What are Network Topologies

A network is a logical extension of data communication system. In computer network, two or more computers are link together with the carrier and data communication devices for the purpose of data communication and resources sharing. The manner in which computer nodes are geometrically arranged and connected is known as the topology of network.

LAN networks are commonly characterized in terms of topology. Two major classes of LAN topologies are unconstrained and constrained topology. Three basic constrained LAN topologies known as Bus, Ring and Star topologies. Specific topologies depends on the geographical scope of network. Network limited to a company’s or universities local facilities. LAN networks has horizontal topologies and interconnect computers that are peers with similar capabilities.

Network that covers long distances are called WAN and often has vertical topologies. because some hosts may have greater data processing capabilities and less capable hosts funnel data to them for processing. In practice, a particular application may be pure topology or a mix of several topologies.

Horizontal OR Constrained Topologies

Star Topologies

Star topologies is the simplest topology. In this topology, there is  central switching node which is connected to nodes through multiple communication line. Nodes are not link directly to each other and can communicate only via the central switching node. The routing function performed by the central node which centrally controls communication between any two nodes by establishing a logical path between them. The central node can be active device or passive device and transmission medium can be either twisted pair wire, coaxial cable or optical fiber.

Advantages

• Transmission delays between two nodes do not increase by adding new nodes to the network because any two nodes may be connected via two links only.
• If any of the local computer or node fail; the remaining portion of the network does not affect.
• Data security is very high.

Disadvantages

• The system totally depends on the central node. If the central node fails entire network fails.
• Communication line cost is very high.

Ring Topology

In a ring topology, consecutive nodes are connected by point to point links which are arranged to form a single closed path called ring. Each ring processor has communicating subordinates but within the ring there is no master computer for controlling other computers.

A node receives data from one of its two adjacent nodes. The only decision a node has to take is weather the data is for its user or not. Data packet is utilized if addressed to the node otherwise passed it to nearby next node. The data flow may be uni-directional or bi-directional.

Ring Point-to-Point Topology – In point-to-point ring topology there is a broken copper wire and transmission is only through from one node to another node. However in any case, If  any one of the node get fails then ring topology automatically converts to bus topology.

Ring Broadcast Topology – In this type of topology transmission done at once at all the nodes rather than from one node to another node.

Advantages

• The ring network works well where there is no central-site computer system. It is truly distributed data processing system.
• It is more reliable than a star network because communication is not dependent on a single host computer. If one line between any two node break down or if one of the node itself break down alternate routing is possible.
• In this topology there is need to send acknowledgement separately. The frame itself work as a acknowledge frame because after passing through each and every node it returns back to the sender.

Disadvantages

• In a ring network, communication delay is directly proportional to number of the nodes in the network. Hence addition of the nodes in the network increases the communication delays.
• The ring network is not as popular as start network because of its more complicated control software.

Bus OR Linear Topology

In a bus topology all the network nodes are connected to a common transmission medium. As a result, one pair of users on the network can communicate at the same time. When a particular node wants to send a message to another nodes, it appends the destination address to the message and check weather the communication line is free. As soon as communication line is free it broadcasts or places the message on the line. As the message travels on the line each computer check weather it is address to it. The message is picked by the addressee computer which sends the acknowledgment to source computer and free the line.

Advantages

• The main advantage is that the cable runs are shorter then the other topology.
• The reliability of network is high because the failure of a computer in network does not affect the network functioning for other computers.
• Addition of new node to this topology is ea

Disadvantages

• Limitation of bus network include transmission distance restriction in base-band buses, difficulty in implementing delivery priorities and the signal balancing problem in baseband and dual cable broadband buses.