Some cities are in a fine mesh

Now that we’re all wrapped up in the Net, some developers want to teach us another metaphor of omnidirectional connectivity: the mesh.

Proponents of meshing say this approach to wireless broadband communications offers better performance and lower costs than other architectures that are popular today.

A traditional wireless network is strictly a hub-and-spoke affair. Each mobile device communicates over the air with a fixed transceiver, which communicates with a server via landline. By contrast, a wireless mesh relays data over the air in many directions, among multiple nodes. Data makes short “hops” from one node to the next, until it reaches its destination. For each packet, the network chooses the most efficient path based on current network conditions.

Mesh networks come in two flavors: peer-to-peer and fixed. In the peer-to-peer mesh, mobile devices form their own network; each end-user device doubles as a router. A network designer might supplement this untethered network with fixed access points, but that landline connection may not be required, depending on the network’s purpose.

Using a peer-to-peer mesh, soldiers dropped into enemy territory, or firefighters in a skyscraper with spotty conventional radio coverage, could exchange text, voice, graphics and video without relying on fixed infrastructure.

If a disaster crippled the infrastructure that supports other wireless networks, people on a peer-to-peer mesh could still communicate. That would put them in a better position than subscribers whose cell phones fell silent during last summer’s blackout in the northeastern U.S.

A peer-to-peer network resists damage, and is therefore a good choice for public-safety applications, because the wireless devices that compose it are like a swarm of bees, said Rick Rotondo, vice president of technical marketing at MeshNetworks in Maitland, Fla.

“You can flick as many bees as you want, but that swarm just fills in the holes and keeps coming,” Rotondo said. If a router on a peer-to-peer mesh stops working, “the routes are going to heal, and you’re just going to go around to another router. That makes it very hard for a natural disaster or a man-made disaster to deny the network to the first responder.”

MeshNetworks is one of the first vendors to come to market with peer-to-peer mesh networking technology. The company employs a proprietary radio protocol — Quadrature Division Multiple Access (QDMA) — and transmits at 2.4 GHz. Its Mesh Enabled Architecture (MEA) relies on software that routes communications among mobile devices, continually evaluating conditions to choose the best path. Network nodes can be notebooks, handheld computers or other devices that accept MeshNetworks’ communications card.

Because the nodes relay data from one to the next, rather than from a single mobile device to a fixed access point, the peer-to-peer mesh can transmit data at high speeds over much longer distances than a conventional Wi-Fi network, within the same power restrictions, Rotondo said.

The company offers pure peer-to-peer networks and networks that mix mobile and fixed routers. MeshNetworks also offers location services without relying on the global positioning system; instead, it uses triangulation to pinpoint each device on the mesh.

Last summer, the Orange County Fire Rescue Department in Winter Park, Fla., tested prototype location devices from MeshNetworks for tracking firefighters inside a burning building. Bill Godfrey, deputy chief in the department’s training and rescue division, eventually hopes to protect firefighters on the job with devices that would combine location with communications capabilities. Such devices would enable firefighters to relay data among themselves and outside to the department’s Rapid Intervention Team (RIT). RIT personnel would track firefighters on MeshNetworks-enabled computers.

“The ability to know where the firefighter was when he was in trouble, or track the firefighters on the fire ground, would be of great interest to the RIT if they had to go in and make a rescue,” Godfrey said.

The department is now working with MeshNetworks to refine the location technology. For example, the pilot revealed that there is no need to supplement triangulation techniques with a GPS receiver outside the building. Rather than calculate each firefighter’s position on the earth, the system merely needs to track his location relative to the rescuer.

“If I can see their dot on a screen, and I can see the dot representing where I am on a screen, and I can walk my dot to their dot and find them, I’ve got a win,” Godfrey said. The peer-to-peer mesh can do this without GPS.

First responders may augment their communications by using “bread crumbs” — handheld devices that they drop on the floor behind them as they move, creating a path for relaying data to the outside, Rotundo said. However, bread crumbs currently are not part of the system in Orlando.

If MeshNetworks perfects its location technology, Orange County Fire Rescue would use the broadband network to transmit video from inside burning structures and back up its voice radio system, Godfrey said.

At the other end of the continent, another public-safety agency, the San Mateo, Calif., Police Department (SMPD), has embraced the second variety of meshing: the fixed mesh. Like a conventional Wi-Fi network, this uses an array of hot spots. But, instead of wiring every access point to the network backbone, a fixed mesh provides a wired backhaul at a small number of hot spots. The other nodes relay data wirelessly among themselves. Like mobile devices in the peer-to-peer network, these fixed devices use software to monitor network conditions and choose the most efficient way to route data to its destination — to the wireline link, or from that link back to a mobile device.

For users who want a fixed infrastructure, the meshing approach offers attractive savings, said Bert Williams, vice president of marketing at Tropos Communications, the SMPD’s technology vendor. The cost of wiring every access point depends on the technology used, but a T-1 link might cost $300 per month, he said. “If you have 10 cells and you can eliminate backhaul to eight of them, if you want to keep two backhauls for redundancy’s sake, then you’ve just saved yourself $2400 a month.”

Tropos, also in San Mateo, uses standard Wi-Fi radio technology. “The real secret sauce is in the operating software in the system, which has three main components,” Williams said.

A “lightweight control protocol” keeps each access point aware of the RF conditions between itself and other network nodes. The “predictive path optimization” software continually calculates the best path between each node and the wired backhaul. The third element provides Advanced Encryption Standard (AES) and Wired Equivalent Privacy (WEP) encryption. Tropos suggests users further boost their security by implementing a virtual private network (VPN).

Last June, the SMPD started using 17 access points, two of them with wired backhauls to the city’s fiber-optic ring. Powered by the streetlamps on which they hang, the access points form a one-square-mile hot zone in the city’s downtown. The department later added a second square mile of coverage, said Lt. Wayne Hoss of the SMPD’s support division.

The department built the network so officers could access law-enforcement and motor-vehicle databases, file reports, view photographs and perform other tasks on the network without returning to the police station. Each officer carries a Panasonic Toughbook with a Cisco Aironet 350 Series communications card. The department supplements Tropos’ security measures with a VPN and intrusion-detection software.

A federal grant funded the mesh-network project, along with a conventional 800 MHz private radio system. Police officers use the conventional system for computer-aided dispatching, reserving the mesh network for applications that require broadband or secure communications, Hoss said.

Although the 800 MHz system transmits data at only 19.2 kb/s, it covers all of San Mateo. The mesh does not. Officers who need broadband communications must be inside one of the hot zones or near one of the supplemental access points the SMPD has placed on buildings throughout the city. According to Tropos, its technology supports data rates ranging from 1 Mb/s to 11 Mb/s. Users can expect performance on this order when they are standing still or traveling at less than 30 mph, Williams said.

If the cost of Tropos’ network drops enough to let the system cover the whole city, the SMPD someday might use it for routine dispatching as well.

“I would disband my private network and do it exclusively on the broadband side,” Hoss said

Although it doesn’t yet meet all the department’s communications needs, the fixed-mesh network has some clear benefits. For example, a motorist led a California Highway Patrol officer on a high-speed chase off the freeway and onto the streets of San Mateo. The driver stopped the car and ran.

“One of our officers pulled up, ran the license plate of the car, and then was able to go into the DMV system and bring up a photograph of the owner of the registered vehicle,” Hoss recalled.

The owner was the errant driver. The officer transmitted the photo to others who were entering the hot zone to join the pursuit. Police soon spotted and arrested the motorist.

“We probably wouldn’t have been able to pick that driver out of a crowd walking down the street without having the photograph,” Hoss said.