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Creating a Wireless Data Network

Written by Law and Order Staff

Smaller public safety agencies, especially those that aren’t well-funded, are often barred from the benefits of having a wireless data network because of economics and spectrum availability. Radio channels are hard to come by in this era of wireless-everything, and when you can get a channel for data, it requires building out a radio transmission network that is prohibitively costly.

The problem gets more complicated when the agency has to serve both an urban and a rural population. Wide-area Wi-Fi works okay in the city, but its limited range usually makes it impractical for terrain where it’s a long way between streetlights and utility poles. A hybrid solution of both Wi-Fi and commercial data network carriers may provide an affordable solution.

The public safety agencies in the cities of Corpus Christi, TX and Oklahoma City, OK operate in diverse environments, ranging from fairly dense urban streets to rural areas. A city-wide Wi-Fi network would work for the urban zones, but deployment into the rural areas would leave too many dead spots where there was no coverage.

A cooperative effort between RedMoon Broadband, PadCom, Inc, and Tropos Networks has produced a hybrid wireless network that operates over Wi-Fi when that coverage is available, then switches over to a commercial data carrier (Cingular, Sprint, or T-Mobile, for example) when the mobile unit moves out of the Wi-Fi coverage area (in Corpus Christi only).

The handoff between networks is seamless and transparent to the user, thanks to the TotalRoam switching technology from PadCom. Users do typically see a drop in data transmission speeds when they move to the commercial data carrier, which is not as speedy as the Tropos network.

The Wi-Fi portion of the network is provided by Tropos Networks. Wi-Fi, also known as 802.11x (the “x” represents a letter designating the specific flavor of Wi-Fi, like a, b, or g—there are various protocols and bandwidths in use) uses a license-free portion of the radio spectrum for data communications. Most Wi-Fi applications are very short-range, for use in offices and homes so that there is no need to string computer data cables to each machine.

There are also thousands of Wi-Fi “hotspots” in use in airports, coffee houses, and fast food restaurants for customers or anyone that wants to log on, depending on the level of security in use. By using signal amplifiers and directional antennas, a Wi-Fi network can be deployed to work over more than the usual 300 foot radius from the access point. This range is extended even further by using a “mesh” topology. In a mesh configuration, mesh routers relay signals from one to another, extending the network as far as the mesh routers are deployed.

The network is dynamic and self-healing. If one mesh router is disabled, others within range bridge the gap to sustain the coverage area. MeshNetworks, now a product of Motorola known as MOTOMESH™, uses a similar technology. Tropos and Motorola are competing companies in this market sector. One difference between the two companies’ products is that MOTOMESH uses its mobile units as relay stations and access points, where the Tropos network relies on fixed mesh routers.

A common problem with wireless networks that use relay stations is that communications between the relay stations add substantially to the “overhead” or data needed to manage the network. Information regarding the path a data packet takes between relay stations can be re-sent over the entire network, slowing down the throughput of the network.

Tropos uses proprietary technology to insure that no more than 5% of the network’s capacity is consumed with overhead traffic at any given time, no matter how many mesh routers are in the network. Tropos’ mesh routers are typically mounted on streetlight and utility poles, where they have access to a hardwired power connection. About 10% of the boxes will have a hardwired Internet connection, as well, allowing traffic to move between the mesh network and servers on the Internet or the public safety agency’s intranet.

Depending on the number of physical obstructions in the operating area, between 10 and 20 routers or relay stations are required for each square mile of coverage. Corpus Christi has about 300 such mesh routers serving 188 users in the police and fire departments.

No special hardware is needed on the users’ computers to access the Tropos network. Cities are free to shop for the best deals they can get on standard Wi-Fi access hardware. Users may need special wireless access cards to use the commercial data network that supplies connectivity outside of the range of the Wi-Fi network. Most cellular carriers market their own flavor of PC cards designed specifically for their network.

The PC cards are, in essence, cellular telephone transceivers that link the cellular data network with the computer where they reside. The antennas used for mobile cellular data networks and Wi-Fi are generally very small and intended for use in an unobstructed environment, so they may not work well in a car. For this reason, some users have external antennas on their cars that are attached by hardwired connections to the mobile computers.

It’s assumed that agency users will make greater use of the Wi-Fi portion of the network than of the cellular data portion. Wi-Fi networks operate on license-free radio channels, so there is no cost to the user beyond the initial purchase of the hardware. Cellular data networks typically charge by the kilobyte, with costs recurring every month they are used.

The hybrid network solution used by Corpus Christi and Oklahoma City provides connectivity throughout the users’ service area, while minimizing costs to the local government by making the greatest use of free public airwaves.

A system like this could be a headache to administer, given that there are three separate vendors (Tropos, PadCom, and whatever cellular carrier is used) for the network, plus the vendor for the computer hardware in the cars. So that users will have a single point of contact to resolve problems, RedMoon Broadband is the systems integrator for these projects. RedMoon is a Texas company that provides high-speed internet access to business and residential customers, as well as for local governments.

Local governments that build out these wide-area Wi-Fi networks can recoup some of their costs by selling wireless access to citizens and business users. A wireless network that extends over an entire city frees users from data cables, and allows them to use the Internet wherever they happen to be. Classrooms, offices and public gathering places like malls and coffee shops are all potential customers.

A wide-area network of this type will support any number of Virtual Local Areas Networks (VLANs), so security is less of a concern. The public sector users can maintain secure access by use of data encryption and standard protocols such as Wi-Fi Protected Access (WPA). The PadCom TotalRoam system adds an additional security layer for users on that network.

Published in Law and Order, Aug 2005

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Related Companies

MotorolaPadcom IncRedmoon Broadband
 

Related Products

CommunicationsMOTOMESH (computer software)TotalRoam (computer software)Tropos NetworksVLAN (Virtual Local Area Networks)Wireless Data Network
 

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