Topologies in Computer Networks

NETWORK TOPOLOGIES!  

The term topology refers to the way in which a network is laid out physically. It is a combination of two or more links, each connecting a number of devices to it. It shows the logical representation between the links and the nodes that are connected to them. There are four basic topologies of networks: mesh, star, bus and ring.

Fig. : Topologies used in Computer Networks

Bus TopologyMesh Topology

In mesh topology, every device has a dedicated point-to-point link to every other device. Each such a link carries the traffic only between the two devices it connects. To find the total number of physical links in a fully connected mesh network having n nodes, we use the formula:

n (n – 1)

However, if each physical link allows communication in both directions (i.e., communication in duplex mode), we can calculate the number of links by dividing the total value by 2:

n (n – 1) / 2

To accommodate n devices (workstations) in a mesh topology, every device on the network must have n-1 input/output (I/O) ports to connect to the other n-1 stations. The following figure shows a network using mesh topology with n workstations:

Fig. : A fully connected mesh topology

There are certain advantages and disadvantages of using mesh topology for forming a network. They are as follows:

Advantages:

1. The user of dedicated links guarantees no traffic problem that can occur when links are shared by multiple devices.
2. It is robust, i.e., if one link fails, it will not affect the traffic in the entire network.
3. There is an advantage of privacy or security. As messages travel along a dedicated line, only the intended recipient can obtain it.
4. It makes fault identification and fault isolation easy.

Disadvantages:

1. As every device is connected with every other device, installation and reconnection are difficult.
2. The sheer bulk of the wiring can be greater than the available space (in walls, ceilings or floors) can accommodate.
3. The hardware required to connect each link (I/O ports and cable) can be prohibitively expensive.

Due to the above said points, a mesh topology is usually implemented in a limited fashion, for e.g., as a backbone connecting the main computers of a hybrid network that can include several other topologies.

Star Topology

In star topology, each device has a dedicated point-to-point link only to a central controller, usually called a hub. The devices are not directly linked to each other, but through the controller, which acts as an exchange. If one device wants to send data to another, it sends the data to the controller, which then relays the data to the other connected device in the network.

Fig. : A Star topology connecting four stations

Some of the advantages and disadvantages of star topology are discussed below:

Advantages:

1. A star topology is less expensive than a mesh topology, because of less number of links used for connecting the devices. Each device needs only one link and one I/O port to connect it to any other devices.
2. Easy to install and reconfigure. Adding, moving or removing a device from the network involves only on3 Pageviewse connection: the connection between the device and the hub.
3. It is robust in managing the failures in links between its devices. If one link fails, only that link is affected. All other links remain active. Moreover, the hub helps the administrator to identify the fault link easily and to rectify it.

Disadvantages:

1. The major disadvantage of this topology lies in the dependency of the whole network on one single point, the hub. If the hub goes down, the whole system is dead.
2. It requires more cabling than some other topologies, due to the reason that each device must be connected to the central hub.

Due these advantages and disadvantages, this topology is used mainly in LAN, especially in High-speed LANs.

Bus Topology

Bus topology is a multipoint network, which uses a long cable as a backbone to the network, to which all the devices are connected. All the devices are connected to the bus cable by drop lines and taps.

Fig. : A Network using Bus Topology

A drop line is a connection running between the device and the main cable. A tap is a connector that either splices into the main cable or punctures the sheathing of a cable to create a contact with the metallic core. As a signal travels along the backbone, some of its energy is transformed into heat. Therefore, it becomes weaker and weaker as it travels farther and farther.

There is some reflection of signals at the taps, which causes degradation in quality of signals transmitted. This degradation can be controlled by limiting the number and spacing of devices connected to a given length of cable. Adding new devices may therefore require modification or replacement of the backbone. The advantages and disadvantages of this topology are as follows:

Advantages:

1. Ease of installation.
2. Uses less cabling than mesh or star topologies.
3. Used mainly in the design of early LANs and later in Ethernet LANs.

Disadvantages:

1. It involves some difficulty in reconnection and fault isolation.
2. It finds difficulty in adding new devices.
3. It creates noise in the network, when there is a damage or fault in the cable. Moreover, a fault or a break in the media stops all transmission, even between the devices on the same side of the problem due to noise.

Ring Topology

In ring topology, each device has a dedicated point-to-point connection with only two devices that are located at both sides of it. Each device in the ring acts either as a receiver or repeater. When it receives any data, it checks whether the incoming data belong to it or not. If it belongs to it, it just receives the data. Otherwise, it regenerates the signal and passes it on to the system next to it.

The data flow in a ring network is always unidirectional. Though each device is connected to two other devices in the ring, it always receives data from any one of the two devices. A signal is passed along the ring from device to device, until it reaches its destination. The following figure shows a ring topology connection with six stations:

Fig. : A Ring network connecting 6 workstations

Ring topology also has its merits and demerits. The following list displays it:

Advantages:

1. Easy to install and reconfigure. This is due to the fact that the addition or removal of a device in the network requires change in only two places.
2. The fault isolation is simplified. The ring is designed in such a way that it can raise an alarm, when a particular workstation doesn’t receive the signal passed along the ring.

Disadvantages:

1. Transferring the signal always takes place in one direction, which results in taking a long route to deliver the data even to its neighbor at the sending side.
2. In a simple ring, a break in the ring can disable the entire network.

Hybrid Topology:

Hybrid topology is a combination of two or more topologies, among which one acts as a backbone.