In the previous tutorial, we have studied about the various techniques of switching. To take this course ahead, we will now look at the network topology used in computer networks.

What is a Network Topology?

The schematic arrangement of various nodes or computers in a computer network is often referred to as a network topology.

Network Topology could be:

• Physical topology e.g. Mesh topology, Bus topology, Ring topology, Star topology
• Logical topology e.g. IEEE 802.3, IEEE 802.5

We focus upon the physical topologies. Physical arrangement of systems and their connection with the cables is called physical topology.

1. Mesh Topology

Mesh topology include connection of every device to every other device via some channel. If N number of devices are connected with each other in mesh topology, then, total number of ports that is required by each device is N-1.

Mesh Topology

In the above figure, there are five devices which are connected to each other using mesh topology. Thus, a total of 4 ports are required for each node.

Note:

• It is worth noting that if a total of N devices are connected with each other in mesh network topology, then, the total number of dedicated links required to connect all of them = NC2 i.e. $ ( \frac {N * (N-1)}{2} ) $.
• In the above example, we can see that a total of $ ( \frac {5 * (5-1)}{2} ) $ = 10 links are there.

Advantages of Mesh Topology

• Robust.
• Fault can be easily diagnosed.
• Reliable data since data transfer occurs through dedicated channels or links.
• Security and privacy are ensured.

Disadvantages of Mesh Topology

• Difficult to install and configure.
• Since this topology requires bulk wiring, it is a costly arrangement and thus it is preferred
  for lesser no of devices.
• It is costly to maintain.

2. Star Topology

All the computers are connected to a single hub via cables. This hub is said to be the central node in star topology. All other nodes or what we say computers are connected to this hub via cables.
A hub can be passive like broadcasting devices or active and act like an intelligent device. Active hubs may act as a repeater for data flow.

Star Topology

Advantages of Star Topology

• Star network topology leads to a faster performance with few nodes and low network traffic.
• The hubs are easier to upgrade.
• The nodes connected through star topology are easier to troubleshoot.
• Since ‘N’ devices are connected to each other in star topology, then, ‘N’ number of cables is required to connect them. Thus, it is easy to set up, install and modify a star topology.
• Since only the failed node is affected, remaining nodes can still function smoothly.

Disadvantages of Star Topology

• The cost of installation of star topology is still high and it is expensive.
• Performance of star topology is based on the hub’s capacity.
• If the hubs fail in case of a star topology, then, the entire arrangement of networks will fail since all the defined nodes are connected to the hub.

3. Bus Topology

All the computers are connected to a common cable called the bus in the bus topology. The data is transmitted in a single direction from one end of the cable to another end in the bus topology arrangement.

Bus Topology

Advantages of Bus Topology

• Only one cable is required to connect to all the nodes called the ‘backbone cable’. A total of ‘n’ drop lines are required. Thus, the length of the cables required is less as compared to the other topologies.
• It is a cost effective arrangement of nodes.
• It is preferred in small networks.

Disadvantages of Bus Topology

• If the backbone cable is damaged or a failure occurs in this cable, then, the entire network comes to a halt.
• The performance of the network decreases in case of a heavy network.
• The backbone cable has a limited length.
• This network topology is slightly slower than ring topology.

4. Ring Topology

In ring topology, the computer is connected to only its two neighbours instead of all the computers in the network.

Ring Topology

Flow of information and transmission of data in the ring topology:

• One of the computers is selected as monitor station in one network. The monitor takes all the responsibility to perform the operations.
• Let’s call the computers to be stations. A token is circulated around in the ring. In order to transmit the data, the circulating token has to be held by the station that wish to transmit the data. Once the transmission is successful, the token held by the station is released so that it can be used by other stations for transmission.
• The token circulates around the ring while no transmission is being done by any of the stations.
• The token can be released using the two techniques:
  Early Token Release: The token is transmitted soon after data is released.
  Delay Token Release: The token is released after the receiver acknowledges.
  We have studied about these terms in greater depth in the previous tutorial. Please refer to access control methods.

Advantages of Mesh Topology

• It is cheap to install and expand the arrangements in ring topology.
• The transmitting network in case of ring network topology can never be affected by the high intensity of traffic or by adding the number of nodes. This is because only those nodes which have tokens can transmit the data. Thus, there is least possibility of collision.

Disadvantages of Mesh Topology

• Adding or deletion of a specific computer from the network disturbs the entire network activity since the computers are connected to their immediate neighbours only.
• Troubleshooting is difficult in this network topology.
• If one computer fails the entire network fails, the activities come to a halt in ring topology.

Is it possible to configure more than one topology in a network?

• Now with the advancement in technology, it is definitely possible to configure more than one topology in any network. The collection of more than one topology in one network is referred to as Hybrid Topology. We can have hybrids of mesh topology and ring topology, or all the topologies can be configured in one network or any other possible combination is possible.

That’s end of our tutorial on network topology. We hope the idea of network topologies is clear now. In the incoming tutorials, we will get into the various layers of computer networks. Thus, it is expected that you go through the previous tutorials carefully before moving ahead. We will be applying the previous concepts in ISO/OSI Stack of computer network.

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