1. Chapter 4
Transmission Media
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2. Overview Guided - wire Unguided – wireless
In both media, communication is in the form of electromagnetic waves
Characteristics and quality determined by medium and signal
For guided, the medium is more important
For unguided, the bandwidth of the signal produced by the antenna is more important
Key concerns are data rate and distance
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3. Design Factors Bandwidth Transmission impairments Interference
Higher bandwidth gives higher data rate
Transmission impairments
Attenuation
Twisted pair suffers more impairment than coaxcial cable and optical fibre
Interference
Number of receivers
In guided media
More receivers (multi-point) introduce more attenuation
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4. Electromagnetic Spectrum
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5. Guided Transmission Media
Twisted Pair
Coaxial cable
Optical fiber
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6. Transmission Characteristics of Guided Media
Frequency Range
Typical Attenuation
Typical Delay
Repeater Spacing
Twisted pair (with loading)
0 to 3.5 kHz
0.2 1 kHz
50 µs/km
2 km
Twisted pairs (multi-pair cables)
0 to 1 MHz
0.7 1 kHz
5 µs/km
Coaxial cable
0 to 500 MHz
7 10 MHz
4 µs/km
1 to 9 km
Optical fiber
186 to 370 THz
0.2 to 0.5 dB/km
40 km
Point to point transmission
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7. Twisted Pair
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8. Twisted Pair - Description
There are two insulated cupper wires. Many pairs are bundled into a single cable.
Twisting reduces crosstalk interference between adjacent pairs of cable.
Neighboring pairs in a bundle typically have different twist lenghts
On long distance links, twist length varies from 5 to 15 cm.
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9. Twisted Pair – Applications 1
Most common medium
For both analog and digital systems
Telephone network
Between house and local exchange (subscriber loop)
Within buildings
Each telephone is connected to a twisted pair, which goes to private branch exchange (PBX)
Designed to support voice traffic using analog signalling
Also can handle digital data traffic at modest data rates (modem)
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10. Twisted Pair – Applications 2
Mostly used in telephone lines.
Compared to coaxial cable and optical fiber, twisted pair is limited in distance, bandwidth and data rate.
Twisted pair is also used within a building for LAN supporting personal computers.
For long distance digital point-to-point signaling a data rate around a few Mbps is possible.
For a very short distances data rates of up to 1 Gbps have been achieved.
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11. Twisted Pair - Pros and Cons
Cheap
Easy to work with
Low data rate
Short range
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12. Twisted Pair - Transmission Characteristics
Analog
Amplifiers every 5km to 6km
Digital
Use either analog or digital signals
repeater every 2km or 3km
Limited distance
Limited bandwidth
Limited data rate
Susceptible to interference and noise
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13. Attenuation of Guided Media
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14. Unshielded and Shielded TP
Unshielded Twisted Pair (UTP)
Ordinary telephone wire
Cheapest
Easiest to install
Suffers from external EM interference
Shielded Twisted Pair (STP)
Metal braid or sheathing that reduces interference
More expensive
Harder to handle (thick, heavy)
UTP is cheap but subject to electromagnetic interference. STP is expensive but better.
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15. UTP Categories Cat 3 Cat 4 Cat 5 Cat 5E (Enhanced) –see tables Cat 6
up to 16MHz
Voice grade found in most offices
Twist length of 7.5 cm to 10 cm
Cat 4
up to 20 MHz
Cat 5
up to 100MHz
Commonly pre-installed in new office buildings
Twist length 0.6 cm to 0.85 cm
Cat 5E (Enhanced) –see tables
Cat 6
Cat 7
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16. Near-End Crosstalk
Coupling of signal from one pair of conductors to another pair
Metal pins on a connector
Wire pairs in a cable
Coupling takes place when transmit signal entering the link couples back to receiving pair at the same end of the link
i.e. near transmitted signal is picked up by near receiving pair
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17. Comparison of Shielded and Unshielded Twisted Pair
Attenuation (dB per 100 m)
Near-end Crosstalk (dB)
Frequency (MHz)
Category 3 UTP
Category 5 UTP
150-ohm STP 1
2.6
2.0
1.1 41 62 58 4
5.6
4.1
2.2 32 53 16
13.1
8.2
4.4 23 44
50.4 25 —
10.4
6.2
47.5
100
22.0
12.3
38.5
300
21.4
31.3
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18. Twisted Pair Categories and Classes
Category 3 Class C
Category 5 Class D
Category 5E
Category 6 Class E
Category 7 Class F
Bandwidth
16 MHz
100 MHz
200 MHz
600 MHz
Cable Type
UTP
UTP/FTP
SSTP
Link Cost (Cat 5 =1)
0.7 1
1.2
1.5
2.2
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19. Coaxial Cable
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20. Coaxial Cable Applications
Most versatile medium
Television distribution
Cable TV can carry hundreds of TV channels
Long distance telephone transmission
Can carry 10,000 voice calls simultaneously (using FDM)
Being replaced by fiber optic
Short distance computer systems links
Local area networks
Using digital signaling, coax cable can be used to provide high speed channels
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21. Coaxial Cable - Transmission Characteristics
Can be used effectively higher frequencies and data rates
Analog
Amplifiers every few km
Closer if higher frequency
Usable spectrum is up to 500MHz
Digital
Repeater every 1km
Closer for higher data rates
Performance Constraints
Attenuation
Thermal noise
Intermodulation noise
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22. Optical Fiber
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23. Optical Fiber - Benefits
Greater capacity
Data rates of hundreds of Gbps
Smaller size & weight
Lower attenuation
Electromagnetic isolation
Are not affected by external electromagnetic fields (thus not vulnarable to interference, impulse noise, crosstalk)
Little interference with other equipment
High degree of security from eavesdropping
inherently difficult to tap
Greater repeater spacing
10s of km at least
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24. Optical Fiber – Applications (1)
Long-haul trunks
1500 km in length
voice channels
Telephone network, undersea
Metropolitan trunks
12 km
100,000 voice channels
Joining telephone exchanges in a metropolitan or city area
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25. Optical Fiber – Applications (2)
Rural exchange trunks km
5000 voice channels
Links towns and villages
Subscriber loops
Run directly central exchange to a subscriber
Handling not only voice and data, but also image and video
LANs
100 Mbps to 10 Gbps
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26. Optical Fiber - Transmission Characteristics
Act as wave guide for 1014 to 1015 Hz
Portions of infrared and visible spectrum
Two different types of ligth of source are used:
Light Emitting Diode (LED)
Cheaper
Wider operating temp range
Last longer
Injection Laser Diode (ILD)
More efficient
Greater data rate
Wavelength Division Multiplexing
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27. Optical Fiber Transmission Modes 1
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28. Optical Fiber – Transmission Modes 2
Step-index multimode
Rays at shallow angles are reflected and propagated along the fiber
Multiple propagation paths exist
Pulses spread out in time
The need to leave spacing between the pulses limits data rate
Suitable for short distance transmission
Single-mode
By reducing the radius of the core
No distortion (because there is a single transmission path)
Suitable for long distance transmission
Graded-index multimode
Suitable for LANs
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29. Frequency Utilization for Fiber Applications
Wavelength (in vacuum) range (nm)
Frequency range (THz)
Band label
Fiber type
Application
820 to 900
366 to 333
Multimode
LAN
1280 to 1350
234 to 222 S
Single mode
Various
1528 to 1561
196 to 192 C
WDM
1561 to 1620
185 to 192 L
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30. Attenuation in Guided Media
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31. Wireless Transmission
Unguided media
Transmission and reception via antenna
Directional
Focused beam
Careful alignment required
Omnidirectional
Signal spreads in all directions
Can be received by many antennae
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32. Wireless Transmission Frequencies
1GHz to 40GHz
Microwave
Highly directional
Point to point
Satellite
30MHz to 1GHz
Omnidirectional
Broadcast radio
3 x 1011 to 2 x 1014 Hz
Infrared portion of the spectrum
Local point to point and multipoint applications within confined areas
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33. Antennas
Electrical conductor (or system of..) used to radiate electromagnetic energy or collect electromagnetic energy
Transmission
Radio frequency energy from transmitter
Converted to electromagnetic energy
By antenna
Radiated into surrounding environment
Reception
Electromagnetic energy impinging on antenna
Converted to radio frequency electrical energy
Fed to receiver
Same antenna often used for both
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34. Radiation Pattern Power radiated in all directions
Not same performance in all directions
A way to characterize the performance of an antenna
Graphical representation of the radiation propoerties of an antenna as a function of space coordinates
Isotropic antenna is (theoretical) point in space
Radiates in all directions equally
Gives spherical radiation pattern
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35. Parabolic Reflective Antenna
Used for terrestrial and satellite microwave
Parabola is locus of all points equidistant from a fixed line and a fixed point not on that line
Fixed point is focus
Line is directrix
Revolve parabola about axis to get paraboloid
Cross section parallel to axis gives parabola
Cross section perpendicular to axis gives circle
Source placed at focus will produce waves reflected from parabola in parallel to axis
Creates (theoretical) parallel beam of light/sound/radio
On reception, signal is concentrated at focus, where detector is placed
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36. Parabolic Reflective Antenna
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37. Antenna Gain 1 Measure of directionality of antenna
Power output in particular direction compared with that produced by isotropic antenna
Measured in decibels (dB)
Results in loss in power in another direction
Effective area relates to size and shape
Related to gain
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38. Antenna Gain 2 Relationship between antenna gain and effective area
G= 4Ae /2 = 4f 2Ae / c2
G = antenna gain
Ae effective area
f carrier frequency
c speed of light
 carrier wavelength
The effective area of
an isotropic antenna : 2 / 4 (power gain of 1)
a parabolic antenna with face area of A: 0.56A (power gain of 7A/2)
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39. Antenna Gain 3
For a parabolic reflective antenna with a diameter of 2m, operating at 12 GHz, what is the effective area and antenna gain?
A =  r 2 = 
Effective area for parabolic antenna is known as Ae = 0.56 A yielding Ae = 0.56 
The wavelength is  = c / f = (3108)/(12109)=0.025m
The power gain of parabolic antenna is known as 7A/2 yielding (7  ) / (0.025)2 = 35,186 which is 45.46dB
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40. Terrestrial Microwave – Physical Description
Parabolic dish sizing 3 m in diameter
Focuses narrow beam
Line of sight transmission to the receiving antenna
To achieve long distance transmissions, a series of microwave relat towers is used
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41. Terrestrial Microwave - Applications
Long haul communications systems
Alternative to coax cable or optical fiber
Requires far fewer amplifiers or repeaters than coax but requires line of sight transmission
Used for both voice and television transmission
Short point to point links between buildings
Cellular systems
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42. Terrestrial Microwave – Transmission Characteristics
Common frequencies: 1 to 40 GHz
The higher the frequency used, the higher the potential bandwidth
The loss in communications is computed by
L= 10 log10 (4d/)2 dB where
d is the distance
 is the wave length
Repeaters and amplifiers every 10 to 100 km
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43. Terrestrial Microwave - Transmission Characteristics
Attenuation increases with rainfall
Interference might be a problem
Most common bands for long haul communication are 4 GHz to 6 GHz
11 GHz band is coming to use
Higher microwave frequencies
Increased attenuation
Used in shorter distances
Antennas are smaller and cheaper
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44. Satellite Microwave – Physical Description
Satellite is relay station
Satellite receives on one frequency, amplifies or repeats signal and transmits on another frequency
Operate on a number of frequency bands: transponder channels
Requires geo-stationary orbit to remain stationary
Height of 35,784km
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45. Satellite Point to Point Link
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46. Satellite Broadcast Link
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47. Satellite Microwave - Applications
Television
Public Broadcasting Service (PBS)
Programs are transmitted to the satellite, broadcast down to the stations, individual viewers
Direct Broadcast Satellite (DBS)
Direct to the home user
Long distance telephone
Private business networks
Very Small Aperture Terminal (VSAT) System
A powerful hub station acts as a relay for communication of subscribers
Global Positioning Systems
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48. Satellite Microwave – Transmission Characteristics
To avoid interference, between satellites 4 spacing required
This limits the number of satellites
For continuous operation without interference, satellite cannot transmit and receive on the same frequency
Inherently a broadcast medium
There is 270 msec propagation delay
Operation is in the range of GHz
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49. Broadcast Radio Omnidirectional 30 MHz to 1 GHz Line of sight
FM radio
UHF and VHF television
Line of sight
Less sensitive to attenuation from rainfall
Suffer relatively less attenuation
Because of the longer wavelength
Suffers from multipath interference
Reflection from land, water or objects
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50. Infrared Modulate noncoherent infrared light
Tranceivers must be within the line of sight (directly or via reflection)
Blocked by walls
No security problems
No frequency allocation
e.g. TV remote control
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51. Wireless Propagation Signal travels along three routes Ground wave
Follows contour of earth
Up to 2MHz
AM radio
Sky wave
Amateur radio, BBC world service, Voice of America
2 MHz to 30 MHz
Signal reflected from ionosphere layer of upper atmosphere
(Actually refracted)
Line of sight
Above 30Mhz because it is not reflected by the ionosphere
May be further than optical line of sight due to refraction
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52. Ground Wave Propagation
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53. Sky Wave Propagation
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54. Line of Sight Propagation
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55. Refraction
Velocity of electromagnetic wave is a function of density of material
~3 x 108 m/s in vacuum, less in anything else
As wave moves from one medium to another, its speed changes
Causes bending of direction of wave at boundary
Towards more dense medium
Index of refraction (refractive index) is
Sin(angle of incidence)/sin(angle of refraction)
Varies with wavelength
May cause sudden change of direction at transition between medium
May cause gradual bending if ref. idx. gradually changes
Density of atmosphere decreases with height
Results in bending towards earth of radio waves

56. Optical and Radio Line of Sight
Optical line of sight
d = 3.57 ( h )1/2
Radio line of sight
d = 3.57 (K h )1/2 where
d is the distance between an antenna and the horizon in kilometers
h is the antenna height in meters
K is the adjustment factor (microwaves are bent with the curvature of the earth) K is approximately 4/3
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57. Optical and Radio Horizons
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58. Example
The maximum distance between two antennas for LOS transmission if one antenna is 100m high and the other is at ground level is
d = 3.57 (Kh) ½ = 3.57 (133) ½ = 41 km
Now suppose that the receiving antenna is 10m high. To achieve the same distance, how high must the transmitting antenna be? The result is
41 = 3.57 ((Kh1) ½ + (13.3) ½ )
h1 = 46.2 m
Savings over 50 m in the height of transmission antenna
Raising receiving antennas reduces the necessary height of the transmitter
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59. Line of Sight Transmission
Free space loss
Signal disperses with distance
Greater for lower frequencies (longer wavelengths)
Atmospheric Absorption
Water vapor and oxygen absorb radio signals
Water greatest at 22GHz, less below 15GHz
Oxygen greater at 60GHz, less below 30GHz
Rain and fog scatter radio waves
Multipath
Better to get line of sight if possible
Signal can be reflected causing multiple copies to be received
May be no direct signal at all
May reinforce or cancel direct signal
Refraction
May result in partial or total loss of signal at receiver

60. Free Space Loss for Isotropic Antennas
Pt / Pr = (4  d ) 2 / 2 = ( 4  f d ) 2 / c 2
Pt = signal power at the transmittin antenna
Pr = signal power at the receiving antenna
 = carrier wavelength (m)
d = propagation distance between antennas (m)
c = speed of light (3108 m/s)
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61. Free Space Loss for Other Antennas
Pt / Pr = (4 )2 (d)2 / GrGt 2
= (d )2/Ar At
= (cd )2 / f 2 ArAt
Gt = gain of the transmitting antenna
Gr = gain of the receiving antenna
At = effective area of the transmitting antenna
Ar = effective area of the receiving antenna
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62. Free Space Loss
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63. Example
Determine the isotropic free space loss at 4 GHz for the shortest path to a synchronous satellite from earth (35,863 km).
At 4 GHz, the wavelength is (3108)/(4109)=0.075m
LdB=195.6 dB (computed by loss in isotropic antenna)
Now consider the antenna gain of both satellite-and ground-based antennas. Typical values are 44 dB and 48 dB, respectively. The free space loss is
LdB = = dB
Now assume a transmit power of 250 W at the earth station. What is the power received at the satellite antenna?
A power of 250 W translates into 24 dBW, so the power at the receiving antenna is 24 – = dBW.
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64. Multipath Interference
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65. Required Reading
Stallings Chapter 4
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