October 2005

From the Lab: Information Technology

New publications, experiments, and breakthroughs in information technology -- and what they mean

By TR Staff

Long-Distance Wi-Fi
Protocol extends the range of wireless networks

Results: Researchers in India have developed a communications protocol to increase the coverage area of Wi-Fi mesh networks. In a conventional Wi-Fi network -- like the ones that are now common at many urban cafEs and airports -- a base station with a wired connection to the Internet exchanges radio signals with users' portable devices.

In a Wi-Fi mesh network, by contrast, several nodes can exchange radio signals with each other as well as with users. Such a network can provide Wi-Fi coverage for a given geographical area at a lower cost than a series of conventional Wi-Fi networks, because not all of its nodes must be wired to the Internet.

The new protocol enables off-the-shelf Wi-Fi radios to form mesh networks with distances of up to 40 kilometers between their nodes -- compared with one kilometer or less for existing Wi-Fi mesh networks -- while maintaining or even increasing data transfer speeds.

In a simulation of a mesh network with nodes at least seven kilometers apart, the researchers achieved data transmission speeds 20 times as high as those possible with Wi-Fi's existing protocol.

Why It Matters: Wi-Fi networks are cheap and easy to set up, but their transmitters have a range of only about 100 meters. Meshed Wi-Fi networks can cover large urban and rural areas, but they don't solve the problem of Wi-Fi's inherently short range. Current Wi-Fi mesh networks typically require several nodes per square kilometer. Having fewer nodes spaced farther apart can result in lower data speeds and reliability.

Because the new communications protocol, developed by Bhaskaran Raman and Kameswari Chebrolu of the Indian Institute of Technology in Kanpur, increases the range of Wi-Fi while maintaining high data speeds or even increasing them, it may reduce the number of nodes needed and hence the cost of blanketing a large area with wireless Internet access, all without sacrificing performance.

Other technologies, including more-powerful antennas, have been developed to increase the range of Wi-Fi transmitters, but they work for only two nodes at a time; the new protocol enables multiple nodes to communicate with each other over long distances, while reducing interference and thus maximizing data speeds.

Methods: Every wireless network must have a communications protocol called a medium access control (MAC), which coOrdinates which radios can send signals when, so that they all transmit in an orderly fashion. In a mesh network, a node consists of multiple radios, each transmitting independently on a separate link to another node. The current MAC for Wi-Fi mesh networks allows some radios in a node to transmit signals at the same time that other radios are receiving signals, leading to interference that can slow the rate of data transfer and cause other problems. The researchers' MAC makes the radios in the same node either transmit only or receive only at any given time, avoiding interference and increasing data transmission speeds.

Next Step: The researchers plan to test their protocol in an outdoor deployment of a Wi-Fi mesh network covering 32 rural Indian villages. One intended application is two-way video to enable patients to "visit" doctors remotely. -- By Corie Lok

Source: Raman, B., and K. Chebrolu. 2005. Design and evaluation of a new MAC protocol for long-distance 802.11 mesh networks. Presented at the Eleventh Annual International Conference on Mobile Computing and Networking. August 28-September 2. Cologne, Germany.