1. EEL 5718 Computer Communications
Physical layer fundamentals
Chapter 3

2. Outline Overview of physical layer Channel limitation
Modulation/demodulation
Transmission media
Communication interfaces

3. Introduction

4. Physical layer

5. What You Need for Better Understanding

6. Representation of Information
Digital representation
Information that occurs naturally in digital form
data files or image files
Analog information: be digitized
Voice
Music
Video
Most communications networks are digital!

7. Source Coding Networks are handling streams of 0’s and 1’
Source Encoding: compression, according to statistics of 0’s and 1’s, map blocks of bits to more regular “shorter” blocks! Get rid of redundancy
Source Decoding: inverse of source encoding

8. Channel Coding
Channel Encoding: According to channel conditions, add redundancy for more efficient transmission, interleaving may be used too.
Channel decoding: the inverse
Observation: source encoding attempts to eliminate “useless information”, while channel encoding add “useful information”, both deal with redundancies!

9. Modulation/Demodulation
Modulation: maps blocks of bits to well-defined waveforms or symbols (a set of signals for better transmission), then shifts transmission to the carrier frequency band (the band you have right to transmit)
Demodulation: the inverse of modulation
Demodulation vs. Detection: Detection is to recover the modulated signal from the “distorted noisy” received signals

10. Physical Components Transmitter Receiver Transmission media
Guided: cable, twisted pair, fiber
Unguided: wireless (radio, infrared)

11. Signal Types Basic form: A signal is a time function
Continuous signal: varying continuously with time, e.g., speech
Discrete signal: varying at discrete time instant or keeping constant value in certain time interval, e.g., Morse code, flash lights
Periodic signal: Pattern repeated over time
Aperiodic signal: Pattern not repeated over time, e.g., speech

12. Continuous & Discrete Signals

13. Periodic Signals

14. Information Carriers
s(t) = A sin (2pft+ ) * Amplitude: A * Frequency: f --- f=1/T, T---period * Phase:  , angle (2pft+ )

15. Varying Sine Waves

16. Frequency Domain Concept
Signal is usually made up of many frequencies
Components are sine waves
Can be shown (Fourier analysis) that any signal is made up of component sine waves
Can plot frequency domain functions
Time domain representation is equivalent to frequency domain representation: they contain the same information!
Frequency domain representation is easier for design

17. Fourier Representation

18. Addition of Signals

19. Received Signals
Any receiver can only receive signals in certain frequency range (channel concept), corresponding to finite number of terms in the Fourier series approximation:
physically: finite number of harmonics
mathematically: finite number of terms
Transmitted signal design: allocate as many terms as possible in the intended receiver’s receiving range (most of power is limited in the intended receiving band)

20. Spectrum & Bandwidth
Spectrum: the range of frequencies contained in a signal
Absolute bandwidth: width of spectrum
Effective bandwidth: just BW, Narrow band of frequencies containing most of the energy
3 dB BW
Percentage BW: percentage power in the band
DC Component: Component of zero frequency

21. Data Rate and Bandwidth
Any transmission system has a limited band of frequencies
This limits the data rate that can be carried
The greater the BW, the high the data rate
Channel capacity (later)

22. Analog vs Digital
Analog: Continuous values within some interval, the transmitted signal has actual meaning, e.g., AM and FM radio
Digital: Digital=DSP+Analog, raw digital bits are processed and mapped to well-known signal set for better transmission, the final transmitted signal is still analog! You could not “hear” though!

23. Analog Transmission
Analog signal transmitted without regard to content
Attenuated over distance
Use amplifiers to boost signal, equalizers may be used to mitigate the noise
Also amplifies noise

24. Digital Transmission Concerned with content
Digital repeaters used: repeater receives signal, extracts bit pattern and retransmits the bit pattern!
Attenuation is overcome and distortion is not propagated!

25. Advantages of Digital Transmission
Digital technology: low cost, can use low power
Long distance transmission: use digital repeaters
Capacity utilization: get rid of useless information and add useful redundancy for data protection
Security & privacy: encryption
Integration: treat analog and digital data similarly

26. Channel Impairments
Attenuation and attenuation distortion: signal power attenuates with distance
Delay distortion: velocity of a signal through a guided medium varies with frequency, multipath in wireless environments
Thermal noise
Co-channel Interference: wireless
Impulse noise (powerline communications)

27. Channel Capacity
Data rate is limited by channel bandwidth and channel environment (impairments)
Data rate, in bits per second, is the number of bits transmitted successfully per second! Should not count the redundancy added against channel impairments!
It represents how fast can bits be transmitted reliably over a given medium

28. Factors Affecting Data Rate
Transmitted power (energy)
Distance between transmitter and receiver
Noise level (including interference level)
Bandwidth

29. Nyquist Capacity Nyquist BW: 2B, where B is the BW of a signal
Sampling Theorem: Any signal whose BW is B can be completely recovered by the sampled data at rate 2B samples per second
Nyquist Capacity Theorem: For a noiseless channel with BW B, if the M level signaling is used, the maximum transmission rate over the channel is C = 2B log2( M)
Digital Comm: symbol rate (baud rate) vs bit rate

30. Shannon Capacity All channels are noisy!
1948 paper by Clyde Shannon: “A mathematical theory of communications” “The mathematical theory of communications”
Signal-to-noise ratio: SNR=signal power/noise power

31. Shannon Capacity (cont)
Shannon Capacity Theorem: For a noisy channel of BW B with signal-to-noise ratio (SNR), the maximum transmission rate is C = B log2 (1+SNR)
Capacity increases as BW or signal power increases: Shout as you can!
Some exercise: B=3400Hz, SNR=40dB
C=44.8 kbps

32. Shannon Capacity (cont)
Shannon Theorem does not give any way to reach that capacity
Current transmission schemes transmit much lower rate than Shannon capacity
Turbo codes: iterative coding schemes using feedback information for transmission and detection
Sailing towards Shannon capacity!

33. Modulation/Demodulation
Line coding: representation of binary bits without carrier (baseband coding)
Modulation/demodulation: representation of digital bits with carrier (broadband coding)
Analog to Digital Coding

34. Line Coding Unipolar: all signal elements have same sign
Polar: one logic state represented by positive voltage the other by negative voltage
Data rate: rate of transmitted data (bps)
Bit period: time taken for transmitter to emit the bit, the duration or length of a bit
Modulation rate: rate at which the signal level changes, measured in baud (symbols per sec)
Mark and Space: binary 1 and Binary 0

35. Schemes Non-return to Zero-Level (NRZ-L)
Non-return to Zero Inverted (NRZI)
Bipolar-AMI
Pseudo-ternary
Manchester
Differential Manchester
B8ZS (see Stallings’ book) or google it
HDB3 (see Stallings’ book) or google it

36. Nonreturn to Zero-Level (NRZ-L)
Two different voltages for 0 and 1 bits
Voltage constant during bit interval
no transition, i.e. no return to zero voltage
e.g., Absence of voltage for zero, constant positive voltage for one
More often, negative voltage for one value and positive for the other---NRZ-L

37. Nonreturn to Zero Inverted
Nonreturn to zero inverted on ones
Constant voltage pulse for duration of bit
Data encoded as presence or absence of signal transition at beginning of bit time
1: Transition (low to high or high to low)
0: No transition
An example of differential encoding

38. NRZ

39. Differential Encoding
Data represented by changes rather than levels
More reliable detection of transition rather than level
In complex transmission layouts it is easy to lose sense of polarity

40. Multilevel Binary Use more than two levels Bipolar-AMI
0: no line signal
1: positive or negative pulse
pulses for 1’s alternate in polarity
No loss of sync if a long string of ones (zeros still a problem)
No net dc component
Lower bandwidth
Easy error detection

41. Pseudo-ternary 1: absence of line signal
0: alternating positive and negative
No advantage or disadvantage over bipolar-AMI
Change for
1’s
No signal
No signal
Change for
0’s

42. Biphase Manchester Differential Manchester
Transition in middle of each bit period
Transition serves as clock and data
1: low to high, 0: high to low
Used by IEEE 802.3
Differential Manchester
Midbit transition is clocking only
0: transition at start of a bit period
1: no transition at start of a bit period
Used by IEEE 802.5

43. Manchester Coding

44. Spectra
Used for the selection of line codes in conjunction with the channel characteristics: design the system so that most power is concentrated in the allowed range
Figure 3.26

45. Modulation Schemes (Binary)
Public telephone system
300Hz to 3400Hz
Use modem (modulator-demodulator)
Amplitude Shift Keying (ASK)
Frequency Shift Keying (FSK)
Phase Shift Keying (PSK)

46. Binary ASK,FSK, PSK
Bit-stream
ASK
FSK
PSK

47. Binary Keying Schemes

48. Digital Modulation
Binary keying schemes are simple, but not efficient!
Digital modulation uses multiple symbols (waveforms) to improve the efficiency
Information bearers: Amplitude Frequency Phase
Mapping: a block of bits to a waveform

49. QPSK
Quadrature Phase Shift Keying

50. Signal Constellation QPSK and QAM 4 “levels”/ pulse 2 bits / pulseAk Bk
4 “levels”/ pulse
2 bits / pulse
2W bits per second
2-D signal Bk
2-D signal Ak
16 “levels”/ pulse
4 bits / pulse
4W bits per second
Figure 3.33

51. QAM
Quadrature Amplitude Modulation (QAM)

52. Analog Modulation

53. Analog Modulation

54. Analog to Digital Sampling Theorem Quantization
Pulse Coded Modulation (PCM)
Differentially coded Modulation (e.g., Delta Modulation)

55. PCM Voice data limited to below 4000Hz Require 8000 sample per second
Analog samples (Pulse Amplitude Modulation, PAM)
Each sample assigned digital value
8 bit sample gives 256 levels
Quality comparable with analog transmission
8000 samples per second of 8 bits each gives 64kbps

56. Delta Modulation
Signals change continuously, close samples have close values!
Analog input is approximated by a staircase function
Move up or down one level () at each sample interval
Binary behavior
Function moves up or down at each sample interval

57. Delta Modulation - example

58. Spread Spectrum Spread power behind the noise
Spread data over wide bandwidth
Makes jamming and interception harder
Frequency hoping
Carrier changes in a random fashion
Direct Sequence
Each bit is represented by multiple bits in transmitted signal, similar to random noise
EEL 6503: CDMA and Spread Spectrum

59. Transmission Media Guided - wired (cable, twisted-pair, fiber)
Unguided - wireless (radio, infrared, microwave)
For guided, the medium is more important
For unguided, the transmission bandwidth and channel conditions are more important
Key concerns are data rate and distance

60. Electromagnetic Spectrum

61. Guided Transmission Media
Twisted Pair
Coaxial cable
Optical fiber

62. Twisted Pair

63. Twisted Pair (cont) Most common medium
Telephone networks and local area networks (Ethernet)
Easy to work with and cheap
Limited BW and low date rate, short distance and susceptible to interference and noise
New technologies: xDSL-digital subscriber line e.g., ADSL, VDSL
DMT: Discrete Multitone (Cioffi’s successful story)

64. 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)

65. EIA-568-A UTP Categories Cat 3: up to 16MHz (LANs) Cat 4: up to 20 MHz
Voice grade found in most offices
Twist length of 7.5 cm to 10 cm
data rate up to 16 Mbps, found in most office building
Cat 4: up to 20 MHz
Cat 5: up to 100MHz (LANs)
Commonly pre-installed in new office buildings
Twist length 0.6 cm to 0.85 cm
Data rate up to 100 Mbps

66. Coaxial Cable

67. Coaxial Cable (cont) Most versatile medium
Television distribution: TV, CATV
Long distance telephone transmission: can carry 10,000 voice calls simultaneously
Short distance computer systems links, LAN
Higher BW and high date rate
Heavy, not flexible, optical fibers may be a better choice

68. Cable Distribution
Head
end
Unidirectional
amplifier
Figure 3.41

69. Optical Fiber

70. Optical Fiber (cont) Greater capacity: Smaller size & weight
High BW ( >100 THz) and Data rates of hundreds of Gbps
Smaller size & weight
Lower attenuation
Electromagnetic isolation
More secure transmission: infeasible wiretap
Greater repeater spacing
10s of km at least

71. Optical Fiber (cont) Light Emitting Diode (LED)
Cheaper
Wider operating temp range
Last longer
Injection Laser Diode (ILD)
More efficient
Greater data rate
More expensive
Wavelength Division Multiplexing (WDM)

72. Optical Transmission System
Optical fiber
Optical
source
Modulator
Electrical
signal
Receiver
Figure 3.47

73. Transmission Modes

74. Fiber-Coaxial Distribution
Head
end
Upstream fiber
Downstream fiber
Fiber
node
Coaxial
distribution
plant
Bidirectional
Split-Band
Amplifier
Figure 3.42

75. Applications Network backbone Local Area Networks (LAN)
Public Switched Telephone Systems (PSTN): copper wires are replaced by fibers
National Internet Infrastructure: Internet2 etc
Cable Networks
Local Area Networks (LAN)
Fiber Distributed Data Interface (FDDI): 100 Mbps
Gigabit Ethernet
Fiber channels

76. Wireless Transmission
Unguided media: transmission over the air
Transmission and reception via antenna
Directional
Transmission limited in certain direction (flash light)
Careful alignment required
Omni-directional
Transmission power evenly spread over all directions (fireworks)
Can be received by many antennae

77. Frequency Bands 2GHz to 40GHz 30MHz to 1GHz 3 x 1011 to 2 x 1014
Microwave
Highly directional, point to point
Satellite, PCS (2Ghz), future wireless (2.4Ghz, 5Ghz)
30MHz to 1GHz
Omnidirectional
Broadcast radio, cellular (
3 x 1011 to 2 x 1014
Infrared

78. satellite & terrestrial
Radio Spectrum
Frequency (Hz)
104
105
106
107
108
109
1010
1011
1012
FM radio & TV
Wireless cable
AM radio
Cellular
& PCS
satellite & terrestrial
microwave LF MF HF
VHF
UHF
SHF
EHF
104
103
102
101 1
10-1
10-2
10-3
Wavelength (meters)
Figure 3.48

79. Characteristics of Wireless
Flexible
Solution for ubiquity of communications: get service on the move
Spectrum is limited
Channels are notoriously hostile
Power limited
Interference limited
Security is a BIG issue!
More details: EEL6591: Wireless Networks or EEL6935: Wireless ad Hoc Networks

80. Communication Interfaces
EIA RS-232 standard: serial line interface
Specify the interfaces between data terminal equipment (DTE) and data communications equipment (DCE)
DTE: represents a computer or terminal
DCE: represents the modem or the “network card”

81. Connector DTE DCE (a) (b)
                        
(b) 1
Protective Ground (PGND) 1 2
Transmit Data (TXD) 2 3
Receive Data (RXD) 3 4
Request to Send (RTS) 4 5
Clear to Send (CTS) 5
DTE
DCE 6
Data Set Ready (DSR) 6 7
Ground (G) 7 8
Carrier Detect (CD) 8 20
Data Terminal Ready (DTR) 20 22
Ring Indicator (RI) 22
Figure 3.67

82. Interfacing DCE communicates data and control info with DTE
Done over interchange circuits
Clear interface standards required
Specifications
Mechanical
Connection plugs
Electrical
Voltage, timing, encoding
Functional
Data, control, timing, grounding
Procedural
Sequence of events

83. Suggested Reading Chapter 3 Cross-read Tanenbaum, Chapter 2
Stallings’s book: 3, 4, 5, 6