Chapter 6: Data Transmission Business Data Communications, 4e

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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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