Network Topology

The way in which the devices are connected in a network both physically and logically is referred to as Network Topology. If one understands the topology of a network, one can easily determine the functioning of the network i.e., the connection between the devices, the number of devices in the network, the capacity of the network, whether the network is centrally controlled or distributed, etc.

There are two ways of defining network topology. They are:

  • Physical Topology
  • Logical Topology or, Signal Topology

Physical Topology

Physical Topology refers to the way in which the devices in a network are connected physically. When two or more devices are connected together then they form a link. When two or more links are connected together then they form a topology. Each device connected in a topology is referred to as a node. There are four basic types of topologies — mesh topology, ring topology, star topology and bus topology. In addition to these four topologies, there are two more common types of topologies – hybrid topology and tree topology.

Mesh Topology

A topology in which each device has a dedicated point-to-point connection with every other device in the network is called a mesh topology.  There are two types of mesh topology —  Full Mesh (Fully Connected Mesh Topology) and Partial Mesh (Partially Connected Mesh Topology).

In full mesh, each device is directly connected to each and every device in the network via a dedicated point-to-point connection.

(Full Mesh)

In partial mesh, some of the devices are directly connected to other devices in the network, some of the devices are connected to only those devices with which they communicate the most in the network and some of the devices may not be connected to each other via a dedicated point-to-point connection and thus they will not be able to communicate.

(Partial Mesh)

Number of wires needed for a full mesh topology where ‘n’ number of devices have to be connected with each other is = (n(n-1))/2

Number of input-output (I/O) ports required on each devices to be connected in a mesh topology with n-1 other devices = (n-1)

Advantages of Mesh Topology:

  • Due to dedicated links, there is no traffic problem. Each link has to take care of its own traffic and has to only maintain its own network load.
  • Failure of one link doesn’t affect the entire network. This property is known as Robustness.
  • It is more secure and private as the message is sent via a dedicated link, so, only the intended recipient can view the message.
  • Due to dedicated links, fault detection is easier.
  • Messages sent to devices connected in the form of a mesh network can take any of several possible paths from source to destination, if required.

Disadvantages of Mesh Topology:

  • A very large number of cables are required as each device in the network should be connected to all other devices in the same network.
  • A large number of I/O ports are required on each device.
  • It is very expensive to implement due to cost involved in hardware (I/O ports and cables).
  • It requires large space for setup as wires accommodate a large area of the total setup.
  • Adding new devices or re-connecting devices in the network is very time consuming as it has to be connected to each and every device in the network.

Ring Topology

A topology in which each device is connected to it’s immediate neighbouring device (on both sides) via a dedicated point-to-point link, is known as ring topology.  In ring topology the devices are connected in a circular manner and forms a continuous pathway i.e. a ring. A token (signal) continuously travels in the network that stops at each device. Any device can send data when it receives an empty token. The sending device removes the token from the network and then releases the token with the information that needs to be sent and the destination address. The token then travels in the network until it reaches the destination device. The destination device copies the data off the token travels in the network until it reaches back to the sender. The sender then takes the token off the network and sends out a new empty token in the network. A repeater can be connected to each device of the network. A token is a special series of bits that contains control information and the method of transmitting data in a ring topology is known as token passing. Each network can have ONLY ONE token.

The direction of communication can be either clockwise or anti-clockwise.

Advantages of Ring Topology:

  • Adding and configuring new devices is easier. This doesn’t impact the performance of the network.
  • All the devices have equal access to the resources.
  • No centralized controller is required to control the connectivity among the nodes.
  • There is a rare chance of data collision as each node can release the data packet after receiving an empty token. Also, the data flows at a very high speed in one direction ONLY.
  • Under heavy traffic conditions, ring topology is better than bus topology.

Disadvantages of Ring Topology:

  • The data travels in one direction in the topology (unidirectional flow of data). So, if 5 devices are connected in a ring topology namely: A, B, C, D and E and the direction of flow of data is in the direction from A to B and henceforth, then if we want to send the data from A to E then the data will travels via B, C and D before reaching E despite of the fact that A and E were directly connected. Thus, ring topology is slower than star topology.
  • Fault in any link disables the entire network i.e. if one device stops working then the entire network stops working. Suppose the data needs to be sent from A to C and device B is not working, then the communication will not be able to take place. This will also be the case if D wants to send data to C.
  • In case of a fault, it is difficult to troubleshoot the network as each link has to be tested for communication failure.
  • If a packet is sent to device that doesn’t exist in the network then no more communication can take place as the node will not receive an empty token; unless the token is automatically set to discard the data packet after a certain number of hops or after a certain period of time.
  • Communication delay is directly proportional to the number of nodes connected. It is maximum if any device has to send data to it’s previous device and it is minimum if any device has to send data to its next immediate neighbour.

Star Topology

‘A star topology is the most popular and widely used network topology.’ A topology in which every device is connected to a central controller (either hub or switch) is known as star topology. The hub or switch is the common point of contact for all devices in the network as the devices are not linked to one another directly. Each device has only a single dedicated point-to-point link to the central controller. Each and every data packet passes through the central controller before reaching the destination device i.e. the data packets from the source device is sent to the central controller which then transmits/relays the data packet to the destination device.

Advantages of Star Topology:

  • It is less expensive than mesh topology as each device requires only one I/O port to connect with any number of devices. It also requires less cabling than mesh.
  • It is easier to install new devices, reconfigure and remove existing devices as the device only needs to be configured with the central controller and the connection of wires needs to be done only between the device and the central controller.
  • If one of the devices/links fails, then the entire network is not affected.
  • It is easier to detect faults in the network. The central controller can be used for monitoring the network for failures.
  • There is no network interruption when adding or removing devices.

Disadvantages of Star Topology:

  • The central controller represents a single point of failure. The whole network depends on the central controller i.e. if the central controller goes down, the entire network stops working.
  • It is more expensive than linear bus topology due to the cost of the central controller.
  • It requires more cable length than linear bus topology.

Bus Topology

A topology in which all the devices are connected to a common cable (also known as bus or backbone) is known as bus topology. This common cable acts as a backbone to link all the devices in the network. The communication in this topology takes place via the bus. The devices are connected to the common cable by drop line and tap. The connecting cable running between the device and the main cable is referred to as drop line. The point where the drop line is connected to the common bus is known as tap. A tap is basically a connector that is used to connect the drop line cable and the bus. A terminator is added at both ends of the cable to prevent the signal from traveling back in the bus again. The function of the terminators is to dump the signal/packet.

Advantages of Bus Topology:

  • It is easy to install and is often used for small networks.
  • It is cheaper than all other topologies as it consists of only one bus and the other small connecting wires.
  • It uses less cables than star mesh topology or mesh topology.
  • If one device fails, it doesn’t affect the entire network.

Disadvantages of Bus Topology:

  • The bus length and the number of devices that are connected to the bus are limited. When the signal travels along the bus, some of the signals are lost as the energy of the signal is lost in the form of heat. So, the signal becomes weaker as it travels farther. So, there is a limit in the cable length and hence the number of devices that could be connected.
  • The transmission speed decreases with the increase in the number of devices as the power and bandwidth of the bus is limited and shared between all the devices.
  • It is difficult to add new devices as the central bus needs to be modified or replaced to add new taps. Also, there is limit to the number of taps and the distance between them.
  • The common bus is the single point of failure in the network. If the bus stops working or breaks, then the entire network stops working.
  • Finding faults is difficult in this topology. Finding faults require checking and exploring each and every device and connecting wires.

Hybrid Topology

Hybrid topology is designed by the combination of two or more basic topologies. The resulting topology will have the characteristics of all the topologies that were used. The combination of topologies is based on the requirement of the end user or the organization.

The hybrid network shown below consists of two star topologies connected using a bus topology. This hybrid network is known as ‘Extended Star’ and is one of the widely used hybrid networks.

Tree Topology

A tree topology is a combination of star topology and bus topology. It is also known as ‘Extended Star Topology’. There are few special cases in which a tree topology is more effective:

  • when the communication needs to be done between two networks.
  • a network structure which requires three or more levels of hierarchy.

Advantages of Tree Topology:

  • Expansion of network (i.e. adding or reallocating devices) is very easy.
  • Fault detection and correction is very easy.
  • The whole network is divided into small segments i.e. star networks which are easy to maintain and manage. Each individual segment has a dedicated point-to-point link to the device at higher hierarchy level (the device maybe hub, switch or router)
  • If any segment is not working, the other segments and the entire network will not be affected.

Disadvantages of Tree Topology:

  • Expansion of network (i.e. adding or reallocating devices) is very easy.
  • Fault detection and correction is very easy.
  • The whole network is divided into small segments i.e. star networks which are easy to maintain and manage. Each individual segment has a dedicated point-to-point link to the device at higher hierarchy level (the device maybe hub, switch or router)
  • If any segment is not working, the other segments and the entire network will not be affected.

Logical Topology or, Signal Topology

The logical topology or signal topology of a network describes the flow of data within the physical topology and the various ways in which the connected devices are communicating. There are two types of logical topology – Shared Media and Token Based.

Shared Media

In a shared media topology, all the devices in the network have the ability to access the physical media in the topology whenever they need. Shared media is used in bus and star topology. The main disadvantage of using this topology is the occurrence of collisions in the network. If two devices are sending information (packets) at the same time on the same wire, then the packets collide and the information contained in the packets loses its integrity and in the worst case scenario the packets are discarded. By the use of proper and high-quality communication cables that supports multiple data stream transfers simultaneously, collisions can be minimized to a large extent.

Token Based

In a token-based network, there is token that travels around the network. When a device needs to send data to another device, it grabs the token, attaches the data to it along with the destination address, and then releases the token in the network. When the token arrives at the destination device, it copies the information off the token and releases the token back into the network with the acknowledgement of the information received. The token then continues to travel in the network until it gets back to the sender. When the sender receives the token back, it takes the token off the network and sends out a new empty token to be used by any other device. The ring topology uses token-based approach.

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