NETWORK TOPOLOGY

Definition - What does Network Topology concept mean?

Network topology refers to the physical or logical layout of a network. It defines the way different nodes are placed and interconnected with each other; alternately, network topology may describe how the data is transferred between these nodes. There are two types of network topologies: physical and logical. Physical topology emphasizes the physical layout of the connected devices and nodes, while the logical topology focuses the pattern of data transfer between network nodes.

 Three topology : Bus, ring and star

Bus Topology

This topology consists of a Backbone cable connecting all nodes on a network without intervening connectivity devices. The single cable is called Bus and can support only one channel for communication; as a result, every node shares the bus’s total capacity. On a Bus Topology network, devices share the responsibility for getting data from one point to another. But only the intended recipient actually receives and processes the transmitted message while a device wants to send the broadcast message to all the devices that connected to the shared cable.
At the ends of each bus network are 50-ohm resistors known as terminators. The terminators stop signals after they have reached the end of the cable. Without these terminators, signals on a bus network would travel endlessly between the two ends of the network; this phenomenon is called signal bounce, and new signals could not get through.
Ethernet bus topology is actually easy to install and does not require much cabling and only a main shared cable is used for network communication. 10BASE2 and 10BASE-T are two popular types of the Ethernet cables used in the Bus topology. Also, Bus network works with very limited devices. Performance issues are likely to occur in the Bus topology if more than 12-15 computers are added in a Bus Network. In addition, if the Backbone cable fails then all network becomes useless and communication fails among all the devices.

 

Ring Topology 

In ring network, each node is connected to the two nearest nodes so that the entire network forms a circle. In a ring network, all the communication messages are transmitted clockwise in one direction, around the ring. Each workstation accepts and responds to packets addressed to it, and then forwards the other packets to the next workstation in the network. Each workstation just acts as a repeater for the transmission, in other words, all workstations participate in delivery makes the ring topology an active topology. A ring topology has no “ends” and transmitted data stops at its destination. Twisted-pair or fiber-optic cabling is commonly used as the physical medium.
Any damage of the cable of any cable or device can result in the breakdown of the whole network. In addition, just as in a bus topology, the more workstations that must participate in data transmission, the slower the response time. Due to these shortcomings, ring topology now has become almost obsolete. FDDI, SONET or Token Ring Technology can be used to implement Ring Technology. Ring topologies can be found in office, school or small buildings.

 

Star Topology 

In the computer networking world the most commonly used topology in local area networking is the star topology. Star topologies can be implemented in home, offices or even in a building. All the computers in the star topologies are connected to central devices like hub, switch or router. The functionality of all these devices is different. Computers in a network are usually connected with the hub, switch or router with the unshielded twisted-pair (UTP) or Shielded Twisted-pair cables. Star topology networks can support a maximum of only 1024 addressable nodes on a logical network.
Star topology requires more cabling than ring and bus. It also requires more configurations. However, because each node is separately connected to a central connectivity device, they are more fault-tolerant. A single malfunctioning workstation cannot disable an entire star network. A failure in the central connectivity device can take down a local area networking segment. Because they include a centralized connection point, star topology can easily be moved, isolated or interconnected with other networks through hubs, switches to form more complex topologies. Most Ethernet networks are based on the star topology.

  Advantages And Disadvantages

Bus Topology

Advantages
▸ It is easy to set up, handle, and implement.
▸ It is best-suited for small networks.
▸ It costs very less.

A bus topology is characterized by a main trunk or backbone line with the networked computers attached at intervals along the trunk line, as shown in Figure 2.5. This topology type is a passive topology because the computers on a bus just sit and listen. When they "hear" data on the wire that belongs to them, they accept that data (they actually listen using their network interface cards). When they are ready to transmit, they make sure no one else on the bus is transmitting and then send their packets of information.

Disadvantages
▸ The cable length is limited. This limits the number of network nodes that can be connected.
▸ This network topology can perform well only for a limited number of nodes. When the number of devices connected to the bus increases, the efficiency decreases.
▸ It is suitable for networks with low traffic. High traffic increases load on the bus, and the network efficiency drops.
▸ It is heavily dependent on the central bus. A fault in the bus leads to network failure.
▸ It is not easy to isolate faults in the network nodes.
▸ Each device on the network "sees" all the data being transmitted, thus posing a security risk.

Ring Topology

Advantages
▸ The data being transmitted between two nodes passes through all the intermediate nodes. A central server is not required for the management of this topology.
▸ The traffic is unidirectional and the data transmission is high-speed.
▸ In comparison to a bus, a ring is better at handling load.
▸ The adding or removing of network nodes is easy, as the process requires changing only two connections.
▸ The configuration makes it easy to identify faults in network nodes.
▸ In this topology, each node has the opportunity to transmit data. Thus, it is a very organized network topology.
▸ It is less costly than a star topology.

Disadvantages
▸ The failure of a single node in the network can cause the entire network to fail.
▸ The movement or changes made to network nodes affect the entire network's performance.
▸ Data sent from one node to another has to pass through all the intermediate nodes. This makes the transmission slower in comparison to that in a star topology. The transmission speed drops with an increase in the number of nodes.
▸ There is heavy dependency on the wire connecting the network nodes in the ring.

Star Topology

Advantages
▸ Due to its centralized nature, the topology offers simplicity of operation.
▸ It also achieves isolation of each device in the network.
▸ Adding or removing network nodes is easy, and can be done without affecting the entire network.
▸ Due to the centralized nature, it is easy to detect faults in the network devices.
▸ As the analysis of traffic is easy, the topology poses lesser security risk.
▸ Data packets do not have to pass through many nodes, like in the case of a ring network. Thus, with the use of a high-capacity central hub, traffic load can be handled at fairly decent speeds.

Disadvantages
▸ Network operation depends on the functioning of the central hub. Hence, central hub failure leads to failure of the entire network.
▸ Also, the number of nodes that can be added, depends on the capacity of the central hub.
▸ The setup cost is quite high.

  Understanding Network Topologies In The Easiest Way

 

Topology	Bus	Ring	Star
Physical	A bus network consists of a single central cable (backbone), to which all computers and other devices connect	A cable forms a closed loop (ring) with all computers and devices arranged along the ring	All the computers and other devices on the network connect to a central device which is hub/switch, thus forming a star.
Logical	The signals are sent as electrical pulses that travel along the length of the cable in all directions.	The signal transfer in a one direction	The signal transfer from the device to hub or switch before transfer to another device
Advantages	Bus networks are inexpensive and easy to install. Computers and other devices can be attached and detached at any point on the bus without disturbing the rest of the network. Failure of one device usually does not affect the rest of the bus network	The transmission of data is relatively simple as packets travel in one direction only. Cable faults are easily located, making troubleshooting easier. The uses of token passing enables all devices in a ring topology to share the network resources fairly.	If one device fails, only that device is affected. Devices can be added to or removed from network with little or no disruption. Easy to troubleshoot and isolate problem.
Disadvantages	There might be disruption when computer or other devices are added or removed. Because all systems on the network connect to a single cable or backbone, a break in the cable will prevent all systems from accessing the network. It is difficult to identify the problem if the entire network shuts down.	A failure in any cable or device breaks the loop and can take down the entire network. Data packets must pass through every computer therefore, this makes it slower. A ring network can span a larger distance than a bus network, but it is more difficult to install.	If the hub or switch fails, the entire network will be inoperable. Requires more cable than most of the other topologies. More expensive because of the cost of the hub/switch.