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

2. Remaining five chapters:
Chapter 7: Data Link: Flow and Error control,
Link management
Data Link
Chapter 8: Improved utilization: Multiplexing
Physical Layer
Chapter 6: Data Communication: Synchronization, Error detection and correction
Chapter 4: Transmission Media
Transmission Medium
Chapter 5: Encoding: From data to signals
Chapter 3: Signals, their representations, their transmission over media, Impairments

3. Agenda Overview Guided Transmission Media
Twisted Pair
Coaxial Cable
Optical Fiber
Unguided (open space, wireless) Transmission
Antennas
Terrestrial Microwave
Satellite Microwave
Broadcast Radio
Infrared

4. Overview
Media:
Guided - wire
Unguided - wireless
Transmission characteristics and quality determined by:
Signal
Medium
For guided, the medium is important
For unguided, the antenna is important

5. Design Issues Key communication objectives are:
High data rate
Low error rate
Long distance
Bandwidth: Tradeoff - Larger for higher data rates
- But smaller for low link cost
Transmission impairments
Attenuation: Twisted Pair > Cable > Fiber (best)
Interference and Cross talk: Twisted Pair > Cable > Fiber (best)
Worse with unguided… (the medium is shared!)
Number of receivers
In multi-point links of guided media:
Attenuation increases with increased number of connected receivers

6. Part of the Electromagnetic Spectrum
1 KHz
1 MHz
1 GHz
1 THz
Guided
Unguided l
V = l f

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

8. Guided Transmission Media
Twisted Pair
Coaxial cable
Optical fiber

9. Transmission Characteristics of Guided Media: Overview
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
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

10. Twisted Pair (TP)

11. UTP Cables
unshielded

12. Twisted Pair - Applications
The most commonly used guided medium
Telephone network (Analog Signaling)
Analog Data (original purpose) : Between houses and the local exchange, e.g. 5 km (subscriber loop)
Digital Data: Transmitted using modems, low data rates
Within buildings (short distances) (Digital Signaling)
To private branch exchange (PBX) (64 Kbps)
For local area networks (LAN) (10-100Mbps)
Example, Ethernet:
10BaseT: Unshielded Twisted Pair, 10 Mbps,100m range

13. 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
Limited distance range (due to large attenuation)
Susceptible to interference and noise

14. Twisted Pair - Transmission Characteristics
Analog Transmission Mode
For analog signals only
Amplifiers every 5km to 6km
Bandwidth up to 1 MHz (several voice channels): ADSL (Ch 8)
Digital Transmission Mode
Using either analog or digital signals
Repeaters every 2km or 3km
Data rates up to few Mbps (1Gbps: over very short distances)
Impairments:
Attenuation: A strong function in frequency, can be modified with loading coils
EM Interference: Crosstalk, Impulse noise, Mains interference

15. Attenuation in Guided Media

16. Attenuation in Twisted pairs (unloaded)
Thinner Wires
Wire Gauge
Wire Diameter
300
1 KHz
3400
1 MHz
Telephone Voice

17. Ways to reduce EM interference
Shielding the TP with a metallic braid or sheathing
Twisting reduces low frequency interference
Tighter twisting  Better performance
Using different twisting lengths for adjacent pairs to 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 metal braid or sheathing:
Reduces interference
Reduces attenuation at higher frequencies
 Better Performance:
Increases data rates used
Increases distances covered
But becomes:
More expensive
Harder to handle (thicker, heavier)
Transmit faster and go further!

20. UTP Categories: EIA-568-A Standard (1995) (cabling of commercial buildings for data)
Up to 16MHz
Voice grade
In most office buildings
Twist length: 7.5 cm to 10 cm
Cat 4
Up to 20 MHz
Cat 5
Up to 100MHz
Data grade
Pre-installed in many new office buildings
Twist length: 0.6 cm to 0.85 cm
(Tighter twisting increases cost but improves performance)

21. Near End CrossTalk (NEXT)
Coupling of signal from a wire pair to an adjacent pair
Coupling takes place when a transmitted signal entering a pair couples (leaks) into an adjacent receiving pair on the same (near) end of the link
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
Better
Better
Better
Better
Not Usable
Not Usable

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 rage

25. Physical Description

26. Coaxial Cable Applications
Frequency Division Multiplexing
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

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 out
Remaining limitations:
Attenuation
Thermal and noise
Inter modulation noise (especially for broadband operation)
Because of FDM

28. Attenuation in Guided Media

29. Coaxial Cable - Transmission Characteristics
Analog Transmission
Amplifiers every few kms
Closer spacing for higher frequency
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 at the other end
As fiber bends around corners, the light stays within the fiber
Lowest losses (attenuation) with ultra pure fused silica glass… but difficult to manufacture
Reasonable losses and performance with multi-component glass and with plastic
Cost, Difficulty of Handling
Attenuation (Loss)
Pure
Glass
Multi-component Glass
Plastic
Best Performance

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 core & Cladding)
Multiple Fiber Cable
(Note multiple cladding)

32. Reflection and refraction of light
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,
Shallower
Incidence 1
rarer 2
v2 = c/n2 n2
denser 1 2 n1
critical
n1 > n2 1
v1 = c/n1
Total internal reflection
Refraction
Critical angle refraction

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 boundary
for
i >
critical
n1 > n2

34. Attenuation in Guided Media
Which side is the IR?
Compare attenuation ranges!

35. Optical Fiber - Benefits
Greater channel capacities over larger distances
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
frequency interval
Electromagnetic isolation
Fiber is not affected by external EM fields:
No interference, impulse noise, crosstalk
Fiber does not radiate (light ray trapped inside):
Not a source of interference
Difficult to tap (data security)
But what could happen at the repeater?

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

37. Optical Fiber - Applications
Long-haul trunks between cities
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 traffic
LANs, Example:
10BaseF 10 Mbps, 2000 meter segment
Exchange
City
City
Competition:
Fiber, Coaxial, m Waves
Main
Exchange
Compare segment length with twisted pair and coaxial!

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
Single ray
Step Index
Graded Index
Multiple rays

39. Optical Fiber Transmission Modes
Dispersion:
Spread in ray
arrival time
Refraction
Deep reflection
Shallow reflection n2
i <
critical n1
Large
Core
n1 > n2
Cladding
2 ways to reduce
dispersion: n1 n2
Smaller
v = c/n
Make n1 lower away from center…this speeds up deeper rays
and compensates for their larger distances, arrive together with shallower rays
Smallest

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

41. The transmission system is not just the medium (fiber)
The transmission system is not just the medium (fiber)! We have also light Sources and detectors…
Light Sources
Light Emitting Diode (LED)
Lower cost, longer life
Wider operating temp range
Injection Laser Diode (ILD)
More efficient (more light power for same electric power input)
Faster switching  Higher data rate

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

43. Optical Fiber – Four Transmission bands (windows) in the Infrared (IR) region
Selection 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
l values shown are in vacuum

44. Wireless Transmission
Free-space is the transmission medium
Need efficient radiators, called antennas / aerials
Signal fed from transmission line (wireline) and radiated 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 (just below visible light)
Localized communications (confined spaces)

46. Antennas
Electrical conductor (or system of conductors) used to radiate / collect electromagnetic energy into/from the environment (TX/RX operation)
Transmission
Radio frequency electrical energy obtained from transmitter
Converted into electromagnetic energy
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 TX and RX in 2-way communication systems

47. Radiation Pattern
Power radiated in different directions, usually not with the same efficiency:
Isotropic (Omni-directional) antenna
A hypothetical point source in space
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 ‘gain’ in that direction
relative to isotropic
Radiation Patterns
Point Source
Isotropic
Directional

48. Parabolic Reflective Antenna
Focus
Parabola
(The Dish!)
Axis

49. Parabolic Reflective Antenna (The Dish!)
Used for terrestrial and satellite microwave
On Transmission: Source placed at the focus will produce waves that get reflected from parabola parallel to the parabola axis
Creates a (theoretically) parallel beam to the parabola axis that does not spread (disperse) in space ( Zero radiation off axis)
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 ( Zero o/p off axis)
i.e. Directionality in both TX, RX operation
The larger the antenna
(in wavelengths) the better
the directionality
 High frequency is advantageous
Focus
Parabola

50. Antenna Gain, G A measure of directionality of the antenna
Power output in a given 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 other directions (Total power is fixed)
Gain Gq depends on the effective area (Ae) of the antenna:
Depends on size and shape of the antenna
The Radiation pattern

51. Antenna Gain, G in the direction of maximum radiation
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 of antenna circle = p r2
Higher frequencies  higher gains

52. Terrestrial Microwave
Parabolic dish
i.e. Focused beam
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 affected by antenna alignment (effect of wind!)
Applications:
Long haul telecommunications Many channels over long distances between large cities, possibly through intermediate relays:Competes with coaxial cable and fiber
Short links between buildings:
CCTV links
Links between LANs in different buildings
Cellular Telephony

53. Terrestrial Microwave: Transmission Properties
GHz: f range used depends on application
Advantages of higher frequencies :
Larger bandwidth, B  higher data rate (Table 4.6)
Smaller l  smaller (lighter, cheaper) antenna for a given antenna gain (see gain eqn)
But Higher f  larger attenuation due 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 (22 GHz) (Attenuation is not a big problem for the short distances, reduces antenna size)

54. Terrestrial Microwave: 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 = dBs/km

55. Satellite Microwave
Satellite is used as a relay station for the link to avoid limitation on distance due to earth curvature
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

56. Satellite Point to Point Link
Relay
Downlink
Uplink
Earth curvature (overcome)

57. Satellite Broadcast Link
Direct Broadcasting Satellite

58. Transmission Characteristics
1-10 GHz
Frequency Trade offs:
Lower frequencies: Noise and interference
Higher frequencies: Smaller antenna, but rain attenuation,
Downlink/Uplink frequencies recently going higher: /6 GHz  12/14  20/30 (due to better receivers)
Delay = 0.25 s  noticeable for telephony
Inherently a broadcasting facility

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

60. Multi-Path effects due to omni-directionality
Omni-Directional Broadcasting
Antenna
TV ghost images

61. Infrared Non coherent infrared light used as carrier
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