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# 4.5 WiFi network structure

A WiFi network can operate in one of two different modes: ad hoc mode or infrastructure mode

In an ad hoc network, stations communicate with each other directly, without the need for any intermediary or central control. This means that when one WiFi device comes within range of another, a direct communication channel can be set up between them. This is known as peer-to-peer communication. Additional devices can join the network, all communicating with each other in a broadcast fashion. (In this context, 'broadcast' means that a message sent by one node will arrive at every other node in the network, regardless of the destination address.)

Figure 14 provides a diagram of a possible geographical layout of an ad hoc network. This figure is similar to the diagrams of network topologies you met in Section 3.3, in that it is a representation of a network layout viewed as though you were floating above it and looking down. In Figure 14, the dark circles represent WiFi stations. The concentric circles around them show that the communication channel is radio waves. In this case, the diagram indicates non-directional antennas. However, since the focus of this diagram is the network layout, then the detail of whether directional or non-directional antennas are used isn't important.

Figure 14 Representation of a WiFi ad hoc network

An ad hoc network is likely to be temporary – for example, a network set up for a business meeting where people want to share information stored on portable devices like lap-top computers and personal digital assistants (PDAs). (If you looked up the term 'ad hoc' in your dictionary you probably found a definition something like 'for a specific purpose, impromptu, not pre-planned'.) An ad hoc network is independent of, and isolated from, any other network.

In infrastructure mode (Figure 15), stations communicate with each other via a wireless access point (AP) which also acts as a connector between a wired network and the wireless network. The access point is effectively a base station that controls the communication between the other stations. Access points form part of a wired network infrastructure and are not mobile. (If you looked up the word 'infrastructure' in your dictionary the definition given probably used terms like 'underlying foundation, basic structure, substructure'.)

Figure 15 Representation of a WiFi infrastructure network
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