)
= cycles per second
Voice/Audio Analog Signals
Easily converted from sound frequencies
(measured in loudness/db) to electromagnetic
frequencies, measured in voltage
Human voice has frequency components
ranging from 20Hz to 20kHz
For practical purposes, the telephone system
has a narrower bandwidth than human voice,
from 300 to 3400Hz
Image/Video: Analog Data to
Analog Signals
Image is scanned in lines; each line is
displayed with varying levels of intensity
Requires approximately 4Mhz of analog
bandwidth
Since multiple signals can be sent via
the same channel, guardbands are
necessary, raising bandwidth
requirements to 6Mhz per signal
amplitude (volts)
Digital Signaling
represented by square waves or pulses
1 cycle
time
(sec)
frequency (hertz)
= cycles per second
Digital Text Signals
Transmission of electronic pulses
representing the binary digits 1 and 0
How do we represent letters, numbers,
characters in binary form?
Earliest example: Morse code (dots and
dashes)
Most common current form: ASCII
Digital Image Signals
Analog facsimile

similar to video scanning
Digital facsimile, bitmapped graphics

uses pixelization
Object-oriented graphics
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image represented using library of objects
e.g. Postscript, TIFF
Pixelization and Binary
Representation
Used in digital fax, bitmapped graphics
1-bit code:
00000000
00111100
01110110
01111110
01111000
01111110
00111100
00000000
Transmission Media
the physical path between transmitter
and receiver (“channel”)
design factors affecting data rate

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bandwidth
physical environment
number of receivers
impairments
Impairments and Capacity
Impairments exist in all forms of data
transmission
Analog signal impairments result in
random modifications that impair signal
quality
Digital signal impairments result in bit
errors (1s and 0s transposed)
Transmission Impairments:
Guided Media
Attenuation

loss of signal strength over distance
Attenuation Distortion

different losses at different frequencies
Delay Distortion

different speeds for different frequencies
Noise

distortions of signal caused by interference
Transmission Impairments:
Unguided (Wireless) Media
Free-Space Loss

Signals disperse with distance
Atmospheric Absorption

Water vapor and oxygen contribute to signal loss
Multipath

Obstacles reflect signal creating multiple copies
Refraction
Noise
Pr
Pt
=
Ar At f 2
(cd)2
Ar At f 2
Pr =
Pt
2
(cd)
Pr : Power received
Pt : Power transmitted
Ar : Area of Receiving antenna
At : Area of Transmitting antenna
f : Carrier frequency
d : Distance between antenna
c : 3000,000 km/s
Types of Noise
Thermal (aka “white noise”)

Uniformly distributed, cannot be eliminated
Intermodulation

When different frequencies collide (creating
“harmonics”)
Crosstalk

Overlap of signals
Impulse noise
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Irregular spikes, less predictable
Channel Capacity
The rate at which data can be
transmitted over a given path, under
given conditions
Four concepts
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Data rate
Bandwidth
Noise
Error rate
Shannon Equation
C = B log2 (1 + SNR)
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B = Bandwidth
C= Channel

SNR = Signal-to-noise ratio
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