1. COE 341: Data & Computer Communications Dr. Marwan Abu-Amara
Chapter 4:
Transmission Media

2. COE 341 – Dr. Marwan Abu-Amara
Agenda
Overview
Guided Transmission Media
Twisted Pair
Coaxial Cable
Optical Fiber
Wireless Transmission
Antennas
Terrestrial Microwave
Satellite Microwave
Broadcast Radio
Infrared
COE 341 – Dr. Marwan Abu-Amara

3. COE 341 – Dr. Marwan Abu-Amara
Overview
Media
Guided - wire
Unguided - wireless
Transmission characteristics and quality determined by:
Medium
Signal
For guided, the medium is more important
For unguided, the bandwidth produced by the antenna is more important
Key concerns are data rate and distance
COE 341 – Dr. Marwan Abu-Amara

4. COE 341 – Dr. Marwan Abu-Amara
Design Issues
Key communication objectives are:
High data rate
Low error rate
Long distance
Bandwidth: Tradeoff - Larger for higher data rates
- But smaller for economy
Transmission impairments
Attenuation: Twisted Pair > Cable > Fiber (best)
Interference: Worse with unguided… (medium is shared!)
Number of receivers
In multi-point links of guided media:
More connected receivers introduce more attenuation
COE 341 – Dr. Marwan Abu-Amara

5. Electromagnetic Spectrum
COE 341 – Dr. Marwan Abu-Amara

6. Study of Transmission Media
Physical description
Main applications
Main transmission characteristics
COE 341 – Dr. Marwan Abu-Amara

7. Guided Transmission Media
Twisted Pair
Coaxial cable
Optical fiber
COE 341 – Dr. Marwan Abu-Amara

8. 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
COE 341 – Dr. Marwan Abu-Amara

9. COE 341 – Dr. Marwan Abu-Amara
Twisted Pair
COE 341 – Dr. Marwan Abu-Amara

10. COE 341 – Dr. Marwan Abu-Amara
UTP Cables
COE 341 – Dr. Marwan Abu-Amara

11. COE 341 – Dr. Marwan Abu-Amara
UTP Connectors
COE 341 – Dr. Marwan Abu-Amara

12. COE 341 – Dr. Marwan Abu-Amara
Note: Pairs of Wires
It is important to note that these wires work in pairs (a transmission line)
Hence, for a bidirectional link
One pair is used for TX
One pair is used for RX
COE 341 – Dr. Marwan Abu-Amara

13. Twisted Pair - Applications
Most commonly used guided medium
Telephone network (Analog Signaling)
Between house and local exchange (subscriber loop)
Within buildings (Digital Signaling)
To private branch exchange (PBX)
For local area networks (LAN)
10Mbps or 100Mbps
Range: 100m
COE 341 – Dr. Marwan Abu-Amara

14. Twisted Pair - Pros and Cons
Cheap
Easy to work with
Cons:
Limited bandwidth
Low data rate
Short range
Susceptible to interference and noise
COE 341 – Dr. Marwan Abu-Amara

15. Twisted Pair - Transmission Characteristics
Analog Transmission
Amplifiers every 5km to 6km
Digital Transmission
Use either analog or digital signals
Repeater every 2km or 3km
Limited distance
Limited bandwidth (1MHz)
Limited data rate (100Mbps)
Susceptible to interference and noise
COE 341 – Dr. Marwan Abu-Amara

16. Attenuation in Guided Media
COE 341 – Dr. Marwan Abu-Amara

17. Ways to reduce EM interference
Shielding the TP with a metallic braid or sheathing
Twisting reduces low frequency interference
Different twisting lengths for adjacent pairs help reduce crosstalk
COE 341 – Dr. Marwan Abu-Amara

18. 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)
COE 341 – Dr. Marwan Abu-Amara

19. COE 341 – Dr. Marwan Abu-Amara
STP: Metal Shield
COE 341 – Dr. Marwan Abu-Amara

20. COE 341 – Dr. Marwan Abu-Amara
UTP Categories
Cat 3
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
COE 341 – Dr. Marwan Abu-Amara

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 back to an adjacent receiving pair at the same end
i.e. near transmitted signal is picked up by near receiving pair
Disturbing pair
Disturbed pair
Transmitted Power, P1
Coupled Received Power, P2
“NEXT” Attenuation = 10 log P1/P2 dBs
The larger … the smaller the crosstalk (i.e. the better the performance)
NEXT attenuation is a desirable attenuation - The larger the better!
COE 341 – Dr. Marwan Abu-Amara

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
COE 341 – Dr. Marwan Abu-Amara

23. 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
COE 341 – Dr. Marwan Abu-Amara

24. COE 341 – Dr. Marwan Abu-Amara
Coaxial Cable
Physical Description:
COE 341 – Dr. Marwan Abu-Amara

25. COE 341 – Dr. Marwan Abu-Amara
Physical Description
COE 341 – Dr. Marwan Abu-Amara

26. Coaxial Cable Applications
Most versatile medium
Television distribution
Cable TV
Long distance telephone transmission
Can carry 10,000 voice calls simultaneously (though FDM multiplexing)
Being replaced by fiber optic
Short distance computer systems links
Local area networks (thickwire Ethernet cable)
COE 341 – Dr. Marwan Abu-Amara

27. Coaxial Cable - Transmission Characteristics
Analog
Amplifiers every few km
Closer if higher frequency
Up to 500MHz
Digital
Repeater every 1km
Closer for higher data rates
COE 341 – Dr. Marwan Abu-Amara

28. Attenuation in Guided Media
COE 341 – Dr. Marwan Abu-Amara

29. COE 341 – Dr. Marwan Abu-Amara
Optical Fibers
An optical fiber is a very thin strand of silica glass
It is a very narrow, very long glass cylinder with special characteristics. When light enters one end of the fiber it travels (confined within the fiber) until it leaves the fiber at the other end
Two critical factors stand out:
Very little light is lost in its journey along the fiber
Fiber can bend around corners and the light will stay within it and be guided around the corners
An optical fiber consists of three parts
The core
Narrow cylindrical strand of glass with refractive index n1
The cladding
Tubular jacket surrounding the core with refractive index n2
The core must have a higher refractive index than the cladding for the propagation to happen
COE 341 – Dr. Marwan Abu-Amara

30. Optical Fibers (Contd.)
Protective outer jacket
Protects against moisture, abrasion, and crushing
Individual Fibers:
(Each having core & Cladding)
Single Fiber Cable
Multiple Fiber Cable
COE 341 – Dr. Marwan Abu-Amara

31. 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
rarer 2
v2 = c/n2 n2
denser 1 2 n1
critical
n1 > n2 1
v1 = c/n1
Refraction
Critical angle refraction
Total internal reflection
COE 341 – Dr. Marwan Abu-Amara

32. COE 341 – Dr. Marwan Abu-Amara
Optical Fiber
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
COE 341 – Dr. Marwan Abu-Amara

33. Attenuation in Guided Media
COE 341 – Dr. Marwan Abu-Amara

34. Optical Fiber - Benefits
Greater capacity
Data rates of hundreds of Gbps
Smaller size & weight
Lower attenuation
An order of magnitude lower
Relatively constant over a larger frequency interval
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)
Greater repeater spacing
10s of km at least
COE 341 – Dr. Marwan Abu-Amara

35. Optical Fiber - Applications
Long-haul trunks
Metropolitan trunks
Rural exchange trunks
Subscriber loops
LANs
COE 341 – Dr. Marwan Abu-Amara

36. Optical Fiber - Transmission Characteristics
Act as wave guide for light (1014 to 1015 Hz)
Covers portions of infrared and visible spectrum
Light Emitting Diode (LED)
Cheaper
Wider operating temp range
Last longer
Injection Laser Diode (ILD)
More efficient
Greater data rate
COE 341 – Dr. Marwan Abu-Amara

37. Optical Fiber Transmission Modes
Dispersion:
Spread in arrival
time
Refraction
Deep reflection
Shallow reflection n2
i <
critical n1
Large
Core
Cladding
2 ways: n1 n2
Smaller
v = c/n
n1 lower away from center…this speeds up deeper rays
and compensates for their larger distances, arrive together with shallower rays
Smallest
COE 341 – Dr. Marwan Abu-Amara

38. 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
Can be reduced by:
Limiting the distance
Thinner fibers and a highly focused light source
 Single mode: High data rates, very long distances
Graded-index thicker fibers: The half-way solution
COE 341 – Dr. Marwan Abu-Amara

39. Optical Fiber – 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
COE 341 – Dr. Marwan Abu-Amara

40. COE 341 – Dr. Marwan Abu-Amara
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
COE 341 – Dr. Marwan Abu-Amara

41. Attenuation in Guided Media
COE 341 – Dr. Marwan Abu-Amara

42. Wireless Transmission
Free-space is the transmission medium
Need efficient radiators, called antenna, to take signal from transmission line (wireline) and radiate it into free-space (wireless)
Famous applications
Radio & TV broadcast
Cellular Communications
Microwave Links
Wireless Networks
COE 341 – Dr. Marwan Abu-Amara

43. Wireless Transmission Frequencies
Radio: 30MHz to 1GHz
Omni-directional
Broadcast radio
Microwave: 2GHz to 40GHz
Microwave
Highly directional
Point to point
Satellite
Infrared Light: 3 x 1011 to 2 x 1014
Localized communications
COE 341 – Dr. Marwan Abu-Amara

44. COE 341 – Dr. Marwan Abu-Amara
Antennas
Electrical conductor (or system of..) used to radiate/collect electromagnetic energy
Transmission
Radio frequency electrical 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 TX and RX in 2-way communication systems
COE 341 – Dr. Marwan Abu-Amara

45. COE 341 – Dr. Marwan Abu-Amara
Radiation Pattern
Power radiated in all directions
Not same performance in all directions
Isotropic antenna is (theoretical) point in space
Radiates in all directions equally
Gives spherical radiation pattern
Used as a reference for other antennae
Directional Antenna
Concentrates radiation in a given desired direction
Used for point-to-point, line of sight communications
Gives “gain” in that direction
relative to isotropic
Radiation Patterns
Isotropic
Directional
COE 341 – Dr. Marwan Abu-Amara

46. Parabolic Reflective Antenna
Used for terrestrial and satellite microwave
Source placed at focus will produce waves reflected from parabola parallel to axis
Creates (theoretical) parallel beam of light/sound/radio
In practice, some divergence (dispersion) occurs, because source at focus has a finite size (not exactly a point!)
On reception, signal is concentrated at focus, where detector is placed
The larger the antenna (in wavelengths) the better
the directionality
COE 341 – Dr. Marwan Abu-Amara

47. Parabolic Reflective Antenna
Axis
COE 341 – Dr. Marwan Abu-Amara

48. COE 341 – Dr. Marwan Abu-Amara
Antenna Gain, G
Measure of directionality of antenna
Power output in particular direction compared with that produced by isotropic antenna
Measured in decibels (dB)
Increased power radiated in one direction causes less power radiated in another direction (Total power is fixed)
Effective area, Ae, relates to size and shape of antenna
Determines antenna gain
COE 341 – Dr. Marwan Abu-Amara

49. 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
COE 341 – Dr. Marwan Abu-Amara

50. Terrestrial Microwave
Parabolic dish
Focused beam
Line of sight
Curvature of earth limits maximum range  Use relays to increase range (multi-hop link)
Long haul telecommunications
Higher frequencies give higher data rates but suffers from larger attenuation
COE 341 – Dr. Marwan Abu-Amara

51. 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
COE 341 – Dr. Marwan Abu-Amara

52. COE 341 – Dr. Marwan Abu-Amara
Satellite Microwave
Satellite is relay station
Satellite receives on one frequency (uplink), amplifies or repeats signal and transmits on another frequency (downlink)
Requires geo-stationary orbit
Height of 35,784km
Applications
Television
Long distance telephone
Private business networks
COE 341 – Dr. Marwan Abu-Amara

53. Satellite Point to Point Link
Relay
Downlink
Uplink
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54. Satellite Broadcast Link
COE 341 – Dr. Marwan Abu-Amara

55. COE 341 – Dr. Marwan Abu-Amara
Broadcast Radio
Omni-directional
No dishes
No line of sight requirement
No antenna alignment
Applications
FM radio
UHF and VHF television
Suffers from multipath interference
Reflections (e.g. TV ghost images)
COE 341 – Dr. Marwan Abu-Amara

56. COE 341 – Dr. Marwan Abu-Amara
Infrared
Modulate non-coherent infrared light
Line of sight (or reflection)
Blocked by walls
No licensing required for frequency allocation
Applications
TV remote control
IRD port
COE 341 – Dr. Marwan Abu-Amara