Network Topologies

Network Topology The topology of a network specifies the geometric arrangement of the network. The complete physical structure of the cable (or data-transmission media) , workstations, nodes, routers and gateways are called the physical topology. The way or path data flows through the network is called the logical topology.

Types of Network Topologies Network topologies are categorized into the following basic types: Bus Ring Star Tree Mesh

Bus topology A bus topology uses one long cable (backbone) to which network devices are attached by using drop lines and tap. A terminator is required at both ends of the cable. A signal from the source travels in both directions to all machines connected on the bus cable until it finds the address on the network that is the intended recipient. If the machine address does not match the intended address, the machine ignores the data. Alternatively, if the data does match the machine address, the data is accepted. Bus topology is a passive topology. Ethernet and LocalTalk networks use a linear bus topology.

Advantages & Disadvantages Easy to understand, install and use for small networks. Cabling cost is less. Easy to expand by joining two cables with a BNC barrel connector. Repeaters can be used to boost the signal and increase the distance. Any one computer or device being down does not affect the others. Disadvantages: Heavy network traffic slows down the bus speed. Only one computer can transmit and other have to wait till their turn comes. Can’t connect a large no. of computers. A fault or break in the bus cable stops all transmission.

Ring Topology Ring topologies consist of several computers joined together to form a circle. Messages move from one computer to the next in one direction only. When a computer receives a message addressed to it, the message is copied and sent back with a modification to indicate that it was received. Ring topology is an active topology, which means that computers connected to a ring topology are responsible for moving data from themselves to the next computer. Thus, if one computer goes down, it affects the rest of the network. Some ring network do token passing.

Advantages & Disadvantages No one computer monopolise the network because every computer is given equal access to the token. Rings are used in high-performance networks where large bandwidth is necessary. Require less cabling so less expensive. Fault isolation is simplified. In a ring a signal is circulating at all times. Disadvantages: Traffic is unidirectional. If one node goes down, it takes down the whole network. Slow in speed. Adding or removing the computers disturbs the network activity.

Star Topology It uses a central device, called a hub, with drop cables extending in all directions. Each networked device is connected point-to-point to the hub that resends the message to all the computers in a broadcast star network or only to the destination computer in a switched star network. 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.

Advantages & Disadvantages Fast as compare to ring topology. Easy to modify and add new computers without disturbing the rest of the network. Single computer failures do not bring down the whole network. Eliminates traffic problem. Disadvantages: If central hub fails, it takes down the whole network. Slow in speed. Require more cabling so wiring cost tend to be higher. More expensive because of the cost of the concentrators(hub or switch).

Tree Topology Tree topology is a variation of a star. However not every computer plugs into the central hub, majority of them are connected to a secondary hub which in turn is connected to the central hub. Also known as hierarchical network.

Mesh Topology Every device has a dedicated point-to-point link to every other device.

Hybrid Topology It is combination of two or more network topologies.

Hub