# Mesh, Bus, Ring and Star Topologies

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Mesh, Bus, Ring and Star topologies

The term Topology refers to the physical or logical shape or layout of a network. Communication between different nodes within a network is determined by its topology. Mesh, Bus, Ring and Star are four of the most common network topologies, each with advantages and disadvantages in relation to each other.

Mesh Topology

The Mesh topology is true to its name in that there are multiple redundant interconnections between network nodes. A mesh network can employ one of two connection arrangements, full mesh or partial mesh. Full mesh topology is when each node is directly connected to every other node on the network. Partial mesh topology occurs when some nodes are connected to every other, but others are only connected to those with which they will exchange the most data.

Mesh networks are reliable, stable, and redundant. If a node can no longer operate, all the others are not affected as they can still communicate either directly or through one or more intermediate nodes. Mesh networks can be very expensive due to the large amount of cable and connections required.

Bus Topology

Bus topology refers to a local area network (LAN) arrangement where each node or device is connected to a main cable or link called a bus. A bus network is simple yet very reliable. Since nodes themselves are not relied upon for communication, the failure of a single node is not problematic for the rest of the network. For a major issue to occur, there must be a problem with the bus itself.

Despite its simplicity and reliability the bus network does have several limitations. The cable length of the bus is limited due to data loss. Also, a bus network may have performance issues if the nodes are located at scattered points and due not lie near a common line.

Ring Topology

Ring networks are those in which the nodes are connected in a closed loop configuration. Nodes adjacent to each other are directly connected while others are indirectly connected with the data passing through one or more intermediate nodes.

The ring topology may be the best choice when system requirements are lesser and nodes are at scattered points. If nodes are reasonably close, the implementation cost can be lower than other topologies when cable routes are chosen wisely.

While cost effective, the ring network can have reliability issues. If a break in cable occurs data speed can suffer due to an increased data path. If two breaks occur, some nodes may be completely cut off from the others.

Star Topology

Star topology refers to a network where each node is directly connected to a common central computer. Each individual node is connected to the other through the central computer. Star networks work best when nodes are at scattered points.

Star network are relatively easy to maintain and provide a reasonable amount of reliability. If a single node fails only the node itself is isolated. All other nodes will continue to function as normal but will not be able to communicate with the isolated device.

Star networks provide little or no inherent redundancy. If the central computer fails the entire system will fail.

Ethernet, Token Ring, FDDI and Wireless

Ethernet

Ethernet was first designed in 1973 by Xerox researcher Bob Metcalfe. It has since become the most widely deployed network technology in the world. A local area technology, Ethernet is used to connect devices within close proximity with each other.

Ethernet relies upon a common medium to which each device or node is connected. Signals are broadcasted throughout the medium and received by all devices. Only the device(s) with the correct destination address will process the signal.

Ethernet has several limitations mostly resulting from the shared common medium and data loss due to cable length. As electrical signals travel across cable they weaken, thus requiring that network cables remain short enough that devices at opposite ends can receive clear signals with minimal delay. Additionally, the protocols used within Ethernet only allow a single device to transmit at any given time. This results in limits to the number of devices that can coexist in a single network.

Token Ring

A Token Ring network is one in which nodes are connected in a ring or star topology and bit or token scheme is used to prevent data collision. The Token Ring is the second most widely used protocol after Ethernet. Token Ring provides for data transfer rates of either 4 or 16 Mbps.

Token Ring networks work by continuously circulating empty information frames on the ring. When a device whishes to send a message it inserts a token, a message, and destination identifier into an empty frame. Each node then examines this frame. If the device determines it is the destination for the message, it copies the message from the frame and changes the token. When the frame returns to the originator and sees that the token has been changed it removes the message from the frame.

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