data and computer communications chapter 4...
TRANSCRIPT
Data and Computer Communications
Chapter 4 –Transmission Media
Computer EngineeringKyungpook National University
Design Factors Determining Data Rate and Distance
• Higher bandwidth gives higher data rate
BandwidthBandwidth
• Impairments, such as attenuation, limit the distance
Transmission impairmentsTransmission impairments
• Overlapping frequency bands can distort or wipe out a signal
InterferenceInterference
• More receivers introduces more attenuation
Number of receiversNumber of receivers
Computer EngineeringKyungpook National University
Transmission Characteristics of Guided Media
Frequency Range
Typical Attenuation
Typical Delay
Repeater Spacing
Twisted pair (with loading)
0 to 3.5 kHz 0.2 dB/km @ 1 kHz
50 µs/km 2 km
Twisted pairs (multi-pair cables)
0 to 1 MHz 0.7 dB/km @ 1 kHz
5 µs/km 2 km
Coaxial cable
0 to 500 MHz
7 dB/km @ 10 MHz
4 µs/km 1 to 9 km
Optical fiber 186 to 370 THz
0.2 to 0.5 dB/km
5 µs/km 40 km
Energy of 10 Mhz signal drops by 7dB every Km
Computer EngineeringKyungpook National University
Twisted Pair Can use either analog or digital signals
Needs a repeater every 2-3km for digital signal Needs an amplifiers every 5km to 6km for analog signal
Limited distance, limited bandwidth (1Mhz), Limited data rate (100Mhz)
Susceptible to interference and noise
Near End Crosstalk Coupling of signal from one pair to another Occurs when transmit signal entering the link couples back to
receiving pair (near transmitted signal is picked up by near receiving pair)
Computer EngineeringKyungpook National University
Unshielded vs Shielded TP Unshielded twisted pair (UTP)
Ordinary telephone wire Cheapest and easiest to install Suffers from external EM (Electromagnetic) interference
Shielded twisted pair (STP) Metal braid that reduces interference More expensive Harder to handle (thick, heavy)
Computer EngineeringKyungpook National University
Coaxial Cable
Frequency characteristics superior to twisted pair
Performance limited by attenuation and noise
Analog signals
• Amplifiers are needed every few kilometers -closer if higher frequency
• Usable spectrum extends up to 500MHz
Digital signals
• Repeater every 1km - closer for higher data rates
Computer EngineeringKyungpook National University
Optical Fiber Greater capacity
Data rates of hundreds of Gbps
Lower attenuation Greater repeater spacing
10s of km at least
Widely used in long distance telecommunications
Has a cylindrical shape with three sections – core, cladding, jacket
Smaller size & weight Electromagnetic isolation
Computer EngineeringKyungpook National University
Optical Fiber Uses total internal reflection to transmit light
Effectively acts as wave guide for 1014 to 1015 hz Can use several different light sources
Light emitting diode (LED) Cheaper, wider operating temp range, lasts longer
Injection laser diode (ILD)More efficient, has greater data rate
Computer EngineeringKyungpook National University
Optical Fiber Transmission Modes
Computer EngineeringKyungpook National University
Electromagnetic Wave Wide-sense : radio wave including light (infrared, visible
light, ultraviolet…) Narrow-sense: electromagnetic wave with wavelength
longer than infrared (3kHz~3THz) Radio Waves Act No. 2: radio wave is defined by
electromagnetic wave propagating through space with frequency lower than 3THz
1864: Maxwell mathematically predicted existence of electromagnetic wave
1874: Hertz experimentally proved existence of electromagnetic wave
1896: Marconi succeeded in transatlantic radio telegraph
Electromagnetic Wave
Computer EngineeringKyungpook National University
Electromagnetic Wave
① Current flow generates electrical field
② Magnetic field inducts electric field
③ Electric field inducts magnetic field
④ …
propagationv = 3*108m/sElectric field
Magnetic field
Electric field is generated by variation of currents Electric field inducts magnetic field, magnetic field
again inducts electric field, and vice versa Electric field and magnetic field propagate as a
wave through space perpendicular to each otherat speed of light
Current flow
Computer EngineeringKyungpook National University
Frequency & Wavelength Frequency : number of vibrations for 1 sec (Hz) Wavelength : distance occupied by one cycle Assuming signal velocity v, f = v/
v = 3*108 m/s (speed of light in free space) Wavelength is inversely proportional to frequency
v m for 1sλ
Computer EngineeringKyungpook National University
Characteristics of Wave
Propagation characteristics depend on wavelength (or frequency)
Straightness Concentrative radiation to destination
Diffraction Can arrive at back of obstacle Strong diffraction => small radio shadow behind
obstacles
Penetrability
Computer EngineeringKyungpook National University
Propagation vs Frequency High frequency wave (short wavelength)
Large attenuation Small coverage
Weak diffraction => Large radio shadow behind obstacles Strong straightness/penetrability Suitable to high data rate applications (communication of bulk data)
Low frequency wave (long wavelength) Large coverage
Good receiving sensitivity
Strong diffraction => Small radio shadow behind obstacles Weak straightness/penetrability Suitable to long distance applications (maritime or air communication)
Propagation quality depends on signal attenuation, diffraction, penetrability in order of importance
Computer EngineeringKyungpook National University
Signal Attenuation Free space loss Multipath Interference
Multiple interfering signals from reflections, diffractions, scatterings
Refraction The velocity of an electromagnetic wave changes with
movement between media with different densities Density of atmosphere decreases with height, resulting in
gradual bending of radio waves toward the earth
Atmospheric absorption From water vapour and oxygen absorption
Computer EngineeringKyungpook National University
Free Space Loss Attenuation is proportional to
square of distance
Attenuation is proportional to square of frequency High frequency wave suffers
from severe attenuation Lower frequency wave has
larger coverage
Computer EngineeringKyungpook National University
Multipath Interference Reflection occurs when
signal encounters a surface that is large relative to the wavelength of the signal
Diffraction occurs at the edge of an impenetrable body that is large compared to the wavelength of radio wave When a radio wave encounters
an edge, waves propagate in different directions as the source
Scattering occurs if the size of an obstacle is on the order of the wavelength of the signal or less An incoming signal is scattered
into several weaker outgoing signals
Figure 10.7 Sketch of Three Important Propagation Mechanisms:Reflection (R), Scattering (S), Diffraction (D)
R
R
D
S
lamppost
Computer EngineeringKyungpook National University
ReceivedLOS pulse
Receivedmultipath
pulses
Time
Time
Transmittedpulse
Transmittedpulse
ReceivedLOS pulse
Receivedmultipath
pulses
Figure 10.8 Two Pulses in Time-Variant Multipath
Multiple secondary pulses due to reflection, diffraction, and scattering may arrive at the same time as the primary pulse
Act as a form of noise to the subsequent primary pulse
Effect of Multipath
Computer EngineeringKyungpook National University
Example
Penetrability1.7 Ghz800 Mhz
Diffraction800 Mhz
Small shadow
1.7 Ghz
Large shadow
B
A
A: 800 MhzB: 1.7 Ghz
Signal attenuation (Coverage)
Computer EngineeringKyungpook National University
Example Sound
Medium = air 20~20K Hz Speed = 340m/s Very strong diffraction but weak straightness
Can hear well behind obstacles Men’s voice spreads farther than women
Ultrasound : sound with freq over 20Khz Sound with strong straightness Used for geographic exploration or fish detection
Can easily return after hitting against ocean floor or school of fish
Light No medium (propagates in just space) Very strong straightness but weak diffraction
Shadow behind obstacle X-ray
Computer EngineeringKyungpook National University
Example AM
531~1,602KHz Wavelength of KBS AM (711KHz) = 421.6m Large coverage Strong diffraction
No poor reception area Can hear even in mountains or tunnels
One AM transmitting tower is enough for Taegu city FM
88.1~107.9MHz Wavelength of MBC FM 95.9MHz = 3.126m Weak diffraction
Disconnected in underpass or tunnel High quality music broadcast since more data are used to
express music TV
GHz Very strong straightness but weak diffraction Large dead spot
Computer EngineeringKyungpook National University
Electromagnetic Spectrum Radio wave is public and limited resource : frequency usage allocation is managed under international and national control
Table of Frequency Allocation : ITU (International Telecommunication Union) coordinates distribution, allocation, and usage of frequency band
Computer EngineeringKyungpook National University
Summary
Guided transmission mediaTwisted pairCoaxial cableOptical fiber
Wireless mediaFrequency vs propagationAttenuation in wireless propagation Diffraction, straightness, penetration, refraction Multipath interference