Network Topologies:

Topology refers to the way in which a network is laid out physically. Geographic orientation and arrangement of networking components is known as LAN topology. While selecting the network topology the following point must be considered.

• The cost of physical interconnections.
• The time delay during the communication.
• Reliability and possibilities of failure.
• Network controlling strategy or protocol.

The different kinds of network topology used in computer networking are Bus/linear, Ring, Star, Tree and Mesh.

Bus / Linear topology:

A bus topology uses one long cable (backbone) to which network devices are directly attached by using short drop cable. As all computers share this bus, a computer checks for any information that might be coming down the backbone before sending its message.

A message passes the other computers on the way to destination. Each computer then checks the address of each message to see if it matches its own. They don't move data affect the rest of the network. Backbone must be terminated at both ends to prevent echoing back signal.

Advantages of Bus topology:

• Easy to install and cheap.
• Easy to add new nodes.
• Less cable required.
• It does not affect the network if one computer goes down.

Disadvantages of Bus topology:

• Since the entire station share the channel, only one station can transmit at any one time.
• If the backbone cable goes down, the entire network system breaks down.
• It covers small geographical area.
• Possibility of data collision.
• Difficult to identification the fault.

Ring Topology:

Ring topology consists of several computer joined together to form a circle. It is called loop topology. Messages move from one computer to the next in one direction only. When a computer receives a message addressed to it, the message is copied and sent back with modification to indicate that it was received. Computers connected to a ring topology are responsible for moving data from themselves to next computer. Thus, if one computer fails, it affects the rest of the network.

Advantages of Ring topology:

• All computers are given equal access to the network.
• It is easy to install.
• Short cable connection which cause an increase in network reliability.
• Less chance of data collision because data travel in one direction.

Disadvantages of Ring topology:

• The failure of one station will cause the system to break down as the link will be broken.
• Adding and removing nodes disturbs the entire network.
• Difficult to configuration.
• In a ring network communication delay is directly proportional to the number of nodes in the network. Hence the addition of new node increases the communication delay.

Star topology:

In star topology, each device (node) has a dedicated point to point link only to a central controller, usually called hub. The computers are not directly linked to one another.

Unlike other topology, star topology does not allow direct traffic (flow of data) between computers. The controller acts as an exchange. If one computer wants to send data to another, it first sends the data to the controller which then relays the data to the other computer. If one link  fails, only that link is affected and all other links are active.

Advantages of Star topology:

• Easy to install and configuration.
• Easy to find out the fault.
• Easy to modify and expansion.
• Single computer failure does not affect other computer in network.
• High data transmission is possible.

Disadvantages of Star topology:

• If the central controller (hub) fails whole network goes down.
• Require more cable than most topologies.
• More expensive than linear bus topologies because of the cost of the concentrators.
• Difficult to expand.

Tree Topology:

In tree topology, the nodes are connected to each other in such a way that forms like a tree structure. Typically to form a tree network, multiple star topologies are combined together. This type of network has combined features of bus and star topology. One tree topology the hubs of each star topology are connected to the central hub that controls the entire network. However, some nodes can be directly connected to the central hub.

Advantages of tree topology:

• Easy to extend because it is divided into many sub-units.
• Easy to add new nodes to branches.
• Supported by several hardware and software venders.
• It supports various types of cable.

Disadvantages of tree topology:

• It is expensive.
• If the main root or hub fails to operate, then the entire network will go down.
• More difficult to configure and wire than other topologies
• Less reliable.

Mesh Topology:

In mesh topology, each computer has a dedicated point to point link to other computers. The term dedicated means that the link carries traffic only between the two devices. Hence, a fully connected topology of 'n' computers has n(n-1)/2 links. If one link fails, it does not affect entire system. It has privacy or security. Large amount of cable is required.

Advantages of Mesh topology:

• It provides alternative routes between nodes.
• Point-to-point link makes fault isolation easy.
• Fastest data transmission.
• Failure in one of the computer does not affect the entire network.
• More reliable.

Disadvantages of Mesh topology:

• Difficult to install/setup.
• It is expensive and difficult to expand.
• It requires large cabling.