Network Topology

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Revision as of 01:48, 13 April 2009

Network topology refers to the configuration of the physical and logical connections between the elements (links, nodes, etc.) of a network. Network topology does not refer to the physical layout of a network, nor does it refer to transmission rates, signal types or other characteristics of computer networks. Networks are normally organized by one (or more) of five basic types of topologies: bus, ring, star, tree, and mesh. In some instances two networks are interconnected forming a sixth type of topology: hybrid.

Types of Network Topologies

Bus

The bus topology uses a single transmission medium on which all messages are broadcasted. Each node receives all messages, but only the intended destination accepts it. Only one message can be sent at a time.

Advantages

• Requires a minimal amount of connections.
• Simple to configure.

Disadvantages

• A fault in the main connection can bring down the entire network.
• Very unsafe. Each computer receives all messages on the network. A computer can still accept messages even if its address does not match the destination address.
• Is slow, and prone to collision problems.

Ring

In a ring topology each node is connected to exactly two other nodes forming a closed loop. Messages are passed along the ring in a single direction until they reach their destination.

Dual-ring network

Dual-ring networks contain two separate rings networks which pass messages in opposite directions. Generally, only one ring is operating at a time. This is to create redundancy in the network if one of the rings fails.

Advantages

• Dual-ring networks are more reliable than bus, and other similar topologies. If a fault occurs in one of the ring, the network does not fail since there is another ring as a backup.
• Is relatively simple to configure.

Disadvantages

• Single-ring networks provide no more reliability than the bus topology.
• Due to all nodes wanting to send messages along the ring, issues concerning fairness.

Star

The star topology consists of a hub, router, etc., connecting each other node of the network. Each message passes through the hub and is redirected to its destination.

Advantages

• Easy to detect errors and faults. Each node is separate from all other nodes through the hub, so it is easy to isolate and fix the problem.
• Any break in a connection between the hub and a node will not cause a failure in the network.
• Minimizes the number of possible collisions.

Disadvantages

• If the hub has a fault, the entire network can fail.

Tree

Combining the characteristics of multiple star networks along a single bus network with a root node forms the tree topology. The hub/central node of each star network is connected to the root node of the tree creating multiple tiers on the network. The root node of the tree connects with the central node of the star network, which connects to more nodes in lower tiers.

Advantages

• Easy to expand.
• Allows for much larger number of nodes than bus or star networks individually, without having the same speed and size limitations.
• If there is a fault in the root node, the connections within each of the second tier networks will not be lost.

Disadvantages

• If there is a fault in the root node, the connection between each of the second tier networks can be lost.
• Is more complex some other topologies, making it more difficult to configure.

Mesh

Mesh topologies present multiple paths that a message can take to reach their destination. Each node is connected to one or more other nodes. If each node is connected to every other nodes then it is called fully connected or a full mesh. Messages are passed along the network by using a routing algorithm or table which determines the next nodes the message will take to reach its destination.

Advantages

• Creates inherent failure protection through the redundancy of connections between nodes. There is generally multiple paths from one node to another, so a network fault will generally not cause a node from being disconnected from the network.
• May be easy to expand. Only a small number of nodes may need to have their routing algorithms/tables updates to expand the network.

Disadvantages

• Can be much more complex than other topologies. As the size of a mesh network grows the number of connections can grow exponentially, with the most extreme growth rate occurring in full meshes, having n(n-1)/2 connections.
• If the routing algorithm/table is not properly implemented on any number of nodes a number of errors can occur, such as lost messages or messages continuously passing through a cycle.
• May be very difficult to expand. As highlighted in a full mesh, all nodes will need to have their routing algorithms/tables updates to include the new nodes.

Hybrid Networks

Hybrids networks are networks that combine two or more of the basic types of topologies, such that it creates a new type of network. For example, connecting two bus networks together via a central node is a hybrid network, whereas just combining two bus networks is not. A practical example of hybrid networks are many home networks, which combine LAN and WLAN. This combines elements of bus, ring, and star topologies to create one network.

Centralization

A network is centralized if there exists a central node, usually a hub or router, through which messages are passed. Both star and tree topologies contain these central nodes. All messages are passed through the central node and are then redirected to the destination node, or at least passed towards the destination node in the case of a tree. Each central node is responsible for knowing the address and routing information of each other node connected directly to it on the network. The peripheral nodes only pass messages to the central node which then routes them towards the messages destination. Centralized networks are far easier to expand upon. When a new node, or set of nodes are added to an existing network only the central node(s) need to be updated in terms of routing capabilities. Peripheral nodes take no part in routing messages thus is it not necessary to update them.

Decentralization

A decentralized network does not contain a central node, messages are passed directly from node to node. Bus, ring, and mesh topologies are all decentralized. Decentralization is particularly important in mesh networks. Since mesh networks are decentralized, it allows them to have multiple redundant connections. Decentralized networks are generally more difficult to expand upon. Since in the bus topology all messages are broadcasted decentralization does not inhibit its ability to expand. However, in the ring and mesh topologies, routing information will need to be changed for each node affected by the changes. This is a particular problem with full meshes. Each node needs to be provided with new routing information when the network is changed.