1. Computer Communication Networks
Data Transmission, Media
Signal Encoding Techniques
Data Communication Techniques
Data Link Control, ATM
Multiplexing, Switching, Routing
Spread Spectrum, Wireless Networks
Local and Wide Area Networks
Data Transmission and Media

2. William Stallings Data and Computer Communications 7th Edition
Chapter 3
Data Transmission

3. Terminology (1) Transmitter Receiver Medium Guided medium
e.g. twisted pair, optical fiber
Unguided medium
e.g. air, water, vacuum

4. Terminology (2) Direct link No intermediate devices Point-to-point
Only 2 devices share link
Multi-point
More than two devices share the link

5. Terminology (3) Simplex One direction e.g. Television Half duplex
Either direction, but only one way at a time
e.g. police radio
Full duplex
Both directions at the same time
e.g. telephone

6. Frequency, Spectrum and Bandwidth
Time domain concepts
Analog signal
Varies in a smooth way over time
Digital signal
Maintains a constant level then changes to another constant level
Periodic signal
Pattern repeated over time
A-periodic signal
Pattern not repeated over time

7. Analogue & Digital Signals

8. Data Rate and Bandwidth
Any transmission system has a limited band of frequencies
This limits the data rate that can be carried

9. Analog and Digital Data Transmission
Entities that convey meaning
Signals
Electric or electromagnetic representations of data
Transmission
Communication of data by propagation and processing of signals

10. Analog and Digital Data
Continuous values within some interval
e.g. sound, video
Digital
Discrete values
e.g. text, integers

11. Analog and Digital Signals
Means by which data are propagated
Analog
Continuously variable
Various media
wire, fiber optic, space
Digital
Use two DC components

12. Advantages & Disadvantages of Digital Tx/Rx
Cheaper
Less susceptible to noise
Greater attenuation
Pulses become rounded and smaller
Leads to loss of information

13. Attenuation of Digital Signals

14. Components of Speech Frequency range (of hearing) 20Hz-20kHz
Speech 100Hz-7kHz
Easily converted into electromagnetic signal for transmission
Sound frequencies with varying volume converted into electromagnetic frequencies with varying voltage
Limit frequency range for voice channel Hz

15. Conversion of Voice Input into Analog Signal

16. Binary Digital Data From computer terminals etc. Two dc components
Bandwidth depends on data rate

17. Conversion of PC Input to Digital Signal

18. Data and Signals
We usually use digital signals for digital data and analog signals for analog data
Can use analog signal to carry digital data
Modem
Can use digital signal to carry analog data
Compact Disc audio

19. Analog Signals Carrying Analog and Digital Data

20. Digital Signals Carrying Analog and Digital Data

21. Analog Transmission
Analog signal transmitted without regard to content
May be analog or digital data
Attenuated over distance
Use amplifiers to boost signal
Also amplifies noise

22. Digital Transmission Concerned with content
Integrity endangered by noise, attenuation etc.
Repeaters used
Repeater receives signal
Extracts bit pattern
Retransmits
Attenuation is overcome
Noise is not amplified

23. Advantages of Digital Transmission
Digital technology
Low cost LSI/VLSI technology
Data integrity
Longer distances over lower quality lines
Capacity utilization
High bandwidth links economical
High degree of multiplexing easier with digital techniques
Security & Privacy
Encryption
Integration
Can treat analog and digital data similarly

24. Transmission Impairments
Signal received may differ from signal transmitted
Analog - degradation of signal quality
Digital - bit errors
Caused by
Attenuation and attenuation distortion
Delay distortion
Noise

25. Attenuation Signal strength falls off with distance Depends on medium
Received signal strength:
must be enough to be detected
must be sufficiently higher than noise to be received without error
Attenuation is an increasing function of frequency

26. Delay Distortion Only in guided media
Propagation velocity varies with frequency

27. Noise (1) Additional signals inserted between transmitter and receiver
Thermal
Due to thermal agitation of electrons
Uniformly distributed
Intermodulation
Signals that are the sum and difference of original frequencies sharing a medium

28. Noise (2) Crosstalk A signal from one line is picked up by another
Impulse
Irregular pulses or spikes
e.g. External electromagnetic interference
Short duration
High amplitude

29. Channel Capacity Data rate In bits per second
Rate at which data can be communicated
Bandwidth
In cycles per second of Hertz
Constrained by transmitter and medium

30. Required Reading
Stallings Chapter 3

31. William Stallings Data and Computer Communications 7th Edition
Chapter 4
Transmission Media

32. Overview Guided - wire Unguided - wireless
Characteristics and quality determined by medium and 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

33. Design Factors Bandwidth Higher bandwidth gives higher data rate
Transmission impairments
Attenuation
Interference
Number of receivers
In guided media
More receivers (multi-point) introduce more attenuation

34. Electromagnetic Spectrum

35. Guided Transmission Media
Twisted Pair
Coaxial cable
Optical fiber

36. Transmission Characteristics of Guided Media
Frequency Range
Typical Attenuation
Typical Delay
Repeater Spacing
Twisted pair
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

37. Twisted Pair

38. Twisted Pair - Applications
Most common medium
Telephone network
Between house and local exchange (subscriber loop)
Within buildings
To private branch exchange (PBX)
Multi-pair TP For local area networks (LAN)
10Mbps or 100Mbps

39. Twisted Pair - Pros and Cons
Cheap
Easy to work with
Low data rate
Short range

40. Twisted Pair - Transmission Characteristics
Analog
Amplifiers every 5km to 6km
Digital
Use either analog or digital signals
repeater every 2km or 3km
Limited distance
Susceptible to interference and noise

41. Near End Crosstalk Coupling of signal from one pair to another
Coupling takes place when transmit signal entering the link couples back to receiving pair
i.e. near transmitted signal is picked up by near receiving pair

42. 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)

43. 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

44. 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

45. Coaxial Cable

46. Coaxial Cable Applications
Most versatile medium
Television distribution
Ariel to TV
Cable TV
Long distance telephone transmission
Can carry 10,000 voice calls simultaneously
Being replaced by fiber optic
Short distance computer systems links
Local area networks

47. 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

48. Optical Fiber

49. Optical Fiber - Benefits
Greater capacity
Data rates of hundreds of Gbps
Smaller size & weight
Lower attenuation
Electromagnetic isolation
Greater repeater spacing
10s of km at least

50. Optical Fiber - Applications
Long-haul trunks
Metropolitan trunks
Rural exchange trunks
Subscriber loops
LANs

51. Optical Fiber - Transmission Characteristics
Act as wave guide for 1014 to 1015 Hz
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
Wavelength Division Multiplexing

52. Optical Fiber Transmission Modes

53. 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

54. Wireless Transmission Frequencies
2GHz to 40GHz
Microwave
Highly directional
Point to point
Satellite
30MHz to 1GHz
Omni-directional
Broadcast radio
3 x 1011 to 2 x 1014
Infrared
Local

55. 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 Tx and Rx

56. 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

57. Parabolic Reflective Antenna
Used for terrestrial and satellite microwave
Parabola is locus of point equidistant from a line and a 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

58. Parabolic Reflective Antenna

59. Antenna Gain 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

60. Terrestrial Microwave
Parabolic dish
Focused beam
Line of sight
Long haul telecommunications
Higher frequencies give higher data rates

61. Satellite Microwave Satellite is relay station
Satellite receives on one frequency, amplifies or repeats signal and transmits on another frequency
Requires geo-stationary orbit
Height of 35,784km
Television
Long distance telephone
Private business networks

62. Satellite Point to Point Link

63. Satellite Broadcast Link

64. Broadcast Radio Omni-directional FM radio UHF and VHF television
Suffers from multipath interference
Reflections

65. Infrared Line of sight (or reflection) Blocked by walls
e.g. TV remote control, IRD port

66. Wireless Propagation Signal travels along three routes Ground wave
Follows contour of earth
AM radio
Sky wave
Amateur radio, BBC world service, Voice of America
Signal reflected from ionosphere layer of upper atmosphere
Line of sight
Above 30Mhz
May be further than optical line of sight due to refraction
More later…

67. Ground Wave Propagation

68. Sky Wave Propagation

69. Line of Sight Propagation

70. 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
May cause sudden change of direction at transition between media
May cause gradual bending if medium density is varying
Density of atmosphere decreases with height
Results in bending towards earth of radio waves

71. Optical and Radio Horizons

72. Line of Sight Transmission
Free space loss
Signal disperses with distance
Greater for lower frequencies (longer wavelengths)
Atmospheric Absorption
Water vapour 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
Signal can be reflected causing multiple copies to be received
May reinforce or cancel direct signal
Refraction
May result in partial or total loss of signal at receiver

73. Free Space Loss

74. Multipath Interference

75. Required Reading
Stallings Chapter 4

76. Q&A ?