Data and Computer Communications Chapter 4 …netopia.knu.ac.kr/2016_2nd_data_com/04-TransmissionMedia.pdfKyungpook National University Computer Engineering Transmission Characteristics

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


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


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)


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)


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


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


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


Optical Fiber Transmission Modes


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


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


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


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


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


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


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


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


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


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)


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


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


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


Summary

Guided transmission mediaTwisted pairCoaxial cableOptical fiber

Wireless mediaFrequency vs propagationAttenuation in wireless propagation Diffraction, straightness, penetration, refraction Multipath interference

Documents

Data and Computer Communications Chapter 4 …netopia.knu.ac.kr/2016_2nd_data_com/04-TransmissionMedia.pdfKyungpook National University Computer Engineering Transmission Characteristics