1. Chapter 4: Transmission Media
COE 341: Data & Computer Communications (T061) Dr. Radwan E. Abdel-Aal
Chapter 4:
Transmission Media

2. Agenda Overview Guided Transmission Media Wireless Transmission
Twisted Pair
Coaxial Cable
Optical Fiber
Wireless Transmission
Antennas
Terrestrial Microwaves
Satellite Microwaves
Broadcast Radio
Infrared

3. Overview
Media:
Guided – wire or fiber
Unguided - wireless
Transmission characteristics and quality determined by:
Signal
Medium
For guided, the medium is more important
For unguided, the bandwidth provided by the antenna is more important

4. Design Issues Key communication objectives are:
High data rate
Low error rate
Long distance
Bandwidth economy: Tradeoff
- Want larger BW for higher data rates: C  B
- But limited by economy: Larger BW is costly e.g. Coaxial Vs TP
Transmission impairments
Attenuation: Twisted Pair > Cable > Fiber (best)
Interference:
Worse with unguided… (the medium is shared!)
Number of receivers
In multi-point links of guided media:
More connected receivers introduce more attenuation

5. The Electromagnetic Spectrum
Microwaves
Light
10 GHz
10 KHz
100 MHz

6. Standard Multiplier Prefixes 1-18 to 10+18
exa- E 1018 = 1,000,000,000,000,000,000 peta- P 1015 = 1,000,000,000,000,000 tera- T 1012 = 1,000,000,000,000 giga- G 109 = 1,000,000,000 mega- M 106 = 1,000,000 kilo- K 103 = 1,000 milli- m 10-3 = micro = 0.000,001 nano- n 10-9 = 0.000,000,001 pico- p = 0.000,000,000,001 femto- f = 0.000,000,000,000,001 atto- a = 0.000,000,000,000,000,001

7. Electromagnetic Spectrum
Ultra violet,
X-Rays,
Gamma-Rays
Used for Communications

8. Study of Transmission Media
Physical description
Main applications
Main transmission characteristics

9. Guided Transmission Media
Twisted Pair
Coaxial cable
Optical fiber

10. Transmission Characteristics of Guided Media: Overview
Frequency Range
Typical Attenuation
Typical Delay
Repeater Spacing
Twisted pair (with loading coils)
0 to 3.5 kHz
0.2 1 kHz
50 µs/km
2 km
Twisted pair (No loading coils)
e.g. for ADSL
0 to 1 MHz
0.7 1 kHz
5 µs/km
2 km
Coaxial cable
0 to 500 MHz
7 10 MHz
4 µs/km
Up to 9 km
Optical fiber
186 to 370 THz
0.2 to 0.5 dB/km
5 µs/km
40 km
Larger
Operating
Frequencies
Lower
Attenuation
Same delay
(except with loading)
Fewer
Repeaters
Required

11. Twisted Pair (TP) Effect of loading with coils in series at intervals
But attenuation rises rapidly
Outside this narrow band.
No good for ASDL which tries to
get 1 MHz BW from the TP line!
Flat and low attenuation
Over the telephone voice band
( Hz)
(Passive Equalizer)

12. UTP Cables
unshielded

13. Twisted Pair - Applications
Most commonly used guided medium
Telephone network (Analog Signaling)
Between houses and the local exchange (subscriber loop)
Originally designed for analog signaling.
Digital data transmitted using modems at low data rates
Within buildings (short distances): (Digital Signaling)
To private branch exchange (PBX) (64 Kbps)
For local area networks (LAN) (10-100Mbps)
Example:
10BaseT: Unshielded Twisted Pair, 10 Mbps,100m range
Digital signal travels in its base band i.e. without modulating a carrier
(short distances)

14. Twisted Pair - Pros and Cons Compared to other guided media
Low cost
Easy to work with (pull, terminate, etc.)
Cons:
Limited bandwidth
Limited data rate
Large Attenuation
Limited distance range
Susceptible to interference and noise (exposed construction)

15. Twisted Pair - Transmission Characteristics
Analog Transmission
For analog signals only
Amplifiers every 5km to 6km
Bandwidth up to 1 MHz (several voice channels): ADSL
Digital Transmission
For either analog or digital signals (carrying digital data)
Repeaters every 2km or 3km
Data rates up to few Mbps (1Gbps: very short distance)
Impairments:
Attenuation: A strong function in frequency ( Distortion, need for equalization)
EM Interference: Crosstalk, Impulse noise, Mains interference, etc.

16. Attenuation in Guided Media
Thinner
Wires

17. Ways to reduce EM interference
WK 7
Ways to reduce EM interference
Shielding the TP with a metallic braid or sheathing
Twisting also reduces low frequency interference
Different twisting lengths for adjacent pairs help reduce crosstalk

18. STP: Metal Shield

19. Unshielded (UTP) and Shielded (STP)
Unshielded Twisted Pair (UTP)
Ordinary telephone wire: Abundantly available in buildings
Cheapest
Easiest to install
Suffers from external EM interference
Shielded Twisted Pair (STP)
Shielded with foil, metal braid or sheathing:
Reduces interference
Reduces attenuation at higher frequencies (increases BW)
 Better Performance:
Increased data rates used
Increased distances covered
 But becomes:
More expensive
Harder to handle (thicker, heavier)

20. TP Categories: EIA-568-A Standard (1995) (cabling of commercial buildings for data)
Cat 3: Unshielded (UTP)
Up to 16MHz
Voice grade
In most office buildings
Twist length of 7.5 cm to 10 cm
Cat 5: Unshielded (UTP)
Up to 100MHz
Data grade
Pre-installed now in many new office buildings
Twist length 0.6 cm to 0.85 cm
(Tighter twisting increases cost but improves performance)
Newer, shielded twisted pair: (150 W STP)
Up to 300MHz

21. Near End Crosstalk (NEXT)
Coupling of signal from one wire pair to another
Coupling takes place when a transmitted signal entering a pair couples into an adjacent receiving pair at the same end
i.e. near transmitted signal is picked up by near receiving pair
Transmitted Power, P1
Disturbing pair
Coupled Received Power, P2
Disturbed pair
“NEXT” Attenuation = 10 log P1/P2 dBs
The larger … the smaller the crosstalk (The better the performance)
“NEXT” attenuation is a desirable attenuation- The larger the better!

22. Transmission Properties for Shielded & Unshielded TP
Undesirable Attenuation- Smaller is better
Desirable Attenuation- Larger is better!
Signal Attenuation (dB per 100 m)
Near-end Crosstalk Attenuation (dB)
Frequency (MHz)
Category 3 UTP
Category 5 UTP
150-ohm STP 1
2.6
2.0
1.1 41 62
68? 4
5.6
4.1
2.2 32 53 58 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

23. Newer 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
UTP: Unshielded Twisted Pair
FTP: Foil Twisted Pair
SSTP: Shielded-Screen Twisted Pair

24. Coaxial Cable Physical Description: 1 - 2.5 cm Designed for operation
over a wider frequency range

25. Physical Description

26. Coaxial Cable Applications
Most versatile medium:
Television distribution (FDM Broadband)
Cable TV (CATV): 100’s of TV channels over 10’s Kms
Long distance telephone transmission
Can carry 10s of thousands of voice channels simultaneously (though FDM multiplexing) (Broadband)
Now facing competition from optical fibers and terrestrial microwave links
Local area networks, e.g. Thickwire Ethernet cable
(10Base5): 10 Mbps, Baseband signal, 500m segment
(5 time TP distance)

27. Coaxial Cable - Transmission Characteristics: Improvements over TP
Extended frequency range
Up to 500 MHz
Reduced EM interference and crosstalk
Due to enclosed concentric construction
EM fields terminate within cable and do not stray outside causing interference
Remaining limitations:
Attenuation
Thermal and inter modulation noise (FDM)

28. Attenuation in Guided Media

29. Coaxial Cable - Transmission Characteristics
Analog Transmission
Amplifiers every few kms
Closer amplifier spacing for higher operating frequencies
Digital Transmission
Repeater every 1km
Closer repeater spacing for higher data rates

30. Optical Fiber A thin (2-125 mm) flexible strand of glass or plastic
Light entering at one end travels confined within the fiber until it leaves it at the other end
As fiber bends around corners, the light remains within the fiber through multiple internal reflections
Lowest losses (attenuation) with ultra pure fused silica glass… but expensive and more difficult to manufacture
Reasonable losses with multi-component glass and with plastic
Quality, Cost, Difficulty of Handling
Attenuation (Loss)
Pure
Glass
Multi-component Glass
Plastic

31. Optical Fiber: Construction
An optical fiber consists of three main parts
Core
A narrow cylindrical strand of glass/plastic, with refractive index n1
Cladding
A tube surrounding each core, with refractive index n2
The core must have a higher refractive index than the cladding to keep the light beam trapped inside: n1 > n2
Protective outer jacket
Protects against moisture, abrasion, and crushing
Single Fiber Cable
Individual Fibers:
(Each having its core & Cladding)
Multiple Fiber Cable
Important: Each core surrounded by a cladding

32. Reflection and Refraction
At a boundary between a denser (n1) and a rarer (n2) medium, n1 > n2 (e.g. water-air, optical fiber core-cladding) a ray of light will be refracted or reflected depending on the incidence angle
Increasing Incidence angle, 1
Angles
With the
Normal
rarer 2
v2 = c/n2 n2
denser 1 2 n1
critical
n1 > n2 1
v1 = c/n1
Total internal reflection
Refraction
Critical angle

33. Optical Fiber Denser n1 n1 > n2 Refraction at boundary
for Escaping light is absorbed in jacket
i <
critical
Rarer n2
Denser
Denser n1 n1
Rarer i
Total Internal Reflection at
core-cladding boundary for
i >
critical
n1 > n2

34. Attenuation in Guided Media
Larger
Frequency

35. Optical Fiber - Benefits
Greater capacity
Fiber: 100’s of Gbps over 10’s of Kms
Cable: 100’s of Mbps over 1’s of Kms
Twisted pair: 100’s of Mbps over 10’s of meters
Lower/more uniform* attenuation (Fig. 4.3c)
An order of magnitude lower
Relatively constant over a larger range of frequencies*
Electromagnetic isolation
Not affected by external EM fields:
No interference, impulse noise, crosstalk
Does not radiate:
Not a source of interference
Difficult to tap (data security)
* With careful selection of operating band

36. Optical Fiber – Benefits, Contd.
Greater repeater spacing: Lower cost, Fewer Units
Fiber: ’s of Kms
Cable, Twisted pair: 1’s Kms
Smaller size and weight:
An order of magnitude thinner for same capacity
Useful in cramped places
Reduced cost of digging in populated areas
Reduced cost of support structures

37. Optical Fiber - Applications
Long-haul trunks
Telephone traffic over long routes between cities, and undersea:
Fiber & Microwave now replacing coaxial cable
 1500 km, Up to 60,000 voice channels
Metropolitan trunks
Joining exchanges inside large cities:
 12 km, Up to 100,000 voice channels
Rural exchange trunks
Joining exchanges of towns and villages:
 km, Up to 5,000 voice channels
Subscriber loops
Joining subscribers to exchange:
Fiber replacing TP, allowing all types of data
LANs, Example:
10BaseF 10 Mbps, 2000 meter segment
Exchange
City
City
Main
Exchange

38. Optical Fiber - Transmission Characteristics
Acts as a wave guide for light (1014 to 1015 Hz)
Covers portions of infrared and visible spectrum
Transmission Modes:
Single Mode
Multimode
Step Index
Graded Index

39. Optical Fiber Transmission Modes
Dispersion:
Spread in ray
arrival time
Refraction
Deep reflection
Shallow reflection n2
i <
critical n1
Large Spread
Core
Cladding
2 ways: n1 n2
Curved path: n is not uniform- decreasing
Smaller
v = c/n
n1 is made lower away from center…this speeds up deeper rays
and compensates for their larger distances, arrive together with shallower rays
Smallest
Smaller spread  Narrower pulses
Higher data rates supported

40. Optical Fiber – Transmission modes
Spread of received light pulse in time (dispersion) is bad:
Causes inter-symbol interference  bit errors (similar to delay distortion)
Limits usable data rate and usable transmission distance
Caused by propagation through multiple reflections at different angles of incidence
Dispersion increases with:
Larger distance traveled
Thicker fibers with step index
Less focused sources
Can be reduced by:
Limiting the distance
Thinner fibers and a highly focused light source
 Single mode (in the limit): High data rates, very long distances
Or Graded-index multimode thicker fibers: The half-way (lower cost) solution

41. Optical Fiber Transmission System– Light Source + Fiber + Light Detector
Light Sources
Light Emitting Diode (LED)
Incoherent light- More dispersion  Lower data rates
Low cost
Wider operating temp range
Longer life
Injection Laser Diode (ILD)
Coherent light- Less dispersion  Higher data rate
More efficient
Faster switching  Higher data rate

42. Optical Fiber – Wavelength Division Multiplexing (WDM)
A form of FDM (Channels sharing the medium by occupying different frequency bands)
Multiple light beams at different light frequencies (wavelengths) transmitted on the same fiber
Each beam forms a separate communication channel
Separated at destination by filters
Example:
256 channels
@ 40 Gbps each
 10 Tbps total data rate
WDM

43. Optical Fiber – Four Transmission bands (windows) in the Infrared (IR) region
Band selection is a system decision based on:
Attenuation of the fiber
Properties of the light sources
Properties of the light receivers S C L
Bandwidth, THz 33 12 4 7
Note: l in fiber = v/f = (c/n)/f = (c/f)/n = l in vacuum/n
i.e. l in fiber < l in vacuum

44. Wireless Transmission
Free-space is the transmission medium
Need efficient radiators, called antennas
Signal fed from transmission line (wireline) and radiated it into free-space (wireless)
Popular applications
Radio & TV broadcast
Cellular Communications
Microwave Links
Wireless Networks

45. Wireless Transmission Frequency Ranges
Radio: 30 MHz to 1 GHz
Omni directional
Broadcast radio
Microwaves: 1 GHz to 40 GHz
Highly directional beams
Point to point (Terrestrial)
Satellite
Infrared Light: 0.3 THz to 20 THz (below light)
Localized communications (confined spaces)

46. Antennas
Electrical conductor (or system of conductors) used to radiate / collect electromagnetic energy into/from surrounding space
Transmission
Radio frequency electrical energy from transmitter
Converted into electromagnetic energy
Radiated into surrounding space
Reception
Electromagnetic energy impinging on antenna
Converted to radio frequency electrical energy
Fed to receiver
Same antenna often used for both TX and RX in 2-way communication systems

47. Radiation Pattern
Power radiated in all directions, but usually not with the same efficiency
Isotropic antenna
A hypothetical point source in space
(Small dimensions relative to l)
Radiates equally in all directions – A spherical radiation pattern
Used as a reference for other antennae
Directional Antenna
Concentrates radiation in a given desired direction – hence point-to-point, line of sight
communications
Gives antenna ‘gain’ in that direction
relative to isotropic for both TX and RX
Larger dimensions relative to l  Greater directivity
Radiation Patterns
Isotropic
Directional

48. Parabolic Reflective Antenna
WK 8
Used for terrestrial and satellite microwave
Source placed at the focal point will produce waves that get reflected from parabola parallel to the parabola axis
Creates a (theoretically) parallel beam of light/sound/radio that does not spread (disperse) in space
In practice, some divergence (dispersion) occurs, because source at focus has a finite size (not exactly a point!)
On reception, only signal from the axis direction is concentrated at focus, where detector is placed Signals from other directions miss the focus  negligible O/P
The larger the antenna
(in wavelengths) the better
the directionality  so, using
Higher frequency is advantageous
Focus
Parabola

49. Parabolic Reflective Antenna
WK 8
Axis

50. Antenna Gain, G A measure of antenna directionality
Power output of the antenna in a particular direction compared to that produced by a perfect isotropic antenna
Can be expressed in decibels (dB, dBi) (i = relative to isotropic)
Increased power radiated in one direction causes less power radiated in another direction (Total power is fixed)
Effective area Ae:
Related to size and shape of antenna
Determines the antenna gain,
Ae is the effective area

51. Antenna Gain, G: Effective Areas
An isotropic antenna has a gain G = 1 (0 dBi)
i.e.
A parabolic antenna has:
Substituting we get:
Gain in dBi = 10 log G
Important: Gains apply to both TX and RX antennas
A = Actual Area = p r2

52. Propagation Attenuation
As signal propagates in space, its power drops with distance according to the inverse square law
While with a guided medium, signal drops exponentially with distance… giving larger attenuation and lower repeater spacing
d’ = distance in l’s
i.e. loss in signal power over distance traveled, d
Show that L increases by 6 dBs for every doubling of distance d.
For guided medium, corresponding attenuation = a d dBs, a in dBs/km
A disadvantage for operating at higher frequency?

53. Microwave Link Calculations
TX-RX Net attenuation, A = L – G1 – G2, dBs
S dBm = P dBm – A dBs

54. Terrestrial Microwave
Parabolic dish
Focused beam (with antenna gain)
Line of sight requirement:
Beam should not be obstructed
Curvature of earth limits maximum range  Use relays to increase range (multi-hop link)
Link performance sensitive to antenna alignment
Applications:
Long haul telecommunications Many voice/data channels over long distances between large cities, possibly through intermediate relays: Competes with coaxial cable and fiber
Short wireless links between buildings:
CCTV links
Wireless links between LANs in close-by buildings
Cellular Telephony

55. Terrestrial Microwave: Transmission Properties
GHz
Higher f Advantages:
Larger bandwidth, B  higher data rate (Table 4.6)
Smaller l  smaller (lighter, cheaper) antenna for a required antenna gain (see gain eqn.)
But Higher f  larger attenuation due propagation and absorption by rain
So,
Long-haul links (long distances) operate at lower frequencies (4-6 GHz,11 GHz) to avoid large attenuation
Short links between close-by buildings operate at higher frequencies (e.g. 22 GHz) (Attenuation is not a big problem for the short distances, smaller antenna size)

56. Satellite Microwave Satellite is used as a relay station for the link
Satellite receives on one frequency (uplink), amplifies or repeats signal and re-transmits it on another frequency (downlink)
Spatial angular separation (e.g. 3) to avoid interference from neighboring TXs
Require a geo-stationary orbit (satellite rotates at the same speed of earth rotation, so appears stationary):
Height: 35,784km (long link, large transmission delays)
Applications:
Television direct broadcasting
Long distance telephony
Private business networks linking multiple company sites worldwide

57. a. Satellite Point to Point Link
Relay
Downlink
Uplink
Earth curvature
Obstructs line of sight
for large distances

58. b. Satellite Broadcast Link
Direct Broadcasting Satellite

59. Transmission Characteristics
1-10 GHz
Frequency Trade offs:
Lower frequencies: More noise and interference
Higher frequencies: Larger rain attenuation, but smaller antennas
Downlink/Uplink frequencies recently going higher: /6 GHz  12/14  20/ (better receivers becoming available)
Delay = 0.25 s  noticeable for telephony
Inherently a broadcasting facility

60. Broadcast Radio: 30 MHz – 1 GHz
Omni directional (no need for antenna directionality horizontally)
No dishes
No line of sight requirement
No antenna alignment requirement/problems
Applications:
FM radio
UHF and VHF television
Choice of frequency range:
Reflections from ionosphere < 30 MHz -1 GHz < Rain
Propagation attenuation:
Lower than for Microwaves (as l is larger)
Problems caused by omni directionality: Interference due to
multi-path reflections
e.g. TV ghost images

61. Multi-Path effects of omni-directionality
Omni-Directional TV Broadcasting
Antenna
TV ghost images

62. Infrared Data Modulates a non coherent infrared light
Relies on line of sight (or reflections through walls or ceiling)
Blocked by walls (unlike microwaves)
No licensing required for frequency allocation
Applications:
TV remote control
Wireless LAN within a room