1. Data Communication Data and Signals
Behrouz A. Forouzan
Data Communication - Data and Signals

2. Data Communication - Data and Signals
Index
ANALOG AND DIGITAL
PERIODIC ANALOG SIGNALS
DIGITAL SIGNALS
TRANSMISSION IMPAIRMENT
DATA RATE LIMITS
PERFORMANCE
Data Communication - Data and Signals

3. Data Communication - Data and Signals
major functions of physical layer is to move data in the form of electromagnetic signals across a transmission medium
data must be transformed to electromagnetic signals for transmission over medium
Data Communication - Data and Signals

4. Data Communication - Data and Signals
ANALOG AND DIGITAL
Analog and Digital Data
computer file is digital
Can be transmitted through analog or digital signal
sound is analog
Analog and Digital Signal
Analog signals have infinite number of values in a range;
digital signals have only a limited number of values
Data Communication - Data and Signals

5. Data Communication - Data and Signals
ANALOG AND DIGITAL
Data Communication - Data and Signals

6. ANALOG AND DIGITAL Periodic and Nonperiodic Signals
Both analog and digital signals can be:
periodic
non-periodic
repeats a pattern over identical periods
The completion of one full pattern is called a cycle
changes without exhibiting a pattern or cycle
Data Communication - Data and Signals

7. ANALOG AND DIGITAL Periodic and Nonperiodic Signals
In data communications, we commonly use periodic analog signals (because they need less bandwidth, Chapter 5) and nonperiodic digital signals (because they can represent variation in data, (Chapter 6)
Data Communication - Data and Signals

8. PERIODIC ANALOG SIGNALS
Simple
cannot be decomposed into simpler signals
Composite
composed of multiple sine waves
Data Communication - Data and Signals

9. PERIODIC ANALOG SIGNALS Sine Wave
fundamental form of a periodic analog signal
sine wave can be represented by three parameters:
peak amplitude
frequency
phase
Data Communication - Data and Signals

10. PERIODIC ANALOG SIGNALS Sine Wave
Peak Amplitude
absolute value of its highest intensity, proportional to the energy it carries, measured in volts.
The power in your house can be 110 to 120V
peak value of an AA battery is normally 1.5 V
Period and Frequency
Period : amount of time, in seconds, a signal needs to complete 1 cycle. (Second)
Frequency : number of periods in 1 second (HZ)
Phase
Data Communication - Data and Signals

11. PERIODIC ANALOG SIGNALS Sine Wave
Data Communication - Data and Signals

12. PERIODIC ANALOG SIGNALS Sine Wave
Power at home has a frequency of 60 Hz
Data Communication - Data and Signals

13. PERIODIC ANALOG SIGNALS Sine Wave
Peak Amplitude
Period and Frequency
Phase
position of the waveform relative to time 0
measured in degrees or radians
Phase of 180° corresponds to a shift of one-half of a period
Data Communication - Data and Signals

14. PERIODIC ANALOG SIGNALS Sine Wave
Data Communication - Data and Signals

15. PERIODIC ANALOG SIGNALS Sine Wave
Example:
A sine wave is offset 1/6 cycle with respect to time 0. What is its phase in degrees and radians?
1/6 * 360 degree = 60 degree =
Data Communication - Data and Signals

16. PERIODIC ANALOG SIGNALS Wavelength
Distance a signal can travel in one period
depends on both frequency and medium
often used wavelength to describe transmission of light in an optical fiber
in a vacuum, light speed is speed is lower in air and even lower in cable
Data Communication - Data and Signals

17. PERIODIC ANALOG SIGNALS Wavelength
Example:
In coaxial or fiber-optic cable, wavelength is shorter (0.5 micron) because propagation speed is decreased
Data Communication - Data and Signals

18. PERIODIC ANALOG SIGNALS Time and Frequency Domains
Time Domain:
amplitude versus time plot
Phase is not explicitly shown
Frequency Domain:
relationship between amplitude and frequency
concerned with only peak value of the frequency
Amplitude changes during one period are not shown
frequency domain conveys the information in time domain plot
The advantage of the frequency domain is that we can immediately see values of frequency and peak amplitude
Data Communication - Data and Signals

19. PERIODIC ANALOG SIGNALS Time and Frequency Domains
Data Communication - Data and Signals

20. PERIODIC ANALOG SIGNALS Time and Frequency Domains
Example:
frequency domain is more compact and useful when dealing with more than one sine wave
Data Communication - Data and Signals

21. PERIODIC ANALOG SIGNALS Composite Signals
Simple sine
60 Hz as energy
A signal of danger
Composite Sine
Send composite signal to communicate data
Data Communication - Data and Signals

22. PERIODIC ANALOG SIGNALS Composite Signals
Data Communication - Data and Signals

23. PERIODIC ANALOG SIGNALS Composite Signals
periodic composite signal
can be decomposed into a series of simple sine waves with discrete frequencies
not typical of those found in data communications
nonperiodic composite signal
can be decomposed into infinite number of simple sine waves with continuous frequencies having real values
Data Communication - Data and Signals

24. PERIODIC ANALOG SIGNALS periodic Composite Signals
Data Communication - Data and Signals

25. PERIODIC ANALOG SIGNALS periodic Composite Signals
Fundamental frequency: (first harmonic)
frequency of the composite signal
In Previous example , composite signal fundamental frequency is f and it includes 1st, 3rd and 9th harmonic of frequency of f
Data Communication - Data and Signals

26. PERIODIC ANALOG SIGNALS non-periodic Composite Signals
Example:
signal created by a microphone or a telephone
Data Communication - Data and Signals

27. PERIODIC ANALOG SIGNALS Bandwidth
difference between the highest and the lowest frequencies contained in a composite signal
Data Communication - Data and Signals

28. PERIODIC ANALOG SIGNALS Bandwidth
AM Radio
530 to 1700 kHz
each AM radio station is 10kHz bandwidth
FM Radio
ranges from 88 to 108 MHz
each FM radio station is 200-kHz bandwidth
black-and-white TV has 3.85 MHz bandwidth
analog color TV channel has 6-MHz bandwidth
Data Communication - Data and Signals

29. Data Communication - Data and Signals
DIGITAL SIGNALS
information can be represented by an analog or digital signal
if a signal has L levels, each level needs
(log L based2 ) bits
Data Communication - Data and Signals

30. DIGITAL SIGNALS Bit Rate
number of bits sent in 1 s, expressed in bps
digital signals are nonperiodic, thus period and frequency are not appropriate Characteristics
bit rate is used to describe digital signals.
A digitized voice channel bit rate id 64kbps
high-definition TV (HDTV) bit rate is 20 to 40Mbps
Data Communication - Data and Signals

31. DIGITAL SIGNALS Bit Length
wavelength for an analog signal = bit length in digital signal
distance one bit occupies on the transmission medium
Data Communication - Data and Signals

32. Digital Signal as a Composite Analog Signal
Frequency components od the square wave is as follows
usually rare
Data Communication - Data and Signals

33. Transmission of nonperiodic Digital Signals
Baseband Transmission
sending a digital signal over a channel without changing the digital signal to an analog signal
Case1: Low-Pass Channel with Wide Bandwidth (dedicated medium)
Case 2: Low-Pass Channel with Limited Bandwidth
Broadband Transmission (Using Modulation)
changing the digital signal to an analog signal for transmission (band-pass channel)
Data Communication - Data and Signals

34. Digital Signal Baseband Transmition
A digital signal is a composite analog signal with an infinite bandwidth
requires a low-pass channel, with a bandwidth that starts from zero to infinity
Design based on medium (channel) property:
Case 1: Low-Pass Channel with Wide Bandwidth (dedicated medium)
Case 2: Low-Pass Channel with Limited Bandwidth
Data Communication - Data and Signals

35. Digital Signal Baseband Transmition dedicated medium
Example: LAN (medium channel is time-division between users)
entire spectrum of a medium is required (however limited in spectrum)
amplitudes of higher frequencies in digital signal is so small that they can be ignored
So over a dedicated medium, such as a coaxial cable or fiber optic sending digital signals with very good accuracy is possible
Data Communication - Data and Signals

36. Digital Signal Baseband Transmition dedicated medium
Data Communication - Data and Signals

37. Digital Signal Baseband Transmition limited bandwidth medium
we approximate the digital signal with an analog signal
The level of approximation depends on the bandwidth available
Data Communication - Data and Signals

38. Digital Signal Baseband Transmition limited bandwidth medium
Let us assume that we have a digital signal of bit rate N
roughly simulate this signal, we need to consider the worst case, a maximum number of changes in the digital signal or the sequence · ...
So analog signal of frequency f = N/2
however, just this one frequency cannot make all patterns; we need more components
Data Communication - Data and Signals

39. Digital Signal Baseband Transmition limited bandwidth medium
Example: a digital signal with a 3-bit pattern is simulated by using analog signals.
000: frequency f =0 and a phase of 180°
111: frequency f =0 and a phase of 0°
The two worst cases
010: frequenc of =NI2 and phases of 180°
101: frequenc of =NI2 and phases of 0°.
other four cases: can only be simulated with an analog signal with f = NI4 and phases of 180°, 270°, 90°, and 0°.
Data Communication - Data and Signals

40. Digital Signal Baseband Transmition limited bandwidth medium
rough approximation is referred to as using the first harmonic (NI2) frequency
Bandwidth required is N/2 – 0 = N/2 Hz
Data Communication - Data and Signals

41. Digital Signal Baseband Transmition limited bandwidth medium
Better Approximation
To make the shape of the analog signal look more like that of a digital signal, we need to add more harmonics of the frequencies
Data Communication - Data and Signals

42. Digital Signal Baseband Transmition limited bandwidth medium
Data Communication - Data and Signals

43. Digital Signal Baseband Transmition limited bandwidth medium
Data Communication - Data and Signals

44. Digital Signal Baseband Transmition limited bandwidth medium
Example: required bandwidth of a low-pass channel to send 1 Mbps by using baseband transmission?
a. The minimum bandwidth, is B =: bit rate /2, or 500 kHz.
b. A better result can be achieved by using the first and the third harmonics with the required bandwidth B =: 3 x 500 kHz =: 1.5 MHz.
c. Still a better result can be achieved by using the first, third, and fifth harmonics with B =: 5 x 500 kHz =2.5 MHz
Data Communication - Data and Signals

45. Digital Signal Baseband Transmition limited bandwidth medium
Example: We have a low-pass channel with bandwidth 100 kHz. What is the maximum bit rate of this channel?
The maximum bit rate can be achieved if we use the first harmonic. The bit rate is 2 times the available bandwidth, or 200 kbps
Data Communication - Data and Signals

46. Broadband Transmission
Modulation allows us to use a bandpass channel (a channel with a bandwidth that does not start from zero)
This type of channel is more available than a low-pass channel
Data Communication - Data and Signals

47. Broadband Transmission
We have used a single-frequency analog signal (called a carrier);
The result, is not a single-frequency signal; it is a composite signal, (Chapter 5)
At the receiver, the received analog signal is converted to digital
Data Communication - Data and Signals

48. Data Communication - Data and Signals
Broadband
Example: sending of computer data through a telephone subscriber line
designed to carry voice (analog signal) with a limited bandwidth (frequencies between 0 and 4 kHz).
Although this channel can be used as a low-pass channel, it is normally USED AS a bandpass channel.
Because the bandwidth is so narrow (4 kHz), the maximum bit rate can be only 8 kbps
Solution: is to consider the channel a bandpass channel.
Data Communication - Data and Signals

49. Broadband Transmission
Example: digital cellular telephone
bandwidth allocated to a company providing digital cellular phone service is very wide, but bandpass
Data Communication - Data and Signals

50. TRANSMISSION IMPAIRMENT
What is sent is not what is received
causes of impairment are attenuation, distortion, and noise
Data Communication - Data and Signals

51. TRANSMISSION IMPAIRMENT Attenuation
loss of energy in overcoming resistance of the medium.
To compensate for this loss, amplifiers are used
Data Communication - Data and Signals

52. TRANSMISSION IMPAIRMENT Attenuation
use the unit of the decibel to show lost or gained strength of a signal
decibel numbers can be added (or subtracted) when we are measuring several points (cascading)
decibel is negative if a signal is attenuated and positive if a signal is amplified
A loss of 3 dB (-3 dB) is equivalent to losing one-half the power
Gain of 10 dB (+10 dB) is equivalent to increasing 10 times the power
Data Communication - Data and Signals

53. TRANSMISSION IMPAIRMENT Attenuation
dB=-3+7-3=+1
dBm = 10 log Pm
where Pm is the power in milliwatts
Data Communication - Data and Signals

54. TRANSMISSION IMPAIRMENT Attenuation
Example : If the signal at the beginning of a cable with -0.3 dBm/km has a power of 2 mW, what is the power of the signal at 5 km?
Power in source = 2mW = +3 dBm
Loss = 5 * (-0.3dBm) = -1.5dBm
Power in dst = 3 – 1.5 = 1.5dBm = 1.4mW
Data Communication - Data and Signals

55. TRANSMISSION IMPAIRMENT Distortion
signal changes its form or shape
Differences in delay in a compound signal at different frequencies may create a difference in phase
Data Communication - Data and Signals

56. TRANSMISSION IMPAIRMENT Noise
thermal noise,
induced noise,
comes from sources such as motors and appliances
crosstalk,
effect of one wire on the other
impulse noise
spike (a signal with high energy in a very short time) that comes from power lines, lightning, and so on
Data Communication - Data and Signals

57. TRANSMISSION IMPAIRMENT Noise
Signal-to-Noise Ratio (SNR)
SNR= (average signal power / average noise power)
SNR dB(m) = lOlog SNR (m)W
The values of SNR and SNRdB for a noiseless channel are which never achieve this ratio in real life
Data Communication - Data and Signals

58. Data Communication - Data and Signals
DATA RATE LIMITS
Data rate depends on three factors:
The bandwidth available
The level of the signals we use
The quality of the channel (the level of noise)
calculate the data rate:
Nyquist for a noiseless channel.
Shannon for a noisy channel
Data Communication - Data and Signals

59. DATA RATE LIMITS Noiseless Channel: Nyquist Bit Rate
Maximum BitRate = 2 x bandwidth x 10g2 L
bandwidth is the bandwidth of the channel,
L is number of signal levels used to represent data,
BitRate is the bit rate in bits per second
Increasing levels of a signal may reduce the reliability of the system
it can be applied to baseband transmission and modulation not just baseband transmit ion previously described
Data Communication - Data and Signals

60. DATA RATE LIMITS Noisy Channel: Shannon Capacity
Highest Capacity =bandwidth X log2 (1 +SNR)
Bandwidth is the bandwidth of the channel, Hz
SNR is the signal-to-noise ratio, in unit of W not dB
capacity is the capacity of the channel in bits per second bps
no matter how many levels we have, we cannot achieve a data rate higher than the capacity of the channel
Data Communication - Data and Signals

61. DATA RATE LIMITS Noisy Channel: Shannon Capacity
Example: Consider an extremely noisy channel in which the value of the signal-to-noise ratio is almost zero. C=B log2 (1 + SNR) =B log2 (l + 0) =0
Example: regular telephone line. Normally has a bandwidth of 3000 Hz (300 to 3300 Hz). The signal- to-noise ratio is usually C =B log2 (1 + SNR) =3000 log2 (l ) = 34,860 bps
highest bit rate for a telephone line is kbps
Data Communication - Data and Signals

62. DATA RATE LIMITS Noisy Channel: Shannon Capacity
when the SNR is very high
C = B * (SNR dB/3)
Example: SNRdB = 36 and channel bandwidth is 2 MHz.
C= 2 MHz X 36/3 =24 Mbps
Data Communication - Data and Signals

63. DATA RATE LIMITS Using Both Limits
Example: Bandwidth = 1MHz, SNR=63
Shanon for upper limit: C= B log2 (l + SNR) =6Mbps
For better performance we choose something lower, 4 Mbps
use the Nyquist formula to find the number of signal levels. 4Mbps=2x 1 MHz x log2 L  L=4
Data Communication - Data and Signals

64. Data Communication - Data and Signals
PERFORMANCE
Measure performance of the network-how good is it?
Bandwidth
Throughput
Latency (Delay)
Bandwidth-Delay Product
Jitter
Data Communication - Data and Signals

65. PERFORMANCE Bandwidth
Bandwidth in Hertz
range of frequencies contained in a signal
range of frequencies a channel can pass
Bandwidth in Bits per Seconds
number of bits per second that a channel can transmit
increase in bandwidth in hertz means an increase in bandwidth in bps
depends on whether we have baseband transmission or transmission with modulation (Chapter 4 & 5)
Data Communication - Data and Signals

66. PERFORMANCE Throughput
bandwidth is a potential measurement of a link; the throughput is an actual measurement of how fast we can send data both in bps
Because of congestion or sender limitation
Data Communication - Data and Signals

67. PERFORMANCE Latency (Delay)
how long it takes for an entire message to completely arrive at the destination from the time the first bit is sent out from the source
Latency =propagation time +transmission time +queuing time + processing delay
Data Communication - Data and Signals

68. PERFORMANCE Latency (Delay)
Propagation Time
Propagation time = (Distance/Propagation-speed)
propagation speed of electromagnetic signals depends on the medium and frequency of the signaL
in a vacuum, light is propagated with a speed of 3 x 108 m/s
Data Communication - Data and Signals

69. PERFORMANCE Latency (Delay)
Transmition Time
time between the first bit and the last bit leaving the sender
Transmission time = (Message size /Bandwidth)
Message size in bits , Bandwidth in bps
Data Communication - Data and Signals

70. PERFORMANCE Latency (Delay)
Queuing Time
time needed for each intermediate /end device to hold the message before it can be processed
The queuing time is not a fixed factor
When there is heavy traffic on the network, the queuing time increases
Data Communication - Data and Signals

71. PERFORMANCE Bandwidth-Delay Product
in data communications product of the two (Bandwidth and Delay) is important
maximum number of bits that can fill the link
Data Communication - Data and Signals

72. PERFORMANCE Bandwidth-Delay Product
Example: bandwidth of 1 bps, link delay is 5 s  no more than 5 bits at any time on the link
Data Communication - Data and Signals

73. PERFORMANCE Bandwidth-Delay Product
Data Communication - Data and Signals

74. PERFORMANCE Bandwidth-Delay Product
This measurement is important in sending data in bursts and wait for the acknowledgment of each burst before sending the next one.
To use the maximum capability of the link, we need to make the size of our burst 2 times the product of bandwidth and delay;
we need to fill up the full-duplex channel
The sender should send a burst of data of (2 x bandwidth x delay) bits then waits for receiver acknowledgment
Data Communication - Data and Signals

75. Data Communication - Data and Signals
PERFORMANCE Jitter
Jitter is a problem if different packets of data encounter different delays
It is important for time-sensitive applications (audio and video)
Discussed in multimedia section
Data Communication - Data and Signals