Network Topology

28/03/2020

Network topology is the arrangement of the elements (links, nodes, etc.) of a communication network. Network topology can be used to define or describe the arrangement of various types of telecommunication networks, including command and control radio networks, industrial fieldbusses and computer networks.

Network topology is the topological structure of a network and may be depicted physically or logically. It is an application of graph theory wherein communicating devices are modeled as nodes and the connections between the devices are modeled as links or lines between the nodes. Physical topology is the placement of the various components of a network (e.g., device location and cable installation), while logical topology illustrates how data flows within a network. Distances between nodes, physical interconnections, transmission rates, or signal types may differ between two different networks, yet their topologies may be identical. A network’s physical topology is a particular concern of the physical layer of the OSI model.

Examples of network topologies are found in local area networks (LAN), a common computer network installation. Any given node in the LAN has one or more physical links to other devices in the network; graphically mapping these links results in a geometric shape that can be used to describe the physical topology of the network. A wide variety of physical topologies have been used in LANs, including ring, bus, mesh and star. Conversely, mapping the data flow between the components determines the logical topology of the network. In comparison, Controller Area Networks, common in vehicles, are primarily distributed control system networks of one or more controllers interconnected with sensors and actuators over, invariably, a physical bus topology.

Types of Network Topology

The arrangement of a network which comprises of nodes and connecting lines via sender and receiver is referred as network topology. The various network topologies are:-

1. Mesh Topology

In mesh topology, every device is connected to another device via particular channel.

Every device is connected with another via dedicated channels. These channels are known as links.

• If suppose, 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. In the Figure 1, there are 5 devices connected to each other, hence total number of ports required is 4.
• If suppose, N number of devices are connected with each other in mesh topology, then total number of dedicated links required to connect them is NC2 i.e. N(N-1)/2. In the Figure 1, there are 5 devices connected to each other, hence total number of links required is 5*4/2 = 10.

• It is robust.
• Fault is diagnosed easily. Data is reliable because data is transferred among the devices through dedicated channels or links.
• Provides security and privacy.

Problems with this topology

• Installation and configuration is difficult.
• Cost of cables are high as bulk wiring is required, hence suitable for less number of devices.
• Cost of maintenance is high.
1. Star Topology

​ In star topology, all the devices are connected to a single hub through a cable. This hub is the central node and all others nodes are connected to the central node. The hub can be passive ​in nature i.e. not intelligent hub such as broadcasting devices, at the same time the hub can be intelligent known as active ​hubs. Active hubs have repeaters in them.

A star topology having four systems connected to single point of connection i.e. hub.

• If N devices are connected to each other in star topology, then the number of cables required to connect them is N. So, it is easy to set up.
• Each device require only 1 port i.e. to connect to the hub.

Problems with this topology

• If the concentrator (hub) on which the whole topology relies fails, the whole system will crash down.
• Cost of installation is high.
• Performance is based on the single concentrator i.e. hub.
1. Bus Topology

​ Bus topology is a network type in which every computer and network device is connected to single cable. It transmits the data from one end to another in single direction. No bi-directional feature is in bus topology.

A bus topology with shared backbone cable. The nodes are connected to the channel via drop lines.

• If N devices are connected to each other in bus topology, then the number of cables required to connect them is 1 ​which is known as backbone cable and N drop lines are required.
• Cost of the cable is less as compared to other topology, but it is used to built small networks.

Problems with this topology

• If the common cable fails, then the whole system will crash down.
• If the network traffic is heavy, it increases collisions in the network. To avoid this, various protocols are used in MAC layer known as Pure Aloha, Slotted Aloha, CSMA/CD etc.
1. Ring Topology

​ In this topology, it forms a ring connecting a devices with its exactly two neighbouring devices.

A ring topology comprises of 4 stations connected with each forming a ring..

The following operations takes place in ring topology are:-

One station is known as monitor station which takes all the responsibility to perform the operations.

To transmit the data, station has to hold the token. After the transmission is done, the token is to be released for other stations to use.

When no station is transmitting the data, then the token will circulate in the ring.

There are two types of token release techniques : Early token release releases the token just after the transmitting the data and Delay token release releases the token after the acknowledgement is received from the receiver.