Network Topologies

In networking, the term topology refers to the layout of connected devices on a network. Think of it as the logical "shape" of the network wiring. This logical shape does not necessarily correspond to the actual physical layout of the devices on the network. For example, the computers on a home LAN may be arranged in a circle, but it would be highly unlikely to find an actual ring topology there.

'Logical' means how it looks as a pure design concept, rather than how it actually looks physically. e.g. the topology pictures you will see have nice straight lines between bits of the network, they don't try to show all the corners that need to be turned and holes that have to be drilled in a real cable installation.

Each topology has its advantages and disadvantages: usually related to cost, complexity, reliability and traffic "bottlenecks".

The 5 topologies are: Bus ... Ring ... Star ... Tree ... Mesh

Bus topology

Many devices connect to a single cable "backbone". If the backbone is broken, the entire segment fails. Bus topologies are relatively easy to install and don't require much cabling compared to the alternatives.

Ring Topology

In a ring network, every device has exactly two neighbours for communication purposes. All messages travel through a ring in the same direction. Like the bus topology, a failure in any cable or device breaks the loop and will take down the entire segment. A disadvantage of the ring is that if any device is added to or removed from the ring, the ring is broken and the segment fails until it is "reforged" (by dwarfish goldsmiths?)   It is also considerably more expensive than other topologies.

Star Topology

A star network has a central connection point - like a hub or switch. While it takes more cable, the benefit is that if a cable fails, only one node will be brought down.

All traffic emanates from the hub of the star. The central site is in control of all the nodes attached to it. The central hub is usually a fast, self contained computer and is responsible for routing all traffic to other nodes. The main advantages of a star network is that one malfunctioning node does not affect the rest of the network. However this type of network can be prone to bottleneck and failure problems at the central site.

Tree Topology

Pic from http://fcit.usf.edu

Also known as the 'Hierarchical topology', the tree topology is a combination of bus and star topologies. They are very common in larger networks. A typical scenario is: a file server is connected to a backbone cable (e.g. coaxial) that runs through the building, from which switches are connected, branching out to workstations.

Mesh topology

In the topologies shown above, there is only one possible path from one node to another node. If any cable in that path is broken, the nodes cannot communicate.

Mesh topology uses lots of cables to connect every node with every other node. It is very expensive to wire up, but if any cable fails, there are many other ways for two nodes to communicate. Some WANs, like the Internet, employ mesh routing. In fact the Internet was deliberately designed like this to allow sites to communicate even during a nuclear war.

The famous Ethernet 5-4-3 rule
(thanks to http://fcit.usf.edu)

A consideration in setting up a tree topology using Ethernet protocol is the 5-4-3 rule. One aspect of the Ethernet protocol requires that a signal sent out on the network cable reach every part of the network within a specified length of time. Each switch or repeater that a signal goes through adds a small amount of time.

This leads to the rule that:

• between any two nodes on the network there can only be a maximum of 5 segments,
• connected through 4 repeaters/switches, and
• if the segments use coaxial cable, only 3 of them may be populated (trunk) segments .

(A populated segment is one which has one or more nodes attached to it)

This rule does not apply in Ethernet networks where all fibre optic cabling or a combination of a fibre backbone with UTP cabling is used. If there is a combination of fibre optic backbone and UTP cabling, the rule is simply translated to 7-6-5 rule.