Perhaps the most ambitious vision of mesh networking is that of MeshNetworks, a firm based in Maitland, Florida. It has developed its own radio hardware and some clever routing software that makes it possible to blanket an area with broadband wireless coverage using "intelligent access points" (its term for NAPs) and shoebox-sized wireless routers. But what is really clever is that this wireless mesh-network then supports mobile devices, such as handheld computers and laptops. And those devices can also act as routers for other mobile devices, further extending the mesh. Cleverest of all, even when two or more devices are beyond the range of a NAP or a wireless router, they spontaneously form their own local network. MeshNetworks' technology thus combines the mesh architecture with the even more radical approach of "ad hoc" networking.
From the Battlefield
As the name suggests, ad hoc networks consist of multiple devices, each of which also acts as a router for the others. Furthermore, these devices may also be moving, so that the network topology is in constant flux. This poses a number of challenges, not least in routing. Clearly, the quickest way to send a packet of data from one device to another changes as the devices move around, and other devices join and leave the network..
Ad hoc networks are commonly associated with military and emergency applications, both of which have to operate in situations where there is no network infrastructure. For that reason, ad hoc networks are sometimes referred to as "infrastructureless" network architectures. Rescue workers in an earthquake zone, for example, could use handheld radios, each of which also acts as a relay for other nearby radios. Similarly, the robust, self-healing properties of ad hoc networks make them suitable for military use, either by mobile combatants, or to connect up "smart dust" sensors that would be sprinkled across a battlefield from an aircraft.
For many years, says Zygmunt Haas, a researcher at Cornell University, most research into ad hoc networks focused on military applications. Recently, however, interest in the field has increased as its commercial possibilities have started to emerge. "Bluetooth", a short-range wireless protocol that enables mobile phones to talk to nearby handheld computers, printers and other phones, is a simple form of ad hoc networking, though it supports only single "hops" between individual devices.
The advent of WI-FI networking equipment has also provided a foothold. With the right software, it is possible to allow WI-FI-equipped laptops to act as relays for other nearby machines, letting packets make multiple hops from machine to machine to get to and from the Internet. Dave Johnson, an ad hoc researcher at Rice University in Houston, Texas, has built demonstration systems based on WI-FI devices in moving cars that do exactly this. Ad hoc networking might also expand the capabilities of mobile phones. People attending the "Burning Man" festival in the Nevada desert would then be able to call each other, even without any local infrastructure, suggests Charles Perkins, an ad hoc guru at Nokia's research centre in Mountain View, California. As well as working without any infrastructure, ad hoc-capable mobile phones would have other advantages. In a crowded environment, such as a sports arena, phones could pass traffic from other phones to base-stations in adjacent cells, thus boosting capacity. Calls between users within the arena could be handled locally, without loading the cellular network.
The ad hoc/cellular hybrid approach would also improve coverage at the edges of a cellular network, since users just outside the network's range would be able to "hop" their calls into the network via somebody else's phone; in the process, they would extend the effective size of the network, allowing still more distant users to "multihop" their way in. No wonder Nokia and other mobile-infrastructure manufacturers are keeping a close eye on ad hoc networking.
But there are still several problems to overcome. The first is a conflict of interest: do you really want somebody in another row of seats using your phone as a relay and draining your battery? The trade-off, says Dr Haas, is that the service quality improves for all, at the cost of handling each other's traffic. Some proposed ad hoc architectures, he says, include micro-payment schemes to ensure that everybody pulls their weight.
Another difficulty is agreeing on protocols; ad hoc will work only if devices are ubiquitous, and support an agreed standard. But different situations require different standards. This may require hybrid, adaptive protocols, where the network's behaviour adjusts depending on the circumstances.
The ad hoc approach is also favoured by proponents of ultra-wideband (UWB) transmission. UWB marks a radical departure from existing wireless technologies because, rather than transmitting and receiving on a particular radio frequency, it involves transmitting very short pulses on a wide range of frequencies simultaneously at low power. Such pulses, which are typically less than a billionth of a second long, pass unnoticed by conventional radio receivers, but can be detected by a UWB receiver. Information is encoded into streams of pulses, millions of which can be sent every second, by varying their polarity or their timing relative to an apparently random but pre-arranged schedule. (A slightly early pulse might signify a one, and a late pulse a zero.)
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