A mesh network distributes a wireless signal through a system of transmitters called nodes. A node is essentially a wireless router. Special software in the node allows it to communicate with a network of other nodes. The nodes use dynamic routing to determine the best route for data exchanges. The best route can change depending on node locations, data traffic and environmental conditions. Most importantly, some nodes can be mobile.
At least one node needs to be in a fixed position. This is the node that acts as a bridge between the mesh network and a larger network like the Internet. For instance, a radio tower linked to the Internet might serve as the main fixed node for a mine. Most mines have multiple fixed nodes to give the mesh network a strong foundation.
Wireless mesh networks tend to operate under one of two radio frequencies: 2.4 gigahertz (the IEEE 802.11 protocol) or 900 megahertz (the IEEE 802.15 protocol). Both types have problems broadcasting signals around corners, so a node at each corner may be necessary to keep the network stable. The MSHA found that these protocols didn't interfere with other communications systems in mines [source: MSHA].
Nodes don't have to be very large -- many are about the same size as an average wireless router. That makes it easy for miners to attach nodes to heavy equipment. Because nodes can work even while in motion (though at a slower bit rate), various mining vehicles can carry the nodes. You can wire nodes to sensors in a mine to monitor mine conditions as well. If danger arises, the node can broadcast signals to the rest of the network.
It's also possible to build electronic badges that interact with the mesh network. That makes it possible for the network to track the location of individual miners. As the miner moves through the mine, different nodes detect the miner's badge. If part of the mine becomes hazardous, the mining company can see how many miners could be affected and evacuate that section of the mine. Miners can also use devices like personal radios or voice over Internet protocol (VOIP) phones to remain in contact with operations on the surface.
Mesh networks can help prevent accidents, but what happens if part of a tunnel collapses? Unfortunately, mesh network wireless protocols aren't very effective at penetrating earth. For that, you need to use a system that operates on very low frequencies. The Mine Safety and Health Administration (MSHA) tested a through-the-earth (TTE) system that could penetrate 270 feet (82.3 meters) of rock. Data transfer was very slow -- 20 to 30 characters per minute with 80-percent accuracy at depths of up to 630 feet (192 meters). Potentially, a TTE system may be able to broadcast even further through solid earth. But there's a downside in addition to the slow data transfer rate -- the MSHA found that TTE systems could interfere with other communications systems in mines.
Wireless radio networks can give miners a lifeline back to the surface. Considering how dangerous their occupation is, a reliable communication network could mean the difference between life and death.
To learn more about different ways to use wireless networks, take a look at the links on the next page.