Bussiness data communications 4e chapter 6

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 importan

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 ( φ ): relative position in time, measured in degrees

✘ 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

time (sec)

1 cycle

frequency (hertz)

= cycles per second

phase difference

Analog Signaling

✘ represented by sine waves

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

time (sec)

1 cycle

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

✘ Digital facsimile, bitmapped graphics

✘ Object-oriented graphics

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:

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

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

✘ B = Bandwidth

✘ C= Channel

✘ SNR = Signal-to-noise ratio