Lecture 4.2 - Switching Techniques: Circuit Switching lesson 2 IT & Software Notes | EduRev

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IT & Software : Lecture 4.2 - Switching Techniques: Circuit Switching lesson 2 IT & Software Notes | EduRev

 Page 1


 
 
 
 
 
 
 
 
 
 
 
Module  
4 
 
Switched Communication 
Networks 
 
Version 2, CSE    IIT Kharagpur 
Page 2


 
 
 
 
 
 
 
 
 
 
 
Module  
4 
 
Switched Communication 
Networks 
 
Version 2, CSE    IIT Kharagpur 
 
 
 
 
 
 
 
 
 
 
Lesson  
2 
 
Switching Techniques: 
Circuit Switching 
 
Version 2, CSE    IIT Kharagpur 
Page 3


 
 
 
 
 
 
 
 
 
 
 
Module  
4 
 
Switched Communication 
Networks 
 
Version 2, CSE    IIT Kharagpur 
 
 
 
 
 
 
 
 
 
 
Lesson  
2 
 
Switching Techniques: 
Circuit Switching 
 
Version 2, CSE    IIT Kharagpur 
 
 
 
4.2.0 Specific Instructional Objectives  
At the end of this lesson the student will be able to: 
• Explain the need for packet switching 
• Explain how packet switching takes place 
• Explain different types of packet switching techniques 
• Distinguish between virtual-circuit and datagram type packet switching  
• Compare circuit switching with packet switching 
 
 
4.2.1 Introduction 
In the preceding lesson we have discussed about circuit switching. In circuit switching, 
network resources are dedicated to a particular connection. Although this satisfies the 
requirement of voice communication, it suffers from the following two shortcomings for 
data communication: 
 
o In a typical user/host data connection, line utilization is very low. 
o Provides facility for data transmission at a constant rate.  
 
However, for information transmission applications, the circuit switching method is very 
slow, relatively expensive and inefficient. First of all, the need to establish a dedicated 
connection before sending the message itself inserts a delay time, which might become 
significant for the total message transfer time. Moreover, the total channel remains idle 
and unavailable to the other users once a connection is made. On the other hand once a 
connection is established, it is guaranteed and orderly delivery of message is ensured. 
Unfortunately, the data transmission pattern may not ensure this, because data 
transmission is bursty in nature. As a consequence, it limits the utility of the method. The 
problem may be overcome by using an approach known as message switching, which is 
discussed in Sec. 4.2.2. However, message switching suffers from various problems as 
discussed in Sec. 4.2.3. To overcome the limitations of message switching, another 
switching technique, known as packet switching was invented. Various aspects of packet 
switching have been discussed in Sec. 4.2.4.  
 
4.2.2 Message Switching 
In this switching method, a different strategy is used, where instead of establishing a 
dedicated physical line between the sender and the receiver, the message is sent to the 
nearest directly connected switching node. This node stores the message, checks for 
errors, selects the best available route and forwards the message to the next intermediate 
node.  
 
 
 
 
Version 2, CSE    IIT Kharagpur 
Page 4


 
 
 
 
 
 
 
 
 
 
 
Module  
4 
 
Switched Communication 
Networks 
 
Version 2, CSE    IIT Kharagpur 
 
 
 
 
 
 
 
 
 
 
Lesson  
2 
 
Switching Techniques: 
Circuit Switching 
 
Version 2, CSE    IIT Kharagpur 
 
 
 
4.2.0 Specific Instructional Objectives  
At the end of this lesson the student will be able to: 
• Explain the need for packet switching 
• Explain how packet switching takes place 
• Explain different types of packet switching techniques 
• Distinguish between virtual-circuit and datagram type packet switching  
• Compare circuit switching with packet switching 
 
 
4.2.1 Introduction 
In the preceding lesson we have discussed about circuit switching. In circuit switching, 
network resources are dedicated to a particular connection. Although this satisfies the 
requirement of voice communication, it suffers from the following two shortcomings for 
data communication: 
 
o In a typical user/host data connection, line utilization is very low. 
o Provides facility for data transmission at a constant rate.  
 
However, for information transmission applications, the circuit switching method is very 
slow, relatively expensive and inefficient. First of all, the need to establish a dedicated 
connection before sending the message itself inserts a delay time, which might become 
significant for the total message transfer time. Moreover, the total channel remains idle 
and unavailable to the other users once a connection is made. On the other hand once a 
connection is established, it is guaranteed and orderly delivery of message is ensured. 
Unfortunately, the data transmission pattern may not ensure this, because data 
transmission is bursty in nature. As a consequence, it limits the utility of the method. The 
problem may be overcome by using an approach known as message switching, which is 
discussed in Sec. 4.2.2. However, message switching suffers from various problems as 
discussed in Sec. 4.2.3. To overcome the limitations of message switching, another 
switching technique, known as packet switching was invented. Various aspects of packet 
switching have been discussed in Sec. 4.2.4.  
 
4.2.2 Message Switching 
In this switching method, a different strategy is used, where instead of establishing a 
dedicated physical line between the sender and the receiver, the message is sent to the 
nearest directly connected switching node. This node stores the message, checks for 
errors, selects the best available route and forwards the message to the next intermediate 
node.  
 
 
 
 
Version 2, CSE    IIT Kharagpur 
 
 
 
 
Node 1 
Message
Message
Message 
 
 
Node 2 
 
 
Node 3 
 
Node 4  
 
Figure 4.2.1 Message Switching Technique 
 
The line becomes free again for other messages, while the process is being 
continued in some other nodes. Due to the mode of action, this method is also known as 
store-and-forward technology where the message hops from node to node to its final 
destination. Each node stores the full message, checks for errors and forwards it. 
 
In this switching technique, more devices can share the network bandwidth, as 
compared with circuit switching technique. Temporary storage of message reduces traffic 
congestion to some extent. Higher priority can be given to urgent messages, so that the 
low priority messages are delayed while the urgent ones are forwarded faster. Through 
broadcast addresses one message can be sent to several users. Last of all, since the 
destination host need not be active when the message is sent, message switching 
techniques improve global communications. 
 
However, since the message blocks may be quite large in size, considerable 
amount of storage space is required at each node to buffer the messages. A message 
might occupy the buffers for minutes, thus blocking the internodal traffic. 
 
Basic idea: 
• Each network node receives and stores the message  
• Determines the next leg of the route, and  
• Queues the message to go out on that link. 
Advantages: 
• Line efficiency is greater (sharing of links). 
• Data rate conversion is possible. 
• Even under heavy traffic, packets are accepted, possibly with a greater delay in 
delivery. 
• Message priorities can be used, to satisfy the requirements, if any. 
Disadvantages: Message of large size monopolizes the link and storage 
 
4.2.3 Packet Switching 
The basic approach is not much different from message switching. It is also based on the 
same ‘store-and-forward’ approach. However, to overcome the limitations of message 
switching, messages are divided into subsets of equal length called packets. This 
approach was developed for long-distance data communication (1970) and it has evolved 
Version 2, CSE    IIT Kharagpur 
Page 5


 
 
 
 
 
 
 
 
 
 
 
Module  
4 
 
Switched Communication 
Networks 
 
Version 2, CSE    IIT Kharagpur 
 
 
 
 
 
 
 
 
 
 
Lesson  
2 
 
Switching Techniques: 
Circuit Switching 
 
Version 2, CSE    IIT Kharagpur 
 
 
 
4.2.0 Specific Instructional Objectives  
At the end of this lesson the student will be able to: 
• Explain the need for packet switching 
• Explain how packet switching takes place 
• Explain different types of packet switching techniques 
• Distinguish between virtual-circuit and datagram type packet switching  
• Compare circuit switching with packet switching 
 
 
4.2.1 Introduction 
In the preceding lesson we have discussed about circuit switching. In circuit switching, 
network resources are dedicated to a particular connection. Although this satisfies the 
requirement of voice communication, it suffers from the following two shortcomings for 
data communication: 
 
o In a typical user/host data connection, line utilization is very low. 
o Provides facility for data transmission at a constant rate.  
 
However, for information transmission applications, the circuit switching method is very 
slow, relatively expensive and inefficient. First of all, the need to establish a dedicated 
connection before sending the message itself inserts a delay time, which might become 
significant for the total message transfer time. Moreover, the total channel remains idle 
and unavailable to the other users once a connection is made. On the other hand once a 
connection is established, it is guaranteed and orderly delivery of message is ensured. 
Unfortunately, the data transmission pattern may not ensure this, because data 
transmission is bursty in nature. As a consequence, it limits the utility of the method. The 
problem may be overcome by using an approach known as message switching, which is 
discussed in Sec. 4.2.2. However, message switching suffers from various problems as 
discussed in Sec. 4.2.3. To overcome the limitations of message switching, another 
switching technique, known as packet switching was invented. Various aspects of packet 
switching have been discussed in Sec. 4.2.4.  
 
4.2.2 Message Switching 
In this switching method, a different strategy is used, where instead of establishing a 
dedicated physical line between the sender and the receiver, the message is sent to the 
nearest directly connected switching node. This node stores the message, checks for 
errors, selects the best available route and forwards the message to the next intermediate 
node.  
 
 
 
 
Version 2, CSE    IIT Kharagpur 
 
 
 
 
Node 1 
Message
Message
Message 
 
 
Node 2 
 
 
Node 3 
 
Node 4  
 
Figure 4.2.1 Message Switching Technique 
 
The line becomes free again for other messages, while the process is being 
continued in some other nodes. Due to the mode of action, this method is also known as 
store-and-forward technology where the message hops from node to node to its final 
destination. Each node stores the full message, checks for errors and forwards it. 
 
In this switching technique, more devices can share the network bandwidth, as 
compared with circuit switching technique. Temporary storage of message reduces traffic 
congestion to some extent. Higher priority can be given to urgent messages, so that the 
low priority messages are delayed while the urgent ones are forwarded faster. Through 
broadcast addresses one message can be sent to several users. Last of all, since the 
destination host need not be active when the message is sent, message switching 
techniques improve global communications. 
 
However, since the message blocks may be quite large in size, considerable 
amount of storage space is required at each node to buffer the messages. A message 
might occupy the buffers for minutes, thus blocking the internodal traffic. 
 
Basic idea: 
• Each network node receives and stores the message  
• Determines the next leg of the route, and  
• Queues the message to go out on that link. 
Advantages: 
• Line efficiency is greater (sharing of links). 
• Data rate conversion is possible. 
• Even under heavy traffic, packets are accepted, possibly with a greater delay in 
delivery. 
• Message priorities can be used, to satisfy the requirements, if any. 
Disadvantages: Message of large size monopolizes the link and storage 
 
4.2.3 Packet Switching 
The basic approach is not much different from message switching. It is also based on the 
same ‘store-and-forward’ approach. However, to overcome the limitations of message 
switching, messages are divided into subsets of equal length called packets. This 
approach was developed for long-distance data communication (1970) and it has evolved 
Version 2, CSE    IIT Kharagpur 
 
 
 
over time. In packet switching approach, data are transmitted in short packets (few 
Kbytes). A long message is broken up into a series of packets as shown in Fig. 4.2.2. 
Every packet contains some control information in its header, which is required for 
routing and other purposes. 
 
 
 
 
 
 
 
 
 
 
 
Figure 4.2.2 A message is divided into a number of equal length short packets 
  
Main difference between Packet switching and Circuit Switching is that the 
communication lines are not dedicated to passing messages from the source to the 
destination. In Packet Switching, different messages (and even different packets) can pass 
through different routes, and when there is a "dead time" in the communication between 
the source and the destination, the lines can be used by other sources.  
 
There are two basic approaches commonly used to packet Switching: virtual-
circuit packet switching and datagram packet switching. In virtual-circuit packet 
switching a virtual circuit is made before actual data is transmitted, but it is different from 
circuit switching in a sense that in circuit switching the call accept signal comes only 
from the final destination to the source while in case of virtual-packet switching this call 
accept signal is transmitted between each adjacent intermediate node as shown in Fig. 
4.2.3. Other features of virtual circuit packet switching are discussed in the following 
subsection. 
 
4.2.3.1 Virtual Circuit Packet Switching Networks 
An initial setup phase is used to set up a route between the intermediate nodes for all the 
packets passed during the session between the two end nodes. In each intermediate node, 
an entry is registered in a table to indicate the route for the connection that has been set 
up. Thus, packets passed through this route, can have short headers, containing only a 
virtual circuit identifier (VCI), and not their destination. Each intermediate node passes 
the packets according to the information that was stored in it, in the setup phase. In this 
way, packets arrive at the destination in the correct sequence, and it is guaranteed that 
essentially there will not be errors. This approach is slower than Circuit Switching, since 
different virtual circuits may compete over the same resources, and an initial setup phase 
is needed to initiate the circuit. As in Circuit Switching, if an intermediate node fails, all 
virtual circuits that pass through it are lost. The most common forms of Virtual Circuit 
Version 2, CSE    IIT Kharagpur 
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