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Transcript
Lecture 13-15:
Transmission media
Aliazam Abbasfar
Outline
 Transmission media
Communications systems
 Wireline (wired)
 Telephony (voice, fax, modem, DSL)
 Ethernet/LAN
 Cable TV
 Backplane copper links
 Wireless (Electromagnetic)
 Over the air communication
 Radio and TV broadcast
 WLAN
 Cellular
 Radar
 Fiber optics
 High speed long haul data communication
 High traffic data transfer
Transmission media
 Open wire
 Twisted pair
 Coaxial cable
 Optical fiber
 Air
Open-wire
 On utility poles
 share power line routes
 Interference limited (EMI)
 Limits the BW too
 Transposition to reduce interference
 Early twisting scheme
 4 twists per Km
 Environmental effects
Twisted pair cable
 Unshielded twisted pair (UTP)
 Shielded twisted pair (STP)
Transmission parameters
• Lumped model
• for a unit length
• System response
H(f) = e-gL
g = a + j b = ( R + jL)(G + jC)
Loss (dB) = 20 log10e a L = a’ L
phase change = b L
• Loss increases with frequency
Interference
 Interference is the main limitation (BW)
 Twisting reduces interference
 Shielding further reduces interference
 Cross talk (X-Talk)
 Near end cross talk (NEXT)
 Far end cross talk (FEXT)
 Cable length

 Space between pairs 
NEXT
-
 NEXT and FEXT decreases with frequency
 f1.5 and f2
FEXT


Applications
 Communication networks
 Subscriber lines


Analog (Voice)
Digital (ADSL, HDSL, VDSL)
 Links (E1,T1)
 Computer networks
 LAN




Cat
Cat
Cat
Cat
3
4
5
6
<
<
<
<
10 Mbps
100 Mbps
150 Mbps
350 Mbps
Coaxial cable
 Low cross talk
 Increases with frequency
 High speed/Long links
 Long distance communication links
 Cable TV
 WAN/MAN
Wireline media comparison
Medium
Freq. range
Loss (dB/Km)
@ freq
Open wire
0-150 KHz
0.03
0.2
1 KHz
200 KHz
Twisted pair
(Loaded)
0-1 MHz
0.7
(0.2)
1 KHz
(3.5 KHz)
Cat 6
1-250 MHz
21
60
1 MHz
10 MHz
Coax
0-500 MHz
7
10 MHz
Optical Fiber
 Structure
1. Core (8 µm)
2. Cladding (125 µm)
3. Buffer (250 µm)
4. Jacket (400 µm)
 Optical cable
Fiber optics
 Optical propagation
 Refraction index
 ncore > ncladding
 Fiber types
 Step index
 Graded index
 Single mode
 Multi mode
Optical fiber
 Loss
 Absorption
 Scattering
 Connectors
 Low loss : 0.2 dB/Km
 Color dependent
 Dispersion
 Multi modes = different paths
 Refraction index is frequency (color) dependent
 High bandwidth ( > 1 GHz)
 Wavelength division multiplexing (WDM)
 DWDM
 Data rate > TB/s in a single fiber !
Wireless communication
 Frequency allocation needed in shared environment
 To avoid interference
 Spectrum is a very valuable resource
 Band allocation to applications
 Government regulations and policies
 ITU coordinates between nations
 Freq band:
 3-30KHz Very low freq. (VLF)
 30-300KHz Low freq. (LF)
 300K-3MHz Medium freq. (MF)
 3-30MHz High freq (HF)
 30-300MHz Very high freq (VHF)
 300M-3G Ultra high freq (UHF)
 3-3GHz Super high freq. (SHF)
Electromagnetic waves propagations
 Ground waves travels along
the surface of the earth
 ( freq < 2 MHz)
 Sky waves reflected by
ionosphere
 Very variable – seasonal


Angle and loss of reflection
Freq < 30 MHz
 Line of sight (LOS)
 No reflection or refraction
 Non Line of sight
 Local reflections/refractions
Wireless issues
 Path loss
 Fading
 Mobility
 Interference
Satellite systems

LEOs




Lower power
Smaller delay
Need many satellites
Shift towards LEOs in 1990



Global domination
Compete with cellular systems
Failed miserably (Iridium )

Big, power hungry mobile terminals
 Global Positioning System (GPS)



Satellite signals used to pinpoint location
Popular in cars, cell phones, and navigation devices
Natural area for satellite systems is broadcasting



Now operate in 12GHz band
100s of TV and radio channels
All over the world
Reading
 Carlson Ch. 1
 Proakis Ch. 1