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