Ad hoc networking means that each computer talks to each other directly without the "supervision" of a device such as a router. This arrangement is sometimes called peer-to-peer networking. In wireless networking, a strength (and sometimes weakness) of ad hoc networking is that nodes on the network (meaning computers) can join (or leave) the network on-the-fly. So you don't necessarily know the number of computers on a wireless network.
Figure 14.1 shows an example of an ad hoc network that uses wireless networking.
Figure 14.1. An ad hoc wireless network.
If enough computers are involved, an ad hoc, or peer-to-peer network, can begin to form a kind of grid, or mesh. This gives peer-to-peer networks in some applications a great deal of power, although there's no really good way to administer a peer-to-peer network, and security remains an issue.
The Wi-Fi standards specify two different configuration modes, infrastructure and ad hoc. The access point/router style network that I showed you in Chapter 13 is an example of using Wi-Fi in infrastructure mode, whereas (as you'd probably expect) peer-to-peer Wi-Fi access uses ad hoc mode.
In infrastructure mode, communications between two nodes on the network flows through the access point. Computer A actually "talks" to the access point, which in turn talks to computer B. The access point also performs a number of other roles, such as connecting the nodes to the Internet or other WAN (wide area network), connecting multiple wireless networks, connecting the wireless nodes to a wired network, and providing management and security functionality (such as a firewall, as explained in Chapter 17, "Protecting Your Mobile Wi-Fi Computer," and Chapter 18, "Securing Your Wi-Fi Network").
In contrast, in ad hoc mode, computers A and B communicate directly without an intermediary, and none of the other functionality provided by the access point is present.
One drawback of using Wi-Fi in ad hoc mode can be signal strength and range. Ad hoc mode might make sense to use if you had two computers close to one another that you didn't expect to move muchan advantage being that you wouldn't need to spend $50 on an access point.
Another drawback of ad hoc mode is transfer rate. The IEEE 802.11 specification states that when in ad hoc mode, products only need to support a transfer rate of 11Mbps. In some cases products may exceed the specification and run at the 54Mbps rate of 802.11g but this is not required of the manufacturers so it can't be guaranteed.
I wouldn't maintain a fixed Wi-Fi network using ad hoc connections with more than a few computers, and I wouldn't expect to be able to maintain an ad hoc connection if one of my computers were mobile. When my wife carries our Wi-Fi laptop out to the garden, it goes way out of ad hoc range, but well within the broadcast range of our access points. (Nothing inherently causes an ad hoc broadcast to have less range; it's just that usually the computers used for ad hoc broadcasts don't have as good a radio or antenna and don't transmit with as much power, as a dedicated access point.)
Ad hoc connectivity is good for, well, ad hoc situations in life. Using Wi-Fi's ad hoc mode, you can connect two computers when you are on the road even though no wireless networks are present, which can be very useful. For example, on a road trip, a colleague might have a wired network connection and access to the Internet, which I might be able to share using an ad hoc connection with my colleague. (Sharing an Internet connection is explained later in this chapter.)
Ad hoc networks are also good for quickly and easily setting up a Wi-Fi network in situations in which flexibility is essential and a wireless infrastructure is not available (or needed). An example of this might be a temporary meeting or convention, or perhaps a group of consultants working onsite but without access to the "official" network might decide to set up an ad hoc network to exchange files between themselves.
Later in this chapter, I'll also show you a hybrid form of networking, which uses a Wi-Fi access point and an ad hoc mesh to blanket a large area and extend the range of the core configuration.