Network topologies

Network Topologies
What does network topology mean ?
The Bus Topology
The Mesh Topology
The Ring Topology
The Star Topology




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What does network topology mean ?
A network topology refers to either the physical or logical layout of a network installation.

Physical Topology when in the context of networking refers to the physical layout of the devices connected to the network, including the location and cable installation.

The Logical Topology refers to the way it actually operates (transfers data) as opposed to its layout.

There are four main network topologies (and mixtures of the four) in common use today.
Bus
Mesh
Ring
Star
We will take a closer look at these topologies below.
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The Bus Topology
This type of network topology is generally used with Ethernet (discussed later in the tutorial) networks.

A Bus topology can be both Physical and Logical.

The Bus topology is one of the simplest of the four network topologies to use, in its most basic form it is simply a case of running one cable (referred to as the backbone) from the first device/PC in the network to the last device/PC, and then add any further devices/PCs to the existing cable (backbone) between the first and last machines (see fig 1.1 below).
This topology is probably the cheapest network type of all to initially setup, as only one cable is used the installation is fairly simple and economical.

The problems can come when trying to add a device to an existing Bus topology network. To add a device requires physically linking it to the existing backbone which can turn out to be a major job.
Another consideration if using a bus topology for a network is fault tolerance, or the lack of it, this type of network transfers data by passing messages through the same cable, so a break in any part of the cable will bring the whole network down.

Each device will check to see if the message is intended for them, the device to which the data is addressed will copy the contents to its network card's onboard RAM and process it accordingly.
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The Mesh Topology
This type of network topology boasts the highest fault tolerance of all of the network topologies, it is also usually the most expensive.

In a mesh topology each device/PC is connected to every other device/PC in the network by its own cable (see fig 1.2 below), which means vast amounts of cables for any sizeable network.
The Mesh topology provides fault tolerance by having separate cables for each connection, allowing any one cable to break without interferring with the rest of the network.

Unfortunately because each connection needs its own cable a Mesh topology can get very expensive.

Everytime you add a client to a mesh network you have to run cables to each of the other devices.
The amount of cables you will need for a mesh network can be calculated by: CN = (D * (D-1)) / 2
(where CN is Cables Needed, and D is the amount of devices on the network)

If your network was going to have 4 clients/devices then you would need 4 * 3 / 2 = 6 cables

The problems come with a large network, imagine a 40 device network: 40 * 39 / 2 = 780 cables

When you need to add a single device to a Mesh network the job can be very complex, for example if you had a network of 15 devices, to add one device would mean adding 15 cables to the network.
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The Ring Topology
The physical ring topology is rarely used these days, a Ring topology networks the devices by connecting each device to its two neighbouring devices (see fig 1.3 below).
Data is passed one way from device to device, fault tolerance in a physical ring topology is non existant, if one device/cable fails then the whole network goes down.

Adding a new device to an existing physical Ring network can be complicated as any new device needs to go inbetween the existing devices.

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The Star Topology
A physical Star topology connects the devices via a centralised unit such as a Hub or Switch.
A Star topology gives great fault tolerance as any device/cable failure will not affect the rest of the network (unless the centralising device or server (if vital) fails).

Physically adding new devices to a Star network is very simple compared to any of the other topologies, you simply run a cable from the new device to the hub/switch.
Star topologies are very common, especially in Ethernet networks, also, they are commonly mixed with one of the other topologies to create a hybrid topology.
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