Chapter 6:
Data Transmission
Business Data Communications, 4e
Electromagnetic Signals
Function of time
Analog (varies smoothly over time)
Digital (constant level over time, followed by a change to another level)
Function of frequency (more important)
Spectrum (range of frequencies)
Bandwidth (width of the spectrum)
Periodic Signal Characteristics
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
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
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)
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
Voice
Images
Digital data
Text
Digitized voice or images
Analog Signaling
represented by sine waves 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
Digital Signaling
represented by square waves or pulses frequency (hertz)
= cycles per second time
(sec)
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
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
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:
Attenuation
Guided Media
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
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
Irregular spikes, less predictable
Channel Capacity
The rate at which data can be transmitted over a given path, under given conditions
Four concepts
Data rate
Bandwidth
Noise
Error rate
Shannon Equation
C = B log
2
(1 + SNR)
B = Bandwidth
C = Channel
SNR = Signal-to-noise ratio
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