In most modern networks, links and nodes are interconnected (both logically and physically) in either a star arrangement (with each node connected directly to a central switch, hub or server) or a bus configuration (with each node attached to a central line that is connected to a central switching component). Both of these configurations are well understood, inexpensive and generally reliable, but one broken link in either setup can isolate a node, cutting it off from the network.
A newer arrangement, mesh networking, connects each node to at least two other nodes (and potentially to each and every other network node, an arrangement referred to as “fully connected”). This involves more cabling (or more wireless devices) and greater overhead, but it allows the network to heal itself automatically when a break occurs, so there's no interruption of service to any node.
The lack of a hub-and-spoke structure is what distinguishes a mesh network. Also, meshes don't need designated routers; instead, nodes serve as routers for one another. Thus, data is passed from node to node in a process called hopping.
The first and best example of a mesh network is the Internet itself. Information travels across the Net by being forwarded automatically from one router to the next until it reaches its destination. The Internet is often depicted as a “cloud” because there are billions of potential paths a signal can take, and it's impossible to predict in advance what that route will be.
Wireless networking is an ideal vehicle for setting up a mesh network, because it can be done quickly and on an ad hoc basis. Wireless mesh nodes are small radio transmitters that function much like wireless routers, using existing Wi-Fi standards (802.11a, b and g) for communications.
Even in a wireless mesh network, you need a wired access point to reach the Internet. Getting that information back to the access point is called backhaul. Small wireless mesh networks handle backhaul without any special configuration. For larger mesh networks, however, such as those designed for cities or large enterprises, certain nodes must be dedicated as backhaul nodes. The other nodes send all outgoing information to a backhaul node, which sends it to the wired access point without extra hops.
Mesh in Action
In 2007, preparing for the annual Muslim pilgrimage to Mecca, Saudi Arabia's Communications and Information Technology Commission asked Internet service provider Bayanat Al-Oula to create a temporary wireless network and provide the 2 million pilgrims with free Internet connectivity. They chose a network of about 70 meshed routers and rolled it out in less than 60 days.