Your small office or home network may be small right now, but it may grow over time (which is what happened to me!). This section explains some of the ways to look at larger, more complicated networks. This material is somewhat advanced, and may be something to keep in the back of your mind when you are setting up your first Wi-Fi access point. But you may find it provides a useful perspective even as you are getting started.
A network topology means the way a network is arranged and how the devices on the network communicate with each other. A network's physical topology is the way devices are laid out (meaning which devices are connected to each other, and so on). In contrast, a network's logical topology is the way that the signals act on the network media, meaning the way that the data passes through the network from one device to the next independent of the physical interconnection of the devices. If you diagrammed the physical and logical topology for a network, the diagram might look the same, but then again, it might not.
In a true mesh topology, every node on the network is connected to every other node on the network.
I've already shown you some examples of network topologies. For example, the mesh pattern created by an ad-hoc wireless network used by a number of devices is a network topology.
An infrastructure wireless network that uses an access point is operating in a star topology, as shown in the diagram in Figure 15.12.
Two other common kinds of network topologies are the bus topology and the ring topology, shown in Figure 15.13. In the bus topology, all the devices on the network are connected to a central cable or bus (also called backbone). In the ring topology, all the devices are connected to one another in the shape of a closed loop, so that each device is directly connected to the two devices on either side of it, and only those two devices. (Does the ring topology "rule them all?")
Networks based on a star topology are probably easier to set up and manage than any other kind of network.
As a practical matter, most even reasonably complex networks are hybrids that have features of a variety of topologies. For example, one common hybrid is called the tree topology, in which groups of star topology networks are placed along a bus-topology backbone. This is a common arrangement for Wi-Fi networks that cover a large area. Each Wi-Fi access point manages a star topology group, and the Wi-Fi access points are connected using a backbone.
From the viewpoint of Wi-Fi, another interesting hybrid topology involves creating a full mesh network. Yes, as I explained at the beginning of the chapter, you can create a kind of mesh using Wi-Fi devices in ad-hoc mode. But what you have isn't necessarily very reliable, and it also doesn't facilitate full mesh computing in which every device speaks to every other device. (The devices don't necessarily have to speak to each other at the same time, or in real time.)
A useful Wi-Fi mesh network needs to be able to respond to devices entering and leaving the network, it needs to be able to operate with devices in infrastructure mode as well as ad-hoc mode, and it needs to be able to switch access points into bridges as required. Figure 15.14 shows what a mesh network of this sort might look like.
A true Wi-Fi mesh network of the sort shown in Figure 15.14 has a great many advantages. It has no single point of failure, and is therefore self-healing. This kind of network can easily get around obstacles?such as water-laden foliage and barriers to sightlines?that are problematic for other kinds of Wi-Fi networks.
However, to work properly, a Wi-Fi mesh topology requires some specialized software that provides routing functionality and has the ability to switch radios between infrastructure and ad-hoc modes as required.
Because of the benefits of Wi-Fi mesh networking, there's a lot of work going on right now in this area. For an effort to bring Wi-Fi meshes to the third world using open source technology, you might be interested in the Wireless Roadshow project, www.informal.org.uk/wirelessroadshow/. Closer, perhaps, to home, a startup company named Firetide, Inc., www.firetide.com, based in Hawaii, is making the hardware and software needed to deploy robust true mesh topology Wi-Fi networks.