Lecture 2.3 - Transmission Impairments and Channel Capacity - IT & Software PDF Download

 Page 1


 
 
 
 
 
 
 
Module 
2 
 
    Data 
Communication 
Fundamentals 
 
 
Version 2 CSE   IIT, Kharagpur 
Page 2


 
 
 
 
 
 
 
Module 
2 
 
    Data 
Communication 
Fundamentals 
 
 
Version 2 CSE   IIT, Kharagpur 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
  
 
 
 
 
Lesson 
3 
 
 
Transmission 
Impairments and 
Channel Capacity 
 
 
 
Version 2 CSE   IIT, Kharagpur 
Page 3


 
 
 
 
 
 
 
Module 
2 
 
    Data 
Communication 
Fundamentals 
 
 
Version 2 CSE   IIT, Kharagpur 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
  
 
 
 
 
Lesson 
3 
 
 
Transmission 
Impairments and 
Channel Capacity 
 
 
 
Version 2 CSE   IIT, Kharagpur 
Specific Instructional Objectives 
At the end of this lesson the students will be able to: 
• Specify the Sources of impairments 
• Explain Attenuation and unit of Attenuation 
• Specify possible types of distortions of a signal 
• Explain Data Rate Limits and Nyquist Bit Rate 
• Distinguish between Bit Rate and Baud Rate 
• Identify Noise Sources  
• Explain Shannon Capacity in a Noisy Channel 
 
2.3.1 Introduction 
When a signal is transmitted over a communication channel, it is subjected to different 
types of impairments because of imperfect characteristics of the channel. As a 
consequence, the received and the transmitted signals are not the same. Outcome of the 
impairments are manifested in two different ways in analog and digital signals. These 
impairments introduce random modifications in analog signals leading to distortion. On 
the other hand, in case of digital signals, the impairments lead to error in the bit values. 
The impairment can be broadly categorised into the following three types: 
  
•        Attenuation and attenuation distortion 
•        Delay distortion 
•        Noise 
 In this lesson these impairments are discussed in detail and possible approaches to 
overcome these impairments. The concept of channel capacity for both noise-free and 
noisy channels have also been introduced. 
 
2.3.2 Attenuation  
Irrespective of whether a medium is guided or unguided, the strength of a signal falls off 
with distance. This is known as attenuation.  In case of guided media, the attenuation is 
logarithmic, whereas in case of unguided media it is a more complex function of the 
distance and the material that constitutes the medium.  
 
An important concept in the field of data communications is the use of on unit 
known as decibel (dB). To define it let us consider the circuit elements shown in Fig. 
2.3.1. The elements can be either a transmission line, an amplifier, an attenuator, a filter, 
etc.  In the figure, a transmission line (between points P
1 
and P
2
) is followed by an 
amplifier (between P
2 
and P
3
).  The input signal delivers a power P
1 
at the input  of an 
communication element and the output power is P
2
. Then the power gain G for this 
element in decibles is given by G = 10log
2
 P
2
/ P
1
. Here P
2
/ P
1
 is referred to as absolute 
power gain. When P
2
 > P
1
, the gain is positive, whereas if P
2
 < P
1, 
then the power gain is 
negative and there is a power loss in the circuit element. For P
2
 = 5mW, P
1
 = 10mW, the 
Version 2 CSE   IIT, Kharagpur 
Page 4


 
 
 
 
 
 
 
Module 
2 
 
    Data 
Communication 
Fundamentals 
 
 
Version 2 CSE   IIT, Kharagpur 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
  
 
 
 
 
Lesson 
3 
 
 
Transmission 
Impairments and 
Channel Capacity 
 
 
 
Version 2 CSE   IIT, Kharagpur 
Specific Instructional Objectives 
At the end of this lesson the students will be able to: 
• Specify the Sources of impairments 
• Explain Attenuation and unit of Attenuation 
• Specify possible types of distortions of a signal 
• Explain Data Rate Limits and Nyquist Bit Rate 
• Distinguish between Bit Rate and Baud Rate 
• Identify Noise Sources  
• Explain Shannon Capacity in a Noisy Channel 
 
2.3.1 Introduction 
When a signal is transmitted over a communication channel, it is subjected to different 
types of impairments because of imperfect characteristics of the channel. As a 
consequence, the received and the transmitted signals are not the same. Outcome of the 
impairments are manifested in two different ways in analog and digital signals. These 
impairments introduce random modifications in analog signals leading to distortion. On 
the other hand, in case of digital signals, the impairments lead to error in the bit values. 
The impairment can be broadly categorised into the following three types: 
  
•        Attenuation and attenuation distortion 
•        Delay distortion 
•        Noise 
 In this lesson these impairments are discussed in detail and possible approaches to 
overcome these impairments. The concept of channel capacity for both noise-free and 
noisy channels have also been introduced. 
 
2.3.2 Attenuation  
Irrespective of whether a medium is guided or unguided, the strength of a signal falls off 
with distance. This is known as attenuation.  In case of guided media, the attenuation is 
logarithmic, whereas in case of unguided media it is a more complex function of the 
distance and the material that constitutes the medium.  
 
An important concept in the field of data communications is the use of on unit 
known as decibel (dB). To define it let us consider the circuit elements shown in Fig. 
2.3.1. The elements can be either a transmission line, an amplifier, an attenuator, a filter, 
etc.  In the figure, a transmission line (between points P
1 
and P
2
) is followed by an 
amplifier (between P
2 
and P
3
).  The input signal delivers a power P
1 
at the input  of an 
communication element and the output power is P
2
. Then the power gain G for this 
element in decibles is given by G = 10log
2
 P
2
/ P
1
. Here P
2
/ P
1
 is referred to as absolute 
power gain. When P
2
 > P
1
, the gain is positive, whereas if P
2
 < P
1, 
then the power gain is 
negative and there is a power loss in the circuit element. For P
2
 = 5mW, P
1
 = 10mW, the 
Version 2 CSE   IIT, Kharagpur 
power gain  G = 10log 5/10 = 10 ×  -3 = -3dB is negative and it represents attenuation as 
a signal passes through the communication element. 
 
Example: Let us consider a transmission line between points 1 and 2 and let the energy 
strength at point 2 is 1/10
 
of that of point 1.  Then attenuation in dB is 10log
10
(1/10) = -
10 dB. On the other hand, there is an amplifier between points 2 and 3. Let the power is 
100 times at point 3 with respect to point 2. Then power gain in dB is 10log
10
(100/1) = 
20 dB, which has a positive sign.  
  
 
 
 
 
 
 
Figure 2.3.1 Compensation of attenuation using an amplifier 
 
The attenuation leads to several problems: 
Attenuation Distortion: If the strength of the signal is very low, the signal cannot be 
detected and interpreted properly at the receiving end. The signal strength should be 
sufficiently high so that the signal can be correctly detected by a receiver in presence of 
noise in the channel. As shown in Fig. 2.3.1, an amplifier can be used to compensate the 
attenuation of the transmission line. So, attenuation decides how far a signal can be sent 
without amplification through a particular medium. 
 
Attenuation of all frequency components is not same. Some frequencies are 
passed without attenuation, some are weakened and some are blocked.  This dependence 
of attenuation of a channel on the frequency of a signal leads to a new kind of distortion 
attenuation distortion. As shown in Fig. 2.3.2, a square wave is sent through a medium 
and the output is no longer a square wave because of more attenuation of the high-
frequency components in the medium.  
 
 
 
 
 
 
 
 
 
Figure 2.3.2 Attenuation distortion of a square wave after passing through a medium. 
The effect of attenuation distortion can be reduced with the help of a suitable 
equalizer circuit, which is placed between the channel and the receiver. The equalizer has 
opposite attenuation/amplification characteristics of the medium and compensates higher 
Version 2 CSE   IIT, Kharagpur 
Page 5


 
 
 
 
 
 
 
Module 
2 
 
    Data 
Communication 
Fundamentals 
 
 
Version 2 CSE   IIT, Kharagpur 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
  
 
 
 
 
Lesson 
3 
 
 
Transmission 
Impairments and 
Channel Capacity 
 
 
 
Version 2 CSE   IIT, Kharagpur 
Specific Instructional Objectives 
At the end of this lesson the students will be able to: 
• Specify the Sources of impairments 
• Explain Attenuation and unit of Attenuation 
• Specify possible types of distortions of a signal 
• Explain Data Rate Limits and Nyquist Bit Rate 
• Distinguish between Bit Rate and Baud Rate 
• Identify Noise Sources  
• Explain Shannon Capacity in a Noisy Channel 
 
2.3.1 Introduction 
When a signal is transmitted over a communication channel, it is subjected to different 
types of impairments because of imperfect characteristics of the channel. As a 
consequence, the received and the transmitted signals are not the same. Outcome of the 
impairments are manifested in two different ways in analog and digital signals. These 
impairments introduce random modifications in analog signals leading to distortion. On 
the other hand, in case of digital signals, the impairments lead to error in the bit values. 
The impairment can be broadly categorised into the following three types: 
  
•        Attenuation and attenuation distortion 
•        Delay distortion 
•        Noise 
 In this lesson these impairments are discussed in detail and possible approaches to 
overcome these impairments. The concept of channel capacity for both noise-free and 
noisy channels have also been introduced. 
 
2.3.2 Attenuation  
Irrespective of whether a medium is guided or unguided, the strength of a signal falls off 
with distance. This is known as attenuation.  In case of guided media, the attenuation is 
logarithmic, whereas in case of unguided media it is a more complex function of the 
distance and the material that constitutes the medium.  
 
An important concept in the field of data communications is the use of on unit 
known as decibel (dB). To define it let us consider the circuit elements shown in Fig. 
2.3.1. The elements can be either a transmission line, an amplifier, an attenuator, a filter, 
etc.  In the figure, a transmission line (between points P
1 
and P
2
) is followed by an 
amplifier (between P
2 
and P
3
).  The input signal delivers a power P
1 
at the input  of an 
communication element and the output power is P
2
. Then the power gain G for this 
element in decibles is given by G = 10log
2
 P
2
/ P
1
. Here P
2
/ P
1
 is referred to as absolute 
power gain. When P
2
 > P
1
, the gain is positive, whereas if P
2
 < P
1, 
then the power gain is 
negative and there is a power loss in the circuit element. For P
2
 = 5mW, P
1
 = 10mW, the 
Version 2 CSE   IIT, Kharagpur 
power gain  G = 10log 5/10 = 10 ×  -3 = -3dB is negative and it represents attenuation as 
a signal passes through the communication element. 
 
Example: Let us consider a transmission line between points 1 and 2 and let the energy 
strength at point 2 is 1/10
 
of that of point 1.  Then attenuation in dB is 10log
10
(1/10) = -
10 dB. On the other hand, there is an amplifier between points 2 and 3. Let the power is 
100 times at point 3 with respect to point 2. Then power gain in dB is 10log
10
(100/1) = 
20 dB, which has a positive sign.  
  
 
 
 
 
 
 
Figure 2.3.1 Compensation of attenuation using an amplifier 
 
The attenuation leads to several problems: 
Attenuation Distortion: If the strength of the signal is very low, the signal cannot be 
detected and interpreted properly at the receiving end. The signal strength should be 
sufficiently high so that the signal can be correctly detected by a receiver in presence of 
noise in the channel. As shown in Fig. 2.3.1, an amplifier can be used to compensate the 
attenuation of the transmission line. So, attenuation decides how far a signal can be sent 
without amplification through a particular medium. 
 
Attenuation of all frequency components is not same. Some frequencies are 
passed without attenuation, some are weakened and some are blocked.  This dependence 
of attenuation of a channel on the frequency of a signal leads to a new kind of distortion 
attenuation distortion. As shown in Fig. 2.3.2, a square wave is sent through a medium 
and the output is no longer a square wave because of more attenuation of the high-
frequency components in the medium.  
 
 
 
 
 
 
 
 
 
Figure 2.3.2 Attenuation distortion of a square wave after passing through a medium. 
The effect of attenuation distortion can be reduced with the help of a suitable 
equalizer circuit, which is placed between the channel and the receiver. The equalizer has 
opposite attenuation/amplification characteristics of the medium and compensates higher 
Version 2 CSE   IIT, Kharagpur 
losses of some frequency components in the medium by higher amplification in the 
equalizer. Attenuation characteristics of three popular transmission media are shown in 
Fig. 2.3.3. As shown in the figure, the attenuation of a signal increases exponentially as 
frequency is increased from KHz range to MHz range. In case of coaxial cable 
attenuation increases linearly with frequency in the Mhz range.  The optical fibre, on the 
other hand, has attenuation characteristic similar to a band-pass filter and a small 
frequency band in the THz range can be used for the transmission of signal. 
 
 
 
Figure 2.3.3 Attenuation characteristics of the popular guided media 
  
2.3.3 Delay distortion 
The velocity of propagation of different frequency components of a signal are different in 
guided media. This leads to delay distortion in the signal. For a bandlimited signal, the 
velocity of propagation has been found to be maximum near the center frequency and 
lower on both sides of the edges of the frequency band. In case of analog signals, the 
received signal is distorted because of variable delay of different components. In case of 
digital signals, the problem is much more severe. Some frequency components of one bit 
position spill over to other bit positions, because of delay distortion. This leads to 
intersymbol interference, which restricts the maximum bit rate of transmission through a 
particular transmission medium. The delay distortion can also be neutralised, like 
attenuation distortion, by using suitable equalizers. 
  
 
Version 2 CSE   IIT, Kharagpur