Network Topology - Module 5 Broadcast Communication Networks Lesson 1

 Page 1


 
 
 
 
 
 
 
Module 
5 
  
Broadcast Communication 
Networks 
 
Version 2 CSE IIT, Kharagpur 
Page 2


 
 
 
 
 
 
 
Module 
5 
  
Broadcast Communication 
Networks 
 
Version 2 CSE IIT, Kharagpur 
 
 
 
 
 
 
 
 
Lesson 
1  
 
Network Topology 
Version 2 CSE IIT, Kharagpur 
Page 3


 
 
 
 
 
 
 
Module 
5 
  
Broadcast Communication 
Networks 
 
Version 2 CSE IIT, Kharagpur 
 
 
 
 
 
 
 
 
Lesson 
1  
 
Network Topology 
Version 2 CSE IIT, Kharagpur 
Specific Instructional Objectives  
At the end of this lesson, the students will be able to: 
• Specify what is meant by network topology 
• Classify different Network topologies 
• Categorize various Network topologies  
• Explain the characteristics of the following topologies: 
o Mesh 
o Bus 
o Star 
o Ring 
o Tree 
o Unconstrained 
 
5.1.1 Introduction 
Topology refers to the way in which the network of computers is connected. Each 
topology is suited to specific tasks and has its own advantages and disadvantages. The 
choice of topology is dependent upon type and number of equipment being used, planned 
applications and rate of data transfer required, response time, and  cost. Topology can 
also be defined as the geometrically interconnection pattern by which the stations 
(nodes/computers) are connected using suitable transmission media (which can be point-
to-point and broadcast). Various commonly used topologies are discussed in the 
following sections. 
 
5.1.2 Mesh Topology 
In this topology each node or station is connected to every other station as shown in Fig. 
5.1.1. The key characteristics of this topology are as follows: 
 
 
Key Characteristics: 
o Fully connected 
o Robust – Highly reliable 
o Not flexible 
o Poor expandability 
 
 
 
Figure 5.1.1 Mesh Topology 
 
Two nodes are connected by dedicated point-point links between them. So the total 
number of links to connect n nodes = n(n-1)/2; which is proportional to n
2 
.
 
Media used 
for the connection (links) can be twisted pair, co-axial cable or optical fiber.
 
With this 
topology there is no need to provide any additional information, that is from where the 
packet is coming, along with the packet because two nodes have a point-point dedicated 
Version 2 CSE IIT, Kharagpur 
Page 4


 
 
 
 
 
 
 
Module 
5 
  
Broadcast Communication 
Networks 
 
Version 2 CSE IIT, Kharagpur 
 
 
 
 
 
 
 
 
Lesson 
1  
 
Network Topology 
Version 2 CSE IIT, Kharagpur 
Specific Instructional Objectives  
At the end of this lesson, the students will be able to: 
• Specify what is meant by network topology 
• Classify different Network topologies 
• Categorize various Network topologies  
• Explain the characteristics of the following topologies: 
o Mesh 
o Bus 
o Star 
o Ring 
o Tree 
o Unconstrained 
 
5.1.1 Introduction 
Topology refers to the way in which the network of computers is connected. Each 
topology is suited to specific tasks and has its own advantages and disadvantages. The 
choice of topology is dependent upon type and number of equipment being used, planned 
applications and rate of data transfer required, response time, and  cost. Topology can 
also be defined as the geometrically interconnection pattern by which the stations 
(nodes/computers) are connected using suitable transmission media (which can be point-
to-point and broadcast). Various commonly used topologies are discussed in the 
following sections. 
 
5.1.2 Mesh Topology 
In this topology each node or station is connected to every other station as shown in Fig. 
5.1.1. The key characteristics of this topology are as follows: 
 
 
Key Characteristics: 
o Fully connected 
o Robust – Highly reliable 
o Not flexible 
o Poor expandability 
 
 
 
Figure 5.1.1 Mesh Topology 
 
Two nodes are connected by dedicated point-point links between them. So the total 
number of links to connect n nodes = n(n-1)/2; which is proportional to n
2 
.
 
Media used 
for the connection (links) can be twisted pair, co-axial cable or optical fiber.
 
With this 
topology there is no need to provide any additional information, that is from where the 
packet is coming, along with the packet because two nodes have a point-point dedicated 
Version 2 CSE IIT, Kharagpur 
link between them. And each node knows which link is connected to which node on the 
other end.
 
 
 
Mesh Topology is not flexible and has a 
poor expandability as to add a new node n 
links have to be laid because that new node 
has to be connected to each of the existing 
nodes via dedicated link as shown in Fig. 
5.1.2. For the same reason the cost of 
cabling will be very high for a larger area. 
And due to these reasons this topology is 
rarely used in practice. 
 
 
New 
Node 
Figure 5.1.2 Adding a new node in 
Mesh Topology 
 
5.1.3 Bus Topology 
In Bus Topology, all stations attach through appropriate hardware interfacing known as a 
tap, directly to a linear transmission medium, or bus as shown in Fig. 5.1.3. Full-duplex 
operation between the station and the tap allows data to be transmitted onto the bus and 
received from the bus. A transmission from any station propagates the length of the 
medium in both directions and can be received by all other stations. At each end of the 
bus there is a terminator, which absorbs any signal, preventing reflection of signal from 
the endpoints. If the terminator is not present, the endpoint acts like a mirror and reflects 
the signal back causing interference and other problems. 
 
Shared 
Media 
Terminator Tap 
Stations 
 
 
 
 
 
 
 
 
 
 
Figure 5.1.3 Bus Topology 
 
Key Characteristics of this topology are: 
o Flexible 
o Expandable 
o Moderate Reliability  
o Moderate performance 
A shared link is used between different stations. Hence it is very cost effective. One 
can easily add any new node or delete any node without affecting other nodes; this makes 
this topology easily expandable. Because of the shared medium, it is necessary to provide 
Version 2 CSE IIT, Kharagpur 
Page 5


 
 
 
 
 
 
 
Module 
5 
  
Broadcast Communication 
Networks 
 
Version 2 CSE IIT, Kharagpur 
 
 
 
 
 
 
 
 
Lesson 
1  
 
Network Topology 
Version 2 CSE IIT, Kharagpur 
Specific Instructional Objectives  
At the end of this lesson, the students will be able to: 
• Specify what is meant by network topology 
• Classify different Network topologies 
• Categorize various Network topologies  
• Explain the characteristics of the following topologies: 
o Mesh 
o Bus 
o Star 
o Ring 
o Tree 
o Unconstrained 
 
5.1.1 Introduction 
Topology refers to the way in which the network of computers is connected. Each 
topology is suited to specific tasks and has its own advantages and disadvantages. The 
choice of topology is dependent upon type and number of equipment being used, planned 
applications and rate of data transfer required, response time, and  cost. Topology can 
also be defined as the geometrically interconnection pattern by which the stations 
(nodes/computers) are connected using suitable transmission media (which can be point-
to-point and broadcast). Various commonly used topologies are discussed in the 
following sections. 
 
5.1.2 Mesh Topology 
In this topology each node or station is connected to every other station as shown in Fig. 
5.1.1. The key characteristics of this topology are as follows: 
 
 
Key Characteristics: 
o Fully connected 
o Robust – Highly reliable 
o Not flexible 
o Poor expandability 
 
 
 
Figure 5.1.1 Mesh Topology 
 
Two nodes are connected by dedicated point-point links between them. So the total 
number of links to connect n nodes = n(n-1)/2; which is proportional to n
2 
.
 
Media used 
for the connection (links) can be twisted pair, co-axial cable or optical fiber.
 
With this 
topology there is no need to provide any additional information, that is from where the 
packet is coming, along with the packet because two nodes have a point-point dedicated 
Version 2 CSE IIT, Kharagpur 
link between them. And each node knows which link is connected to which node on the 
other end.
 
 
 
Mesh Topology is not flexible and has a 
poor expandability as to add a new node n 
links have to be laid because that new node 
has to be connected to each of the existing 
nodes via dedicated link as shown in Fig. 
5.1.2. For the same reason the cost of 
cabling will be very high for a larger area. 
And due to these reasons this topology is 
rarely used in practice. 
 
 
New 
Node 
Figure 5.1.2 Adding a new node in 
Mesh Topology 
 
5.1.3 Bus Topology 
In Bus Topology, all stations attach through appropriate hardware interfacing known as a 
tap, directly to a linear transmission medium, or bus as shown in Fig. 5.1.3. Full-duplex 
operation between the station and the tap allows data to be transmitted onto the bus and 
received from the bus. A transmission from any station propagates the length of the 
medium in both directions and can be received by all other stations. At each end of the 
bus there is a terminator, which absorbs any signal, preventing reflection of signal from 
the endpoints. If the terminator is not present, the endpoint acts like a mirror and reflects 
the signal back causing interference and other problems. 
 
Shared 
Media 
Terminator Tap 
Stations 
 
 
 
 
 
 
 
 
 
 
Figure 5.1.3 Bus Topology 
 
Key Characteristics of this topology are: 
o Flexible 
o Expandable 
o Moderate Reliability  
o Moderate performance 
A shared link is used between different stations. Hence it is very cost effective. One 
can easily add any new node or delete any node without affecting other nodes; this makes 
this topology easily expandable. Because of the shared medium, it is necessary to provide 
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some extra information about the desired destination, i.e. to explicitly specify the 
destination in the packet, as compared to mesh topology. This is because the same 
medium is shared among many nodes. As each station has a unique address in the 
network, a station copies a packet only when the destination address of the packet 
matches with the self-address. This is how data communications take place among the 
stations on the bus. 
 
As there are dedicated links in the mess topology, there is a possibility of 
transferring data in parallel. But in bus topology, only one station is allowed to send data 
at a time and all other stations listen to it, as it works in a broadcast mode. Hence, only 
one station can transfer the data at any given time. Suitable medium access control 
technique should be used so as to provide some way to decide “who” will go next to send 
data? Usually a distributed medium access control technique, as discussed in the next 
lesson, is used for this purpose. 
As the distance through which signal traverses increases, the attenuation 
increases. If the sender sends data (signal) with a small strength signal, the farthest station 
will not be able to receive the signal properly. While on the other hand if the transmitter 
sends the signal with a larger strength (more power) then the farthest station will get the 
signal properly but the station near to it may face over-drive. Hence, delay and signal 
unbalancing will force a maximum length of shared medium, which can be used in bus 
topology. 
 
5.1.4 STAR Topology 
In the star topology, each station is directly connected to a common central node as 
shown in Fig. 5.1.4. Typically, each station attaches to a central node, referred to as the 
star coupler, via two point-to-point links, one for transmission and one for reception.  
 
 
 
 
 
 
Key features: 
o High Speed 
o Very Flexible 
o High Reliability 
o High Maintainability 
Figure 5.1.4 Star Topology
 
 
In general, there are two alternatives for the operation of the central node.  
o One approach is for the central node to operate in a broadcast fashion. A 
transmission of a frame from one station to the node is retransmitted on all of the 
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