Topology is the way networks are physically connected together. Topology determines the complexity and therefore the cost of network cable installation. Cable installation can often be a major cost factor for network system. Topology also determines the strategy for physically expanding the network.
Linear Bus Topology: –
In this layout, a single main cable connects each node, in what amounts to a single line of computers accessing it from end to end. Each node is connected to two others except the machines at either end of the cable, which are connected only to one other node. The network operating system keeps track of a unique electronic address for each node, and manages the flow of data based on this addressing scheme. This topology has the advantage of not requiring that every computer be up and running in order for the network to function. But because a single cable is dedicated to all the information traffic, performance can be slow at times. This topology is often found in client/server systems, where one of the machines on the network is designated as a file server meaning that it is dedicated solely to the distribution of data files, and is not usually used for information processing.
When the computer wants to send a message to another computer, it would send an addressing message to it and wait for a response. The response would indicate whether that computer is ready to receive the previous computers message. After this communication is finished, it goes to the next one and establishes a link with it.
Advantages of The Bus Topology: –
1) The bus is simple, reliable in very small networks, easy to use, and easy to understand.
2) The bus requires the least amount of cable to connect the computers together and is therefore less expensive than other cabling arrangements.
3) It is easy to extend a bus. Two cables can be joined into one longer cable with a connector, making a longer cable and allowing more computers to join the network.
Disadvantages of The Bus Topology: –
1) Heavy network traffic can slow down a bus considerably. Because any computer can transmit at any time, and computers on most bus networks do not coordinate with each other to reserve times to transmit, a bus network with a lot of computers can spend a lot to its bandwidth with the computers interrupting each other instead of communicating. The problem only gets worse as more computers are added to the network.
2) It is difficult to troubleshoot a bus. A cable break or malfunctioning computer anywhere between two computers or loose connector will also cause reflections and bring down the whole network, causing all network activity to stop.
Circular Topology: –
This layout is similar to the linear bus, except that the nodes are connected in a circle using cable segments. In this layout, each node is physically connected to only two others. Each node passes information along to the next, until it arrives at its intended destination. Since each computer re-transmits what it receives, a ring is an active network and is not subject to the signal loss problems which a bus topology experiences. There is no termination because there is no end to the ring.
Performance is faster on this system because each portion of the cabling system is handling only the data flow between two machines. This type of topology can be found in peer-to-peer networks, in which each machine manages both information processing and the distribution of data files.
Some ring networks do token passing. A short message called a token is passed around the ring until a computer wishes to send information to another computer. That computer modifies the token, adds an electronic address and data, and sends it around the ring. Each computer in sequence receives the token and the information and passes them to the next computer until either the electronic address matches the address of a computer or the token returns to its originator. The receiving computer returns a message to the originator indicating that the message has been received. The sending computer then creates another token and places it on the network, allowing another station to capture the token and begin transmission. The token circulates until a station is ready to send and captures the token.
Advantages of Ring Topology: –
1) Because every computer is given equal access to the token, no one computer can monopolize the network.
2) The fair sharing of the network allows the network to degrade gracefully as more users are added.
Disadvantages of Ring Topology: –
1) Failure of one computer on the ring can affect the whole network.
2) It is difficult to troubleshoot a ring network.
3) Adding or removing computers disrupts the network.
Star Topology: –
Each computer on a star network communicates with a central hub that re-sends the message either to all the computers or only to the destination computer. The hub in a broadcast star network can be active or passive.
An active hub regenerates the electrical signal and sends to all the computers connected to it. This type of hub is often called a multiport repeater. Active hubs and switches require electrical power to run. A passive hub merely acts as a connector point and does not amplify or regenerate the signal. Passive hubs do not require electrical power to run.
You can use several types of cable to implement a star network. Hybrid hub can accommodate several types of cable in the same star network. You can expand a star network by placing another star hub where a computer might otherwise go, allowing several more computers or hubs to be connected to that hub. This creates a hybrid star network.
Advantages of Star Topology: –
1) It is easy to modify and add new computers to a star network without disturbing the rest of the network. You simply run a new line from the computer to the central location and plug it into the hub. When the capacity of the central hub is exceeded, you can replace it with one that has a larger number of ports to plug lines into.
2) The center of a star network is a good place to diagnose network faults. Intelligent hubs also provide for centralized monitoring and management of the network.
3) Single computer failures do not necessarily bring down the whole star network. The can detect a network fault and isolate the offending computer or network cable and allow the rest of the network to continue operation.
4) You can use several cable types in the network with a hub that can accommodate multiple cable types.
Disadvantages of Star Topology: –
1) If the central hub fails, the whole network fails to operate.
2) Many star networks require a device at the central point to rebroadcast or switch network traffic.
3) It costs more to cable a star network because all network cables must be pulled to one central point, requiring more cable than other networking topologies.
Tree Topology: –
This is a network topology containing zero or more nodes that are linked together in a hierarchical fashion. The topmost node is called the root. The root may have zero or more child nodes, connected by edges; the root is the parent node to its children. Each child node can in turn have zero or more children of its own. Nodes sharing the same parents are called siblings. Every node in a tree has exactly one parent node, and all nodes in the tree are descendants of the root node. These relationships ensure that there is always one and only one path from one node to any other node in the tree.
Graph Topology: –
In this method of connection, zero or more nodes are linked together in an arbitrary fashion. Any two nodes in a graph may be connected by a link. Not all the nodes in a graph need to be connected, but if a path can be traced between any two nodes, the graph is a connected one.
Star Bus and Star Ring Topology: –
There are the two most common combination networks.
Star Bus Network: –
The star bus topology combines the bus and the star, linking several star hubs together with bus trunks. If one computer fails, the hub can detect the fault and isolate the computer. If the hub fails, computers connected to it will not be able to communicate, and the bus network will be broken into two segments that cannot reach each other.
Star Ring Network: –
In the star ring, also called the star wired ring, the network cables are laid out much like a star network, but a ring is implemented in the central hub. Outlying hubs can be connected to the inner hub, effectively extending a loop of the inner ring.
Mesh Topology: –
It is the most commonly used topology in wan and is often seen in public network link internet.
According to mesh network theory every device needs to have point to point connection with every other device on the network, but in practice, this network is used in a hybrid approach with only the most important devices interconnected in the mesh. This is so because it is impractical to do so in normal condition we can say that this is peer – to – peer networking.