NETWORK TOPOLOGY:

INTRODUCTION :
The word topology is commonly used to determine the properties of various types of networks. Networks have shapes and the shape that a network takes has much to do with the way it functions. A network is represented as a collection of nodes connected by communication cables. Network topology determines how different nodes are connected and how they communicate. Network topology can be physical or logical. The physical topology refers to the actual layout or appearance of a network and its transmission media. It determines the network shape of the in which various components are connected. The logical topology of a network describes how the data flow through a physical topology. The distinction between the logical and physical topology is necessary because the way something looks can be quite different from the way it functions. The physical topology is how the computers and peripherals are connected and how the cable is running between them. In other words, the way the network looks. The logical topology describes the way in which a network transmits information from one node to the next node. A network's logical topology is not necessarily the same as its physical topology.

While discussing the network topologies we will come across the following terms.

Node: Node is a device that is connected to a network. E. g. Workstation, Server, Printers etc.
Workstation: It refers to any computer connected to a network.
Packet: A packet is a message that is sent over the network from one node to another. A packet contains the address of the source and the destination along with control information.
Server: A computer system that is running a software that enables it to provide various services to other computers that are part of the network. For example: We can have a file server that is a central storage space for the network, printer server that handles various printing tasks.
Network Interface Card (NIC): It enables a computer to communicate with other on a network. There are six basic topologies which include the Bus, Star, Ring, Hybrid, Tree and Mesh topology.

1 BUS TOPOLOGY :
Bus topology is a multipoint topology also called linear or horizontal topology. It uses a single transmission medium called a bus or a trunk to connect all the nodes together i. e. nodes communicate with each other using a common bus. This bus is also called a backbone. Usually a coaxial cable is used to connect nodes in a bus topology.

Nodes in a bus topology are connected to the backbone by a T - connector and droplines (A dropline is a cable running from the cable to the node). Each node is connected to two others except for the nodes at the either end of the backbone cable. A terminator is required at each end of the backbone to absorb the signal so that it does not reflect back across the bus.

ADVANTAGES :
Following are the advantages of the bus topology:

Node (computers and other devices) can be attached and detached at any point on the bus without disturbing the rest of the bus network.
Failure of a node does not affect the rest of the network.
It is inexpensive to install as cabling cost is minimized due to common bus
Less length of cable is required to connect the nodes.
Simple and flexible.

DISADVANTAGES:
Although bus topology is a common network topology buy it still suffers from some limitations.

Number of computers that can be part of network are limited.
Failure of a bus (ie. Network media) brings the entire network to a halt.
There can be security problem since every node may see every message.
With the increase in the number of nodes the speed of the network slows down.
Trouble shooting can be difficult since the fault can be anywhere along the bus.

2. STAR TOPOLOGY:
Star topology is a widely used method for cabling in computer networks. In a Star topology | Each node is connected by a point to point dedicated link to a central hub or switch or any other connecting / routing device known as concentrator. A concentrator is a device that provides a central connection point for cables from various nodes. A concentrator can be a hub or switch.

The nodes are not directly connected to each other, they communicated with each other through the central controller (hub / switch), so the control is centralized.
If a node wants to send data to another node, the data goes to the central controller. If hub is used as a central controller then the received data is broadcast to all the nodes . In this case, the network topology appears to be star but is actually a bus as in case of a hub each node on the network. sees all the packets transmitted over the network just as in case of bus topology. So this is physical star topology and logical bus topology.

ADVANTAGES :
Following are the advantages of Star topology:

Failure of a node only isolates its from the network but does not affect the rest of the network.
Node can be added or removed to / from the network simply by connecting it to the central controller and does not disturbs the rest of the network.
Centralized monitoring and management is possible because data must travel through central controller.
Easy to install and maintain due to centralized monitoring.

DISADVANTAGES:
Following are the disadvantages of networks using Star topology:

Failure of central controller brings the network to a halt.
Crowding of cables, as each node is connected to the central point by individual cable.
As each node is connected to a central controller by individual cable so more cable length is required as compared with a bus or ring topology.
More costly due to more cable length and additional cost of hub / switch.

3. RING TOPOLOGY:
Ring topology is a network topology in which nodes are connected in a closed loop so as to form a ring in such a way that every node is connected to two adjacent nodes. Ring topology does not require terminators as it does not has an open end. It is opposite to a bus topology. A bus has two end points but a ring is a circle with no ends. Moreover, unlike bus topology in which network devices do hot play an active role in the network, each device has additional network functions in case of ring topology.

Each node in the ring can act as a repeater boosting the signal before sending it to the next node, so ring topology is an active topology.

4. ADVANTAGES:
Following are the advantages of using ring topology in networks.

Each node has equal access to the ring because each node is given equal access to the token. No node can monopolize the network.
Each node acts a repeater that amplifies the signals. So very little signal degradation occurs.
Less crowding of cables as compared with star and mesh topologies as nodes are connected by a single loop of cable.
Less cost of cabling as compared with Star topology. But the cost increases if the distance between the nodes increases.
No collisions as all the packets travel in one direction.
Ring topology can span larger distance than other networks (such as bus) as each node regenerates the signal.

DISADVANTAGES :
Following are the disadvantages of ring topology:

It is difficult to add or remove node as adding or removing node disturbs the rest of the network.
Failure of a node on the ring can affect the entire network.
Troubleshooting a ring network is difficult.
As the number of node increases more time is required to delay the messages and network becomes slower.
A break in the ring can disable the entire network. However, many topologies use dual rings so that if one ring fails other can perform its functions.

4. MESH TOPOLOGY :
Mesh topology is a point to point network topology in which devices are connected with many rodundant interconnections between nodes. Mesh topology can employ one of two connection arrangements.

Full mesh topology.
Partial mesh topology .

The Full mesh topology also called true mesh topology is a network topology in which each node is connected by point to point link to every other node i. e. Each node has a direct path to every other node. A network that uses full mesh topology is called fully connected network. This mesh topology provides many redundant paths. If one of the links fail, other can be used for communication. The communication between any two nodes is faster because a message needs to use only one link to travel between two nodes. Many connections may be under utilized if the involved nodes rarely communicate with each other.
True mesh topology is impractical to set up as it requires large number of direct links. For example: In case of 100 nodes, the true mesh topology requires 4650 (i. E. N x (n - 1) / 2 for n = 100) direct connections, so most popular mesh topologies are partial mesh topologies.
In partial mesh topology, some of the nodes are connected by point to point link to all other nodes, but rest of the nodes are connected to only those nodes with which they exchange most of the data. In other words, with partial mesh topology only some of the nodes are organized in full mesh scheme. A network that employs partial mesh topology is called a hybrid mesh topology.

ADVANTAGES:
Following are the advantages of networks using mesh topology.

It provides better performance.
Data can never fail to be delivered as failure of anode does not disturbs the network and rest of the nodes can still communicate using alternative paths.
Less traffic on each link.
Easy to troubleshoot because it is easy to identify the broken link and bypass that link.
Very robust. If a link becomes usable then there are other links available to carry the traffic.

DISADVANTAGES:
Following are the disadvantages of using mesh topology: -

More complex.
Difficult to manage as no central control exists.
Very expensive due to large number of direct connections.
Installation and configuration of a mesh network involving large number of nodes is difficult.

5. TREE TOPOLOGY:
Tree topology is a network topology in which nodes are organized as a tree. It can be viewed as a collection of star configured nodes arranged in a hierarchy, so it is also known as hierarchical topology. This topology is commonly used for corporate networks where individual offices are connected to local main office. The various main offices are then connected to regional offices which are further linked to headquarters.

ADVANTAGES :
Following are the advantages of networks using tree topology.

There is no closed loop like ring topology. Hence, failure of one branch does not affect the rest of the network.
It is easy to expand the tree topology as nodes can be added to the branches with disturbing the transmission of information on the network.
It is supported by several hardware and software vendors.

DISADVANTAGES:

Tree topology is complex to configure in comparison to bus and star topology.
If the backbone fails, complete network comes to a halt.
The number of nodes on each branch is limited.
Tree topology is more expensive to administer.

HYBRID TOPOLOGY :
Hybrid topology is a network topology that uses a combination of two or more basic topologies (bus, star, ring) in such that resulting topology does not have any of the standard forms. For example : A tree topology connected to a tree topology is still a tree topology. But two star networks connected together (known as extended star ) is a hybrid topology.

NETWORK TOPOLOGY: